Newsgroups: comp.lang.java.programmer,comp.lang.java.help,
comp.lang.java.gui,comp.answers,news.answers
Approved: news-answers-request@MIT.EDU
Subject: Java Programmers FAQ
Followup-To: poster
Summary: This posting answers frequently-asked questions by Java programmers
Supersedes: <6l4gts$psh$1@shell15.ba.best.com>
Archive-name: computer-lang/java/programmers/faq
Posting-Frequency: weekly
Last-modified: 1998/6/11
URL:
http://www.afu.com/javafaq.html
Copyright: (c) 1997,1998 Peter van der Linden
Maintainer: Peter van der Linden
Frequently Asked Questions (with answers) for Java programmers
_____________________________________________________
________| |________
\ | Java Programmers FAQ http://www.afu.com | /
\ | Copyright (c) 1997-98 Peter van der Linden | /
/ |_____________________________________________________| \
/___________) (__________\
The Java FAQs here are intended for people who already have some
programming experience, though maybe not in Java.
Go to the FAQ home page at http://www.afu.com
for other Java information and downloads, and the most up-to-date copy of
the FAQ. Report FAQ updates to faqme at the address afu.com.
Please support Java Portability.
The biggest value of Java is its portability.
-
Portability makes it easier for companies to change/upgrade operating systems
and platforms.
-
Portability makes it easier for Java programmers to transfer their skills
to new employers.
Microsoft's strategic objective: "Kill cross-platform Java"
Microsoft does not want users to easily move software to different
platforms and is actively trying to undermine Java.
The Department of Justice's lawsuit quoted a Microsoft memo
describing the strategic objective to "kill cross-platform Java
by grow[ing] the polluted Java market".
See
http://www.usdoj.gov/atr/cases/f1700/1762.htm.
Microsoft is being sued because of unauthorized changes it made Java.
A Federal judge issued a preliminary injunction against Microsoft in March
1998, prohibiting them from labelling their incompatible J++ product as Java.
Speak to your management chain -- how comfortable do they feel using a
Microsoft product that is embroiled in a legal dispute, that introduces
deliberate incompatibilities, and whose stated
goal is to lock you in to one platform? It is a safer
choice to get standard Java from any other source than Microsoft.
You can use these facts to move your company to standard Java.
The best way for programmers to support portable Java is to reject
"polluted" non-standard
tools from the only company pushing them: Microsoft. As a Java programmer
please join the Java Lobby, an independent organization dedicated to representing
non-vendor interests in Java. It's free, and you can sign up by visiting
http://www.javalobby.org for details.
Other ways to encourage portable java:
-
Use development environments from other vendors, or convert Microsoft Visual
J++ to use Sun's JDK, following the instructions at
http://www.orbiter.demon.co.uk/
-
Use Netscape Communicator (not Internet Explorer)
-
If required to use Internet Explorer, use the Java Plug-In to get a standard
Java system inside it.
-
Use a JVM from GNU, Kaffe or Sun (not Microsoft's J++ SDK)
Free standard Java compilers and Java Plug-In can be downloaded from
http://java.sun.com.
Free standard Java Virtual Machines can be downloaded from
http://www.kaffe.org. and
http://www.oryxsoft.com/projects/gvm and
http://www.redhat.com/linux-info/jolt
Free Java AWT software can be downloaded from
http://www.biss-net.com/biss-awt.html and the files are all at
ftp.java-linux.org (the linux site) too.
Free Java software can be downloaded from
http://www.gnu.org/software/java/java.html
Just for the record, the May 1998 Federal case against Microsoft has nothing to
do with innovation, or product design. Microsoft is charged with
-
taking anti-competitive action to exclude competition in browsers,
in order to protect its monopoly in desktop operating systems.
-
using its monopoly to impose restrictive agreements that require PC
manufacturers to accept the Microsoft browser with Windows, and that
hinder the promotion of competing browsers.
Many people think that contracts prohibiting the distribution of
a third party's products are somewhat sleazy. Such contracts are also
in restraint of competition and illegal when used by a monopoly.
This is why Microsoft is facing mounting legal problems in the United
States, in Japan, in Brazil, and in the European Union.
0. A WORD FROM OUR SPONSOR
1. LOOKING FOR INFORMATION
2. COMPILERS, ETC.
3. LANGUAGE ISSUES
4. AWT and Related Topics
5. CORE LIBRARIES
6. I/O
7. NETWORKING & DISTRIBUTED
OBJECTS
8. MULTI-MEDIA
9. SECURITY
10. Java IDIOMS
11. FOR C and C++ AFFICIONADOS
12. FURTHER RESOURCES
13. MOST COMMON JAVA NOVICE PROBLEMS
14. FOR WINDOWS USERS
15. Java GOTCHA'S
-------------------------------------------------------------------------
0. A WORD FROM
OUR SPONSOR
Learning Java
This Java FAQ is maintained as a service to the Java community. Please consider
these books from the FAQ author when you are looking for a programming text.
Sample chapters are on Sun Microsystems webpage at
http://www.sun.com/971124/cover-linden/.
1. Looking
For Information
1.0 Is
Java "Open" or "Proprietary"?
A. Java is a freely-available open
specification. Sun Microsystems publishes the Java specifications and
public interfaces as open specifications that can be independently
implemented
by anyone without charge (termed a "clean-room" implementation in industry
jargon).
Specifically, Sun includes a perpetual, irrevocable, free and royalty-free
license in the front of the Addison-Wesley books containing
the specification.
Further, by submitting Java specifications to be ISO/IEC standards, Sun is
giving up some control in exchange for the world being able to refer to a
Java specification that is guaranteed to be open by ISO/IEC's charter and
rules. Sun promised ISO that it will abide by ISO's rules for developing
a standard, in particular it will take account of industry input. Just as
with other ISO standards, "open" does not mean that anyone who suggests any
new feature will get it into the Java standard; Sun has the casting vote
in the future evolution of Java, but it does not own the specification nor
the sole right to implement to it.
On the spectrum of "single vendor proprietary
technology" to "open to all to implement", Java lies much more towards the
open end than the single vendor end.
Sun Microsystems has invested significant time and money to create its Java
implementation. Sun has licensed its source code to more than 140 companies
and organizations who want to build it into a commercial product of their
own, including competitor Microsoft. Sun has also allowed free download and
use of the Java implementation (binary executables) by anyone. Over
2 million computer users have downloaded the JDK binaries for free in
the twelve month period to December 1997.
The relative openness of Java contrasts with systems that are only available
from one vendor, whose interfaces are developed in secret, without an open
process for others to participate, and whose owners do not allow competing
implementations of the same API. Typically, such systems also
feature "private"
APIs that are published late or not published at all, to allow the single
vendor to gain a competitive advantage for their other products. Typically
such proprietary systems do not make the source code available for inspection
by all.
Some programmers want to see the Java source for free.
They say that nothing is truly open without the source code.
Sun allows this. If you aren't planning on incorporating Java into
a commercial product, or doing any sort of external redistribution, there
is an Internal Noncommercial Use Source License available
at no charge. Please see URL
http://java.sun.com/nav/business/source_form.html
1.1 What is
the best way to refer someone to the FAQ when they ask question I know is
answered there?
A. The Java Programmers FAQ (at http://www.afu.com)
answers your question in section N.n. ...
This gives them the answer, and shows them where to go for future questions
It also demonstrates that the FAQ can answer their questions, supplying an
incentive to go there next time. It's regarded as elementary politeness to
look for the FAQ of a newsgroup and read it, before posting any questions.
In general, FAQs for any newsgroup are available by looking at past postings
in a group, or by searching Deja News (see Q 1.4), or via anonymous FTP at
directories under ftp://rtfm.mit.edu The pathnames are called things like
/pub/usenet-by-group/comp.lang.java.programmer/Java_Programmers_FAQ which
may help you get to the right one directly, as it takes some time to get
a directory listing there. Alternatively, you can look for newsgroup names
on the same ftp site by going to the same site and looking under
/pub/usenet-by-hierarchy/. That has subdirectories such as alt/, ba/, ca/,
comp/, and subdirectories under them such as
/pub/usenet-by-hierarchy/comp/lang/ and so on. This helps you explore the
world of newsgroups with FAQs.
If you do not have anonymous FTP access, you can access the rtfm.mit.edu
archives by mail server as well. Send an E-mail message to
mail-server@rtfm.mit.edu with "help" in the body for more information. "RTFM"
stands for "Read The effing Manual" -- you must expect to put in some time
and effort to master a new area of study.
1.2 Where can I look at the definitive Java Language
Specification?
1.3
Where can I find a full list of Java books and book reviews?
1.4
What if my question is not answered in this FAQ?
1.5
How do I check on known bugs in JDK 1.1?
A. Look at the Java Developer Connection at
http://java.sun.com/jdc.
Note that only bugs that Sun deems "important" are listed there. The criteria
for "important" are not published. After you have checked whether the bug
is already listed, you can submit a bug report through:
http://java.sun.com:80/cgi-bin/bugreport.cgi
You can also send in an RFE (Request For Enhancement) or ease-of-use issue
there. You can even vote on the priority you would assign to a particular
bug fix! Join the Java developer connection (it's free) by going to
http://java.sun.com/jdc Then browse
webpage
http://developer.javasoft.com/developer/bugParade/#votes
1.6
What computers have Java ports? Is there a port to Win 3.1?
A. A list of JDK ports is available from
http://java.sun.com/cgi-bin/java-ports.cgi
There are several Java ports to Win 3.1. IBM's ADK1.02 is available at
the following locations:
Netscape Navigator for Win3.1 has Java support.
Java will never be well-supported
under Win3.1 because Win3.1 lacks the basic features expected of a modern
OS (primarily lengthy filenames and multithreading support).
You could also consider JavaSoft's JavaPC kit
that converts a PC into a thin
client Java system. Details are at
http://java.sun.com/products/javapc/index.html.
JavaPC is probably more for business users than personal PCs.
However, unlike the 16-bit versions of Netscape Navigator and
Microsoft Internet Explorer,
which provide a Java Virtual Machine that is only compliant with the JDK
1.02 API, the JavaPC software allows IS managers to deploy
JDK 1.1 compatible Java applications on PCs running DOS and Windows 3.x.
1.7
Where can I find information on Java 3D?
A.
The Java 3D FAQ
at http://tintoy.ncsa.uiuc.edu/~srp/java3d/faq.html may have the answers
you're looking for. It contains general information about Java 3D, as well
as programming tips.
1.8 What are the folks at project Gnu doing with
Java?
A. Guava (a GPL'd Java compiler) can be found
at
Kaffe (a JVM) can be found at
JCL is a free implementation of Sun's core Java libraries (v1.1),
being developed for the GNU Project http://www.gnu.org.
Information regarding JCL is at
http://www.oryxsoft.com/projects/gvm/
See also
http://www.oryxsoft.com/projects/gvm and
http://www.hungry.com/products/japhar (Now one merged project).
1.9
Where can I find information about Java Certification?
A. Sun is sponsoring an examination which programmers
worldwide can take. Those passing can use the designation "Sun certified
Java Programmer". There is also a second level test, involving writing a
program to spec, and taking a further test. That results in the qualification
"Sun Certified Java Developer". You can find out all about the exam at:
and then search for "sun certified java". It costs $150 to sit the Java
Programmer exam. It is not trivial to pass the Java certification exam. It
requires understanding the objectives of the test, and the material that
is tested for. These are given, along with sample questions, at the URL mentioned
above.
1.10 I'm looking for a Java style guide on naming conventions.
A. Check out the section "Naming Conventions"
in the language specification
Also take a look at Doug Lea's draft coding standard -
See also Q3.25 for some basic rules of thumb.
1.11 Where can I find information about future Java
APIs?
A. JavaSoft has followed a policy of creating
new APIs in consultation with leading industry participants, then posting
the draft specification for public review and comments. Check the JavaSoft
roadmap of new APIs and products at
http://Java.sun.com:80/products/api-overview/index.html
Also, some APIs that are under consideration, possibly for JDK 1.2 are at:
1.12
How can I find links to recent news about Java?
1.13 What IDEs
(Integrated Development Environments) are there?
A. Some popular IDEs include:
1.14 What
is "San Francisco"?
1.15 What large
Office-style or other applications have been written in Java?
A. Well, the first one to consider is IBM's San
Francisco project, mentioned in the answer to Q1.15. There is also Lotus's
e-suite - a suite of Java applets and beans including a spreadsheet and a
word processor. These became available in March 1998. Another office suite
in Java is Applix Anyware at
http://www.applix.com/anyware/index.htm. Applix became available in
downloadable demo form in April 1998.
Yet another is Star Division's Client/Server Office. It is an
office suite with the client part written in Java and able to
run on JavaStations. The server part will run on Solaris, NT,
OS/390 and AS/400.
The older (non-Java) version is
bundled with all Sun wokstations sold in Germany
The Linux version is freely downloadable from
http://www.stardivision.com.
However the niche for single user office productivity applications is pretty
well already dominated by Microsoft products, and it is unrealistic to think
that Java software will unseat shrink-wrapped software simply because it
is written in Java. This is why Corel replanned its Java rewrite of Corel
Office before taking it to FCS. When Corel did that, it also increased its
investment in Java from 33% of R&D budget to 50%, at the expense of Windows.
Most of Java development is taking place for custom applications internal
to a company. Most programmers of any kind have never worked on MS Office,
but work on internal applications, and so it is with Java. These projects
don't have the high profile of major vendor's products, but they are the
mainstay of the industry.
Finally, note that Sun's Java compiler is written in Java. This is a
really big application in widespread use on millions of platforms.
The compile command "javac test.java" is equivalent to
java sun.tools.javac.Main test.java
In reality, the javac script does a few extra things, like
setting the heap size, etc. but nothing major, and nothing
that isn't represented by a command line switch.
1.16 What Java
User Groups are there?
A.
There are scores of Java User groups around the world, mostly in urban
areas, and centers of software technology development. A partial list
with contact information can be found at
http://sunsite.unc.edu/javafaq/usergroups.html.
If you can't find a user group in your area/school, it's easy and
satisfying to start one.
2. COMPILERS, ETC.
2.1
Why did I get an OutOfMemory error when porting
working code from jdk 1.02 to 1.1
2.2 Why do I get a "Statement not reached" error
from javac for no apparent reason?
A. JDK1.0 has a limit of 63 words of storage
for local variables in any method. longs and doubles require two words of
storage, and all other primitive types and all reference types require one
word. If you assign values to more than 63 words of local variables, you
will get a "Statement not reached" error on the statement after you assign
to the variable that contains the 64th word. In JDK 1.1, the low limit was
removed.
2.3 Is
there a lex and yacc or preferably a flex and bison equivalent for Java?
A. There is a lex equivalent called JavaLex and
a yacc equivalent called CUP.
LALR(1) parser
LL(k) parser
2.4 Where can I find a byte code obfuscator?
2.5 Which program is used to create .zip files
compatible with the java* programs? (eg classes.zip, moz3_0.zip)
A. Use the jar-tool from JDK1.1(.1):
jar [ options ] [manifest] destination input-file
[input-files]
E.g.:
jar cvf myJarFile.jar *.class
creates a compressed archive
jar cvfO myJarFile.zip *.class
creates it fullsize (uncompressed) (note the 'O'-option used for JDK1.0.2)
On Unix you can also use:
zip -rn ".class" my_file.zip *
Info-ZIP home page:
http://www.cdrom.com/pub/infozip/
Latest source code:
ftp://ftp.uu.net/pub/archiving/zip/src/zip21.zip
Netscape's command line version of its JAR packager and signing tool is called
"zigbert". They also have a signing tool with GUI written in Java. More info
http://developer.netscape.com/software/signedobj/jarpack.html
If you zip your .class files for JDK 1.0.2 (for 1.1 you'll use a Jar)
1. zip your files uncompressed (can use WinZip 6.2 up);
Unix command: zip -r0 classes.zip <directories>
2. Make sure the main class has no parent directory inside the archive, (in
other words, don't build an archive with foo/bar/myMain.class, unless your
myMain is in a package called foo.bar. Instead start it at myMain.class).
Your packages must be placed in the archive using their corresponding filesystem
pathnames.
3. Put the archive in the same directory as the .html page;
4. Put something like the following tag in the .html file:
<APPLET CODEBASE="."
ARCHIVE=my_zip_file.zip,myOtherZip.zip,thirdfile.zip
CODE="my_main_class.class"
WIDTH=600 HEIGHT=250>
</APPLET>
From JDK 1.1 on, an example of the applet tag used with a jar file is
<APPLET ARCHIVE=myfile.jar
CODE=myapplet.class
WIDTH=600 HEIGHT=250>
</APPLET>
These lines will use an applet called myapplet that can be found in the jarfile
myfile.jar. An example applet tag of a jar file used to hold classes in packages
is
<APPLET ARCHIVE="myclasses.jar"
CODE="linden.net.MyApplet.class"
WIDTH=480
HEIGHT=120>
</APPLET>
You can supply several jar filenames in a comma-separated list. Jar files
are in compressed PKZIP format.
2.6 Can I compile a Java program to a binary
executable, .exe on a PC?
A. Some companies make products that do this.
See the webpages for Symantec http://www.symantec.com, Supercede
http://www.supercede.com,
and Tower Technology http://www.twr.com. The first two
are targetted to Windows. Tower Technology supports several flavors of Unix.
Also, there is a native Java compiler from IBM, known as the HPJ
(High Performance Java) compiler. One user has reported that it
created a 2Mb executable from a 12K java file, and did not run
any faster. If you want to create a native executable to make
executable use and distribution easy, consider a Jar file instead.
Compiling to native code takes away the most significant benefit of Java:
portability of executables. Further, if you want your Java DLL (or .exe)
to interact with C++, you'll have to specify which specific C++ compiler
and/or actually compile some sort of linkage via the appropriate C++ compiler.
Because C++ does not have a standard ABI there is a big problem with
interoperability. Every C++ compiler uses a different object model, a different
way of laying out class members, and a different way of "mangling" names
for the linker.
C is much simpler. The only question here is how structures are "packed"
(i.e., are integers aligned on four-byte bounds?). All the C++ compilers
can interact with C code, thanks to 'extern "C"' declarations.
Consider carefully why you want to compile to a native executable, and whether
there is a Java way to accomplish your goal. There may be a good reason for
compiling to native code, but it needs to be thought through.
2.7 How can I performance profile my Java code?
-
A. java -prof MyClass
produces some basic output in a file called java.prof, showing the
number of times methods were invoked. The output lines are of the form:
| # of calls |
method called |
called by |
time spent |
On a Unix system, you can sort the file with something like
sort -r +82 <java.prof > java.sort
More and better Java tools are a third party opportunity.
One Java profiler is JProbe Profiler, available from
http://www.klg.com/jprobe.
JProbe is said to be easy to use.
Another profiler is OptimizeIt, available from
http://www.optimizeit.com.
Each of these profilers has
performance tuning, which shows which methods took how much time,
and also memory tuning, which shows what objects are in memory and how they
were allocated. Both are important things to know. The latest version of
the CodeWarrior IDE
http://www.metrowerks.com has a
time-based profiler for Java code. Java Workshop from Sun also has a time-based
profiler.
2.8 When I
use javadoc and I click on any java class included in the jdk why do I get
this message?
Netscape is
unable to find the file or directory named:
/E|/Jwrkshop/JDK/bin/java.lang.Throwable.html
A. References to the JDK classes assume that
*all* generated html files are in the same directory and, in fact, that all
files for all classes referenced are generated at the same time. There is
no way to generate files incrementally and have them all reference each other,
as you would like.
As long as you have source for everything involved (including the JDK and
all third-party classes), you can list all of your packages and all of the
others on the javadoc command line and generate the whole set at once, but
it is burdensome. Of course, if you receive any libraries as .class files,
even this workaround will not suffice.
Also javadoc will not generate the image files - you need to get them from
the images directory under the JDK API documentation files. You can just
copy the entire directory into your own doc directory. javadoc is a very
nice concept, with a few implementation flaws.
2.9 I'm working on a project with lots of classes
and I use the JDK. A recompile from scratch takes forever when I do it a
class at a time. How do I recompile
everything?
A. The first way is
Another way is
where "tip.java" is a class "at the tip of the iceberg", i.e. that depends
on (uses) all the other classes. Typically, this may be your main class.
However, "-depend" is known to be buggy and cannot be relied upon. It also
doesn't issue compile commands in parallel to make use of multi-processor
systems.
Without the "-depend" option, the standard "javac files" doesn't look beyond
the immediately adjacent dependencies to find classes lower down the hierarchy
where the source has changed.
The -depend options searches recursively for depending classes and recompiles
it. This option doesn't help when you have dynamically loaded classes whose
names cannot be determined by thecompiler from the dependency graph. E.g.
you use something like
Class.forName(argv[0]);
The author of the code using those classes should make sure that those classes
are mentioned in a Makefile.
2.10 Why do I get the
java.lang.UnsatisfiedLinkError
when I run my Java program containing Native Method invocations?
A. Your program is not able to find your shared
library or DLL.
On Windows 95/NT, make sure that the DLL exists in a path that is included
within the PATH environment variable. (This need is true for both standard
(untrusted) applications and trusted applets. At least, if you use the Java
Plug-in to give yourself standard Java inside a browser).
On Solaris, make sure that the environment variable LD_LIBRARY_PATH includes
the path of your shared library.
2.11 An anonymous class can't seem to access
a private outer method. Why
is that?
A. It's a known bug in the JDK 1.1.4. The code
is:
public class MyDialog {
void Setup() {
addWindowListener( new WindowAdapter() {
public void windowClosing(WindowEvent e) {
myCloseWindow(); } }
); // anon inner class
}
private void myCloseWindow() { // private outer method
dispose();
}
}
This code sends javac into an infinite loop. The workaround is to make the
private method non-private, or to make the inner class a named class.
Sun temporarily put a workaround in the compiler to
silently set the field to package access! Interesting.
2.12 What are the major Java releases and their
contents?
A. There have been three Java releases from Sun
so far, plus a number of bugfix (dot-dot) releases. The releases are:
-
JDK 1.0.2
This was the release FCS in May 1996. It had some security fixes over JDK
1.0.
-
JDK 1.1
This release (Feb 1997)
introduced a new event model in the window system. It also made
JDBC support and beans support a standard feature.
It changed and standardized
the native code interface to JNI. It also introduced inner classes.
-
JDK 1.2
This release (due Fall 1998) made the Swing library a standard feature. Swing
is a set of rich platform-independent graphical components.
2.13
What is the difference between jre and java?
A.
They are functionally equivalent, with minor
differences in the handling of default classpath and options supported.
jre.exe is the runtime stub that
comes with the Java Runtime Environment (it's also in the JDK).
It ignores the CLASSPATH environment setting in favor of
its own internally generated default and whatever is supplied on the
cmd line using -cp or -classpath. It's intended to be a bit simpler
for those who are only ever running Java programs, not developing them.
java.exe is the runtime stub that only comes with the JDK.
It uses the CLASSPATH environment setting as a starting point and
then tacks on its own internally generated entries.
They both serve the same purpose and that's to start a Java VM,
have it run a Java application, then
terminate. The source for jre.exe is provided in the JDK. The
source to java.exe is provided only in the JDK Source distribution.
3.LANGUAGE
ISSUES
-
3.1
Why doesn't my "hello
world" program work?
A. Common causes of failure are:
First, did you write an applet or an application? If an applet, you must
make sure that you did extend the java.applet.Applet class. You must declare
your class that is extended from Applet as "public". If you don't, unfortunately
some browsers will still run the code, while others won't.
-
Your class name and the file name must match exactly, even letter case. If
your class is HelloWorld, your source file must be HelloWorld.java
-
In JDK 1.0.2 the $CLASSPATH environment variable should include "." the current
dir.
In JDK 1.1, users are no longer required to set the CLASSPATH to point to
the classes.zip file, and in fact the README advises users not to set it.
From JDK 1.1.2 on, it is generally an ERROR if the user sets the CLASSPATH
to include classes.zip. CLASSPATH is only needed to
-
point to the roots of the programmers own packages,
-
use rmic
-
use unbundled packages like Swing in JDK 1.1
-
point to native code libraries.
If you're not doing any of these, do not set CLASSPATH. If you have set it,
unset it.
3.2
How can I program linked lists if Java doesn't have pointers?
A. Of all the misconceptions about Java, this
is the most egregious. Far from not having pointers, in Java, object-oriented
programming is conducted exclusively with pointers. In other words, objects
are only ever accessed through pointers, never directly. The pointers are
termed "references" and they are automatically dereferenced for you.
Java does not have pointer arithmetic or untyped casting. By removing the
ability for programmers to create and modify pointers in arbitrary ways,
Java makes memorymanagement more reliable, while still allowing dynamic data
structures. Also note that Java has NullPointerException, not
NullReferenceException.
A linked list class in Java might start like this:
public class LinkedList {
public LinkedList head;
public LinkedList next;
public Object data;
public LinkedList advanceToNext(LinkedList current) { ...
}
Another choice for a linked list structure is to use the built-in class
java.util.Vector which accepts and stores arbitrary amounts of Object data
(as a linked list does), and retrieves it by index number on demand (as an
array does). It grows automatically as needed to accommodate more elements.
Insertion at the front of a Vector is a slow operation compared with insertion
in a linked list, but retrieval is fast. Which is more important in the
application you have?
3.3
What
is the true story about how parameters are passed in Java? Is is by value
or by reference?
-
A. All parameters (values of primitive types,
& values that are references to objects) are passed by value [JLS sect
8.4.1]. However this does not tell the whole story, as objects are always
manipulated through reference variables in Java. Thus one can equally say
that Objects are passed by reference (and the reference variable is passed
by value). This is a consequence of the fact that variables do not take on
the values of "objects" but values of "references to objects" as described
in the previous answer.
Bottom line: primitive type arguments (int, char, etc) _do not_ change when
the corresponding parameter is changed. The fields of object type arguments
_do_ change when the corresponding parameter fields are changed.
3.4
How, then, can
I pass an object to a method, and have the method change the reference so
it points to a different object back in the calling code?
-
A. There are two ways. The obvious way is "just
add another level of indirection" -- wrap the object in another class, whose
purpose is simply to be passed as a parameter, allowing the nested object
reference to be modified.
The second alternative is a clearer variant of this. Pass in a single element
array. Since arrays are objects, this works.
Note that changing a global variable/object inside a method is an egregious
programming practice -- it usually violates basic OOP constructs.
3.5Why
is String a final class? I often want to override it in some way.
-
A. There are several reasons.
The simplest is that being final guarantees that instances of String are
immutable. (The String class implements immutable objects, but if it were
not final it would be possible to write a subclass of String which permitted
instances to be changed.) But that's an unsatisfying answer,
because the real question is "Why must Strings be immutable?"
One reason is efficiency. It's easier to optimize accesses to an object that
is known to be immutable. Strings are very commonly used, even used behind
the scenes by the Java compiler. Efficiency gains in the String class can
yield big dividends. If no one can change a String, then
you never have to worry about who else has a reference to your String.
A more compelling reason is security. Before String was changed to be final
(while Java 1.0 was still in beta) there was a race condition which could
be used to subvert security restrictions. It had to do with having one thread
change a pathname while another thread was about to open it.
There are other ways to solve these problems, but making String final is
the one that the designers chose.
3.6
What happened to "private protected"?
3.7 I extended the class called Frotz, and the
compiler is giving me an error message "No
constructor Frotz()" in the child class. Why?
-
A. When you define a constructor for a class,
unless you explicitly call the superclass' constructor at the start, a call
to the superclass' parameterless constructor is implicitly inserted. The
problem you're seeing is what happens when the superclass doesn't *have*
a parameterless constructor. The solution is usually to call the correct
version of the superclass' constructor with the right parameters.
3.8 Why does <unexpected> happen
in Java floating point?
-
A. There are several unexpected things that seem
to bite programmers with floating point. Invariably this is a result of the
programmer not being fully conversant with FP, rather than some fault in
Java.
If you seem to be having problems with floating point, your problem probably
stems from the fact that floating-point arithmetic is inherently imprecise.
You can expect up to 7 digits of precision with floats and 16 digits with
doubles. However, that does not mean that a number that can be exactly
represented in 7 digits decimal, can be exactly represented as a binary floating
point number. On the contrary, that is usually not the case.
Additionally, when Java converts floating point numbers to a String, as is
done when they are output, enough digits are printed so the number can be
read back in with no loss of precision. For this reason, you may see more
"inaccuracies" in floating point output than you are used to. This policy
actually gives you more consistent results than on a system where floating
point output is deliberately rounded to make the output "pretty".
There is a limitation of FP in JDK 1.0 (fixed in JDK 1.1). Namely, when you
output a floating point number in Java 1.0, the result is system-dependent
and contains no more than six digits after the decimal point. This bug is
fixed in Java 1.1.
For more information and detailed specifications on how Java deals with floating
point, see the following URLs:
http://java.sun.com/products/jdk/1.1/compatibility.html#incompatibilities
http://java.sun.com/doc/language_specification/javalang.doc.html#1466
http://java.sun.com/doc/language_specification/javalang.doc.html#5899
If you want the rounded floating point output that most languages have, use
the new java.text package of Java 1.1 to limit the number of digits that
are output. If you need more precision than about 16 digits, use the BigInteger
and BigDecimal classes of Java 1.1.
3.9
Why do I get the compiler error message
"urltest.java:8: Variable test may not have been initialized."
URL test;
try {
test = new URL("http://osprey.avs.dec.com");
} catch (MalformedURLException e) {
System.out.println("bad URL:" + e.getMessage());
}
System.out.println("this is url " + test);
A. If an exception is raised in the try clause,
test will not be given a value, yet you are using it in the catch clause.
The solution is to declare test with an explicit initial value of null.
3.10
Why do I get this compiler error message?
-
public static void main(String[] args) {
^
Statement expected.
public static final float Conversion_Factor = 39.37;
^
Type expected.
A. Argument declarations and variable declarations
inside methods are never given the "public", or "static" keywords. They are
not public or static because they are local to a method. (They used not to
be allowed the "final" keyword either, but there was not good reason for
that restriction and it was dropped in JDK 1.1). Move your public or static
declarations outside the method. They are usually put at the beginning of
the class.
3.11
Why do I get this compiler error message?
-
MyApplet.java:11: No constructor matching MyCheckbox(myApplet)
found in class MyCheckbox.
bp1 = new MyCheckbox(this);
^
1 error
A. The first thing to check is whether you gave
a return value to the constructor, like this:
If you did, the compiler will think it is an ordinary method, not a constructor.
This is a very common mistake, and quite hard to spot.
3.12
Why do I get the compiler
error message
T.java:96: Can't access protected method clone in
class java.lang.Object. OtherT is not a subclass of
the current class.
class SomeObject implements Cloneable {
public Object clone()
throws CloneNotSupportedException {
return super.clone();
}
}
[i.e. override clone to make it public, and call the superclass clone].
class Foo {
Bar bar;
Foo (Bar b) {
try {bar = (Bar) b.clone();}
catch (Exception e) {}
}
...
class Bar implements Cloneable {
public Object clone()
throws java.lang.CloneNotSupportedException {
return super.clone();
}
}
Another refinement is to note that Object.clone() only throws a
CloneNotSupportedException when the object doesn't implement Cloneable.
Since you control what your classes do and don't implement, you can
ensure that cloneable classes implement the interface and
you don't need to make the overridden clone() throw the exception.
public class X implements Cloneable {
public Object clone() { // no throws
try {
X c = (X)super.clone(); // in case members need cloning
return X;
} catch (CloneNotSupportedException e) {
// should not happen, because of Cloneable
throw new InternalError();
}
}
}
3.13
How do you transform a char into the corresponding int value,
that represents the code value of the char?
A. char c = 'A';
int i = c;
Going the other way is just
This question crops up so frequently because the BASIC language uses functions
to map characters into ints, ASC( 'A' ) => 65 causing BASIC programmers
to seek the corresponding Java functions. Pascal and Ada have similar functions,
and no doubt other languages too.
3.14
If I extend a class with a subclass, are the constructors inherited?
-
A. "Constructor declarations are not members.
They are never inherited and therefore are not subject to hiding or overriding."
The default constructor is not inherited, but provided (see Section 8.6.7,
JLS).
If you don't give your child class any constructors, a default no-arg constructor
that invokes the superclass' constructor is provided for you. If the superclass
doesn't have a no-arg constructor, see 3.7
3.15
How do I allocate
a multidimensional array?
-
A. There are several ways. If you want a rectangular
array, you can allocate the space for the array all at once. The following
creates a 4x5 array:
If you want each row to have a different number of columns, you can use the
fact that a two-dimensional array is actually an array of arrays. The following
code allocates a triangular array:
Note that if you allocate an array of any kind of object (as opposed to primitive
type), all the references will be null by default. These null references
can result in NullPointerExceptions if you try to dereference them.
In other words, after doing:
you can say "if (arr[2] == 0)"
But after doing
you must fill in the object reference before using it, with e.g.
or
before you can say "if (Iarr[2].equals(myInt))"
3.16
Does
it make a difference to the class file in any way, if I import a package
versus use the full name, i.e.
import java.rmi.server.*;
RemoteObject ro;
versus:
java.rmi.server.RemoteObject ro;
A. No, it makes no difference to the class files.
Import is just a shorthand for quoting the full name package and class name
(as in the examples in the question). Importing a class does not cause the
class to be loaded at run time. There is no run time penalty for using the
* form of import. The class file will contain the name of the packages it
uses, and the loader will look for those classes as needed at runtime.
The different forms of import may or may not make a difference to compile
time. Such a difference is likely to be negligible, and should not be a factor
in which form of import you use.
Some people say that stating which classes you are importing can help program
readability. In a program with many * import statements, it may take a programmer
time to find which package an obscure class is imported from. If you explicitly
list each class you import at the top of the
program, you document which package each class you use comes from. These
people suggest that you use
import java.rmi.server.RemoteObject;
in preference to:
import java.rmi.server.*;
Other people say that it is clearer still to use the full package and class
name, at the point where you use classes in other packages. These people
suggest that you use:
java.rmi.server.RemoteObject ro;
But that gets a little lengthy when you instantiate:
java.rmi.server.RemoteObject ro
= new java.rmi.server.RemoteObject();
You always have the option of stating the full package and class name, whether
you use import or not.
Another good reason not to use the * form is when you are importing two packages
that have classes of the same name and you want to use only one of those
classes. E.g.
import com.sun.*;
import com.ms.*;
where there is a class called SerialPort in both those packages. If you use
the * form of import, you import both of the SerialPort classes and then
must fully qualify the class each time you use it, to say which of the two
you mean.
In Java 1.0, if you import a class that has the same name as a class defined
in that source file, you will get an error that the class names clash. In
Java 1.1, the local class will be used when the package name is not given;
use the package name of the imported class to use it.
The best advice is to write the program so that it is as readable as possible.
Where you have a group of well known classes, as in java.awt, there is no
reason not to use "import java.awt.*;"
3.17
What are "class literals"?
A. A feature introduced in JDK 1.1. They are literals of type "Class"
that hold a value representing any class. There are even values to represent
"void" and an array, like this:
Class myCl1 = Character.class;
Class myCl2 = Void.class;
Class myCl3 = Object.class;
Class myCl4 = String[].class;
Class myCl5 = int[][].class;
You might use it like this:
Class cl = thing.getClass();
if (cl.equals(myCl1))
System.out.println("It's a Character class");
A class literal
is the equivalent of
The second can throw an exception, but the first cannot. If you don't know
the name of the class when you write the code, you cannot use the first form.
3.18
How
do I copy the contents of an array (with primitive type contents) to another
array?
-
A. Use the method java.lang.System.arraycopy(Object
src, int src_position, Object dst, int dst_position, int length);
Note that there is no corresponding method to clear an array to 0.0, 0, null,
false, '\u0000' etc (except for allocating a new array, which will have the
default initializations). This has lead several people to suggest the old
COBOL trick of "initialize by rollup". In initialize by roll-up, the programmer
sets the first element of the array to the desired value, and then moves
elements array[0 to n-1] to array[1 to n]. Under some old IBM COBOL compilers,
code was generated to move element 0 to 1, then 1 to 2, and so on, "rolling
up" the desired value through the array. Although it went outside the boundaries
of defined behavior in COBOL, on S/360 computers and their successors. this
could compile into a single machine instruction. The roll-up behavior was
documented by IBM, and the technique was common enough that it was worth
optimizing the trick in microcode.
In any event, initialization by roll-up is defined not to work in Java. If
the src and dst arguments refer to the same array object, then the copying
is performed as if the components at positions srcOffset through
srcOffset+length-1 were first copied to a temporary array with length components
and then the contents of the temporary array were copied into positions dstOffset
through dstOffset+length-1 of the argument array.
If you simply want to clear the same array to the same value many times,
create two arrays. Fill one with the reset value, then use System.arraycopy
to copy it into the work array each time you need to clear the work array.
3.19
What is a fast way to set all
elements of an array to a given value without duplicating the (possibly large)
array?
A. Using a loop that does it one by one is probably
20 to 40 times slower than good-old memset() in C.
A fast way on many VM's is to set the first byte of the array, then use
System.arraycopy repeatedly to fill the next byte, the next two bytes, the
next four bytes, the next eight bytes, etc. (Note these are not overlapping
slices, so the issue raised in Q3.18 does not arise).
public static void bytefill(byte[] array, byte value) {
int len = array.length;
if (len > 0)
array[0] = value;
for (int i = 1; i < len; i += i)
System.arraycopy( array, 0, array, i,
((len - i) < i) ? (len - i) : i);
}
This is faster on Sun's VM than a simple loop, and probably faster even under
JITCs because it only performs log2(array.length) bounds-checks at most.
This is a clever code idiom applying the binary chop algorithm to arrays
whose size is not a power of 2.
3.20
Is there some declaration that I can use to make "acos", "cos",
"sin", etc. (from java.lang.Math) recognizable in my own class, so I don't
have to prefix "Math." to them?
A. No. There is no good alternative. There are
several bad alternatives:
You could wrap the functions in your own class.
double sin(double x) {
return Math.sin(x);
} // etc. for each function
If your class does not extend another class, you could make it extend Math,
to bring the namespace in, but this is very poor OOP style.
If the Math class provided a public constructor -- which it does not -- you
could instantiate a local copy of Math with a shorter name, e.g. "M". There
is a cheating way to do this which works because all the methods are static.
Declare
java.lang.Math M = null;
angle = M.cos(i);
Please don't do this.
If java.lang.Math were not final and your class did not extend another class,
you could have your class extend Math, to bring the namespace in. However,
the final prevents this and it is very poor OOP style to use inheritance
to obtain a trivial name abbreviation (rather than to express a type hierarchy).
The import stament only imports packages, subpackages, and classes. Members
of classes are not imported, so
import java.lang.Math.*;
doesn't work.
3.21
Why does b >>>= 1 give me the same result as b
>>= 1?
A. First of all, note that ">>" is a "signed"
or "arithmetic" shift, namely, it replicates the sign bit on the left as
it shifts.
The ">>>" operator is an "unsigned" or "logical" shift; it does
a shift right and zero fill. However, there are a couple of places where
">>>" looks like it does a signed shift!
The issue occurs when you have a non-canonical type, byte, or short, with
a negative value, e.g.
byte b = -15; // 0xF1
b = (byte) b >>> 4; // why isn't b 0x0f ?
The initial expectation is that an unsigned shift right of 0xF1 would
successively be (in binary) 0111_1000 then 0011_1100 then 0001_1110 then
0000_1111
But that doesn't happen. The rules of arithmetic in Java say that all operands
are converted at least to int before the operation (and possibly to a more
capacious type). That means our byte is promoted to an int, so instead of
shifting 0xf1, we are shifting 0xfffffff1. If you shift right unsigned 4
places, you get 0x0fffffff. When you cast that to a byte it becomes 0xff,
or -1.
The bottom line is that the final result is the same as if you'd performed
the signed shift because the unsigned shift applied to the intermediate int,
not to the original byte. This anomaly means that ">>>" is useless
for negative bytes and shorts. It is probably safer and clearer not to use
it at all, but to mask and shift like this:
// not recommended
byte b = -15;
b = (byte) (b>>>4);
System.out.println("b= "+Integer.toHexString(b) );
// recommended
b= -15;
b = (byte) ( (b & 0xFF) >> 4 );
System.out.println("b= "+Integer.toHexString(b) );
3.22
How do I compare two Strings
together?
if (s1 == s2)
is giving me funny results.
A. The comparison using "==" on objects, like Strings, is asking the
question "do these two objects have the same reference?". That is, do they
have the same address, and hence are the same object? What you really want
to do is ask "do these two Strings have the same *contents*?"
You do that with any of (and there are others too)
if (s1.equals(s2) )
or if (s1.equalsIgnoreCase(s2) )
or if (s1.startsWith(s2) )
or if (s1.endsWith(s2) )
or if (s1.regionMatches(s1_offset, s2, s2_offset, length) )
or if (s1.compareTo(s2) < 0)
Note that you can do this with literals:
if ("apple".equals(s2) ) ...
If you compare these the other way round, like this:
if ( s2.equals("apple") ) ...
and s2 is null, you will get a null pointer exception. Be alert to this.
3.23 How can I be type-safe without having
to create a multitide of subclasses (IntegerLinkedList, StringLinkedList,
etc.)?
A. Generic programming in java (the rough equivalent of C++'s templates)
works reasonably well since all java classes are subclasses of Object. There
is, however one potential problem - there is always a possibility that a
generic container may contain different classes of objects.
This naturally leads to the question of how to do this in a type-safe way.
If you've created a generic LinkedList class, how can you be type safe without
having to create a multitide of subclasses (IntegerLinkedList, StringLinkedList,
etc.)?
One way to handle this would be to offer up an additional constructor in
your generic class that takes a parameter of type "Class" and uses that parameter
along with Class's "isInstance" method to guarantee that Objects added to
the container are the expected type.
Note that the comments in the source for isInstance() refer to a "specified
Class parameter", suggesting that you are supposed to write something like:
This works, but the
documentation for isInstance is clear that the parameter should be an
Object rather than a Class. Also, be aware of "Collections" that are coming
in JDK 1.2. More information about this is available at the Java Developer
Connection at the Java website http://java.sun.com.
3.24
Why is there a standard JNI?
-
A. JNI is the Java Native Interface. It defines the way that a Java
program can call C programs. The industry has agreed on, and Sun has codified,
JNI as the standard. Microsoft shuns the standard and uses a protocol of
its own called Raw Native Interface, RNI.
You might think that once a Java program uses JNI, portability is lost, and
hence it doesn't matter if vendors diverge from the JNI standard. However,
it does affect programmers and especially end-users. When you use native
code, you lock your program into one platform (and other platforms if you
port the native code). But programs that use native code should still be
able to use a JVM, and other Java tools and libraries from any vendor. This
is possible when everyone uses the same native code interface.
Microsoft's non-standard RNI is the reason that programs using the Microsoft
JVM cannot use the standard Java jdbc-odbc library. That library has a piece
written in C. It works for all JVM's except Microsoft's.
The standard JNI thus has two purposes:
-
Source code compatibility between different platforms.
-
Binary code compatibility between different JVMs on the same platform.
3.25
What are the naming
conventions in Java?
3.26
What
are the differences between an interface and an abstract class?
-
A. The principal difference is that an abstract class is used when
you have a type/subtype relationship. An interface does not imply a type/subtype
hierarchy; it merely forces a class to have some methods with a given signature.
That allows some other code to rely on being able to call those methods.
Interfaces allow you to treat an object as an abstraction without having
to know anything about its particulars. If, for example, you wanted to have
a bunch of persistent objects on which you need to provide persistence services,
these objects of many varying kinds would all in their individual ways implement
a Persistent interface that prescribes a save() method. The persistence service
simply treats each different object as a Persistent and calls save() on it.
Each object of all the unrelated classes knows how to save() itself when
told to do so. The service has no need to worry about the actual class of
each object.
You want to use abstract classes when you plan to have related subtypes that
define the abstract behavior. The subclass should have a "kind of" relationship
to the superclass. A user record is a kind of record, a dog is a kind of
mammal, etc. Inheritance provides a "kind of" relationship.
By way of contrast, an interface provides a "plays the role of" relationship.
An image loader could play the role of an Observable thing. Just about any
class could play the role of a "sortable thing". Interfaces therefore often
have a name that is a "something-able thing". A class may only extend one
other class, but it can implement an arbitrary number of interfaces.
Abstract classes can have abstract methods and they can also have fully defined
members. Interfaces can't have any code in them at all, though they can have
final data. Another minor difference is that calls to a subclass method are
faster than calls to an interface method (because the interface method involves
a bit more lookup at runtime about what is actually being called).
3.27
How do static
methods interact with inheritance?
-
A. Static (per-class, rather than per-object) methods do not participate
in overriding (choosing the right method at runtime based on the class of
the object). Probably the best and simplest way to think about this (and
to write your code) is to write every invocation of a static method using
the fully qualified class name so:
class A {
public static method1() {
A.method2();
}
public static method2() {
}
}
class B extends A {
public static method3() {
A.method1();
}
public static method2() {
}
}
Now it is perfectly clear that the static method2() that is called is
A.method2(), not B.method2(). This is true regardless of whether you use
the fully-qualified class name or not, but the use makes it obvious to all.
3.28 How
do I get unsigned ints in Java?
-
A. Java doesn't have unsigned ints. The reason is that this is a
poorly designed area of C -- the rules for what type you end up with when
you mix signed and unsigned in expressions are complicated -- and they
changed between K&R and ANSI C (you might have heard this under
the name "unsigned preserving vs. value preserving"). Worse, they depended
on the underlying hardware, so they varied from platform to platform,
causing
bugs in all kinds of unexpected places. The book "Expert C Programming" goes
into this in more depth (p25). So, to avoid bringing over the hidden
complexities, Java does not bring over unsigned types from C.
Use type char if you are ok with 16-bit unsigned quantities. Otherwise go
to the next larger type and use masking. Specifically, to convert an int
to its unsigned representation, use:
((long)i) & 0xFFFFFFFF
This promotes the signed int to long and chops off the sign-extension,
leaving it as an unsigned 32-bit quantity held in a 64 bit type.
Also worth noting is that if you're going to work with unsigned bytes, int
is a more efficient larger type to use than short or char, since smaller
values have to be promoted to int to do any arithmetic or testing on them.
3.29
How do I send a variable number of arguments to a method,
or get multiple return values back?
-
A. (Easy) Use method overloading to support different parameters.
This makes things easy on the caller but can get out of hand if you want
to support a wide number and variety of parameter types. You should ask yourself
if your code design is well-organized if you need to do this.
(More complicated) Use arrays. It's possible to declare arrays inline as
shown below:
foo("A param", new Object[] {"param3", "param4", new Integer(5)} );
// ...
void foo(String param1, Object param2[]) {
System.out.println(param1);
for (int i = 0; i < param2.length; i++) {
System.out.println(param2[i].toString());
}
}
You can even pass arrays of arrays using this method. Of course,
inside the method you need to be able to decode what the arguments are
and how you use them.
Alternatively you can invent a class that
just contains all the possible fields you might want to pass into a
method (plus booleans to say if each field is set or not), and make
an object of that class be a parameter to the method.
You can return multiple values from a method the same ways; either
have the method return an array or a wrapper object.
Finally, you can just have the function return a Vector. This is
particularly convenient when you're not sure how much you
are going to be returning, based on what occurs in the method.
A Vector is
essentially a dynamically-growable array. Regular arrays can't
grow after you declare them - you have to declare a bigger array
and move the old stuff into it.
Remember the wise words of Prof Alan Perlis however, "if your procedure
has more than about half a dozen parameters, you probably forgot a few."
Perlis meant that passing large numbers of arguments into a function
suggests that your function is badly-organized.
4.AWT
and Related Topics
-
4.0.1Why
do I get this when using JDK 1.1 under X Windows?
-
A. There's a missing font on your system. Move
font.properties from the "lib" subdirectory aside to font.properties.bak
Then it won't look for the font and fail to find it.
The problem occurs because the Motif AWT libraries use the Font "plain Dialog
12 point" as a fall-back default font. Unfortunately, when using a remote
X server sometimes this font isn't
available.
On an X-terminal, the diagnostic may be slightly different, a segv
To determine which fonts you have, issue a command such as
Then pick through the long list of fonts to determine which ones you want
to use. The xfd program will let you look at a font:
-
xfd -fn "your font name here" &
4.0.2
Why is GridBagLayout so hard to use?
A. There are two reasons. First, detailed GUI
layout is difficult. Simple layouts are easy. Second, GridBagLayout was
contributed to Javasoft by a programmer who wanted to support the Java effort.
It was intended as a proof that the AWT offered enough features for programmers
to write their own layout managers. It wasn't designed with human factors
and ease of use in mind. If it bothers you (it bothers me) then just don't
use it. Create your GUI on several panels and use the other layout managers
as appropriate to get the exact effect you want. The official story from
the project leader of the AWT project, as explained to the Mountain View
Java Users' Group on Dec 4 1996, is:
"The case has been made and is now accepted that GridBagLayout is too hard
to use for what it offers. GBL will continue to be supported, and something
better and simpler will eventually be provided as well. This "better GBL"
can be used instead of GBL."
Bottom line: nobody has to spend any effort on GBL, there are better alternatives
available now, and it will be replaced by the SwingSet "SpringLayout" Springs
& Struts style layout manager.
SpringLayout was to be introduced as part of the Java Foundation Classes
with JDK 1.2, but it was dropped from the beta release as the code was not
ready in time. Javasoft generously made the preliminary code available and
invited programmers to hack on it and submit the results.
4.0.3
How
do you change the font type and size of text in a TextArea?
A. myTextArea.setFont(new Font("NAME",
<STYLE>, <SIZE>));
where:
NAME is the name of the font (eg Dialog or TimesRoman).
<STYLE> is Font.PLAIN, Font.ITALIC, Font.BOLD or any additive
combination (e.g. Font.ITALIC+Font.BOLD).
<SIZE> is the size of the font, e.g. 12. e.g. new Font("TimesRoman",
Font.PLAIN, 18);
4.0.4
Can you have different fonts for individual words in a TextArea ?
A. No. If you're trying to write a word processor,
use the Canvas class to render on. Note, this can be done using the
Swing classes.
4.0.5
How much text can be put in a TextArea?
A. TextArea just uses the corresponding widget
of the underlying window system. It will be bounded by the limit imposed
in the native window system. In Windows95 TextAreas can hold about 28Kbytes.
The native widget allows 32Kb, but there is some overhead which reduces the
amount available to the programmer. The limit is removed in JTextComponent
in Swing (JDK 1.2) which dispenses with peer controls.
4.0.6
MyClass
works fine except when I try to set a particular font. I just can't seem
to get it to work in Win95, but I can get it to work on a MacOS and Unix.
A. You probably specified a font name that isn't
available under your Win95; this is one of those cross-platform differences
that can bite you if you over-specify for one platform, like specifying "Arial"
as a font and expecting it to work on something other than Windows.
On both Windows 95 and Solaris 2.6, these fonts
Dialog
SansSerif
Serif
Monospaced
Helvetica
TimesRoman
Courier
DialogInput
ZapfDingbats
are revealed by this code:
import java.awt.*;
class foonly {
static public void main(String s[])
{
String n[]= new Frame().getToolkit().getFontList();
for (int i=0;i<n.length; i++)
System.out.println(n[i]);
System.exit(0);
}
}
In other words, You can get a String array of the names of the fonts by
String[] fonts = Toolkit.getDefaultToolkit().getFontList()
The names of actual fonts like Helvetica, TimesRoman, and Courier
are deprecated in JDK 1.1 in favor of font styles like
SansSerif, Serif, and Monospaced
(respectively). The font style will be mapped into the
nearest actual font on a platform.
The font styles are now mapped into a system font name using
the entries in one of the font.properties files in
$JAVAHOME/lib. There are multiple font.properties files
corresponding to different locales. If you wanted a quick hack
for testing, you could modify the file or add to it so a
different mapping is done to a new font you want to try.
4.0.7
Is it possible to draw a polygon or a line more than 1 pixel wide?
A. JDK 1.1.1 doesn't have support for this. The
standard workaround for drawing a thick line is to draw a filled polygon.
The standard workaround for drawing a thick polygon is to draw several polygons.
There is a useful class at
http://www.apl.jhu.edu/~hall/java/GraphicsUtil.html
It extends the drawxxx and fillxxx methods of java.awt.Graphics. It adds
a Line Width argument to most of the drawxxx methods, a Color argument to
most of the drawxxx and fillxxx methods, and a Font argument to draw String
and drawChars.
4.0.8 I use add(Component) to add Components
to the Container. Is there any way
toexplicitly set the z-order
of these Components?
-
A. JDK 1.0 has no way to explicitly set the z-order
of components. You can try it heuristically, based on the browser you're
using, or you can use CardLayoutManager to ensure the panel you want is at
the front.
In JDK 1.1 the z-order of components ("z-order" means "front-to-back" order,
i.e. which window is in front of which) can be controlled by using the the
method add(Component comp, int index). By default, components are added 0
to N. The method paint of class Container paints its visible components from
N to 0.
4.0.9When
I call repaint() repeatedly, half my requests get lost and don't appear on
the screen. Why is this?
-
A. repaint() just tells the AWT that you'd like a paint to happen.
AWT will fold several adjacent repaint requests into one, so that only the
most current paint is done. One possible workaround might be to use a clip
rectangle and only paint the different areas that have changed.
4.0.10 What is the difference between
Component's setForeground(Color c)
and
Graphics's setColor(Color c) ?
A. First of all, these methods do the same thing:
set the foreground color to the value of the parameter. The difference lies
in where you use them. There is also a Component.setBackground that will
set the background color.
If you are in a constructor or an event handler (e.g. "click here to turn
the canvas blue") you have a Component and should use the setForeground()
method. If you are in a paint() method, that takes a Graphics context as
its argument so you will typically use g.setColor(c).
Unlike a Component, a Graphics object doesn't have a background color and
a foreground color that you can change independently. A Graphics object arrives
in the color(s) inherited from the drawing surface. From then on, any rendering
(drawLine(), drawRect(), fillOval(), etc.) will be
done in the setColor() color. Because they do different things, the Component
and Graphics methods have different names.
4.0.11 When I start
amouse drag inside a Component,
and go outside the Component, still dragging, the mouse events still get
sent to the Component, even though I am outside it. Is this a bug?
-
A. No, it is the specified behavior. The java
API documentation says:
-
"... Mouse drag events continue to get sent to this component even when the
mouse has left the bounds of the component. The drag events continue until
a mouse up event occurs. ..."
It is done for the convenience and ease of the application programmer. It
allows you to handle all drags from the place of origin. If you don't want
this, simply look at the coordinates of the mouseDrag Event, and if they
are outside the Component, ignore them.
4.0.12 What's all this about subclassing Canvas
and overriding paint()
?Can't I just do a
getGraphics() for a component, and draw directly on that?
-
A. You can do that, and it might work up to a
point (or it might not). A problem arises when the window system wants to
refresh that component e.g. because it has been partially obscured and is
now revealed. It calls paint(), and paint() has no knowledge of the other
g.drawing() you have just done.
4.0.13
But couldn't
the AWT just remember what has been drawn to a Graphics context, and replicate
that instead of calling paint()?
-
A. Possibly it could, but how do you unremember something that has
been drawn? How do you start drawing over again with different contents?
You could solve these by creating extra methods, but that is not how it works.
In practice it is a lot simpler to be able to look at the paint method, and
see explicitly all the things that will be done to draw that component. Bottom
line: use paint(), not g=getGraphics(); g.drawString( ...
4.0.14How
can I get the dimensions and resolution of the screen?
-
A.
java.awt.Toolkit.getDefaultToolkit().getScreenSize()
java.awt.Toolkit.getDefaultToolkit().getScreenResolution()
Screen resolution is in dots-per-inch.
Take a look in the Toolkit class for other useful methods.
Toolkit.getDefaultToolkit().getColorModel().getPixelSize()
gets you the color model in terms of bits per pixel.
Math.pow(2,
Toolkit.getDefaultToolkit().
getColorModel().getPixelSize())
gets you the color model in terms of number of colors. Or use this:
1 << Toolkit.getDefaultToolkit().
getColorModel().getPixelSize()
That does a shift left to calculate the power of two.
4.0.15
How do I allow for the size of the title bar and border when I draw a Frame?
4.0.16 When I run
the Swing demo
on Win95 I get an error "Out of environment space"
-
A. That's because you don't have enough space for your DOS environment.
You can fix this with:
-
Right click your MS-DOS Prompt icon or window and choose Properties.
-
Choose "Memory" and on "Initial Environment" choose 4096 instead of "auto".
-
Run Swing again, you'll be OK.
4.0.17 How do I
resize a List? I had a List defined as
List tlist = new List(10);
but the Strings in the list were 80 characters long and only the first 15
were being shown. I was not able to resize the List to display the contents
without using the scroll bar.
A. A List cannot be resized in a constructor,
so add the following to the Applet (or wherever)
public void paint (Graphics g) {
tlist.setSize(200,200);
}
Then before showing panel/frame with the List:
tlist.resize(400,400);
4.0.18 How can my program
tell when a window is resized?
-
A. Override the setBounds(int,int,int,int)
method of Component
to do what you want. Of course, have it call super.setBounds() as well. Note
that setBounds() replaces reshape() which is deprecated.
Note the new APIs call the deprecated APIs instead of the other way round.
For example, Component.setBounds calls Component.reshape, instead of reshape
calling setBounds. This is because the AWT sometimes needs to call these
for its own purposes. If it called the old one which then called the
new one, and you overrode the new one, the AWT would (wrongly) not
call your routine. By having the AWT call the new one (and then the
new one call the old one), any overrides of the new one will correctly
be called by the AWT as needed. If that didn't make sense, forget I
mentioned it.
4.0.19 Why doesn't my window
close when I
click on the X in the title bar?
A. Here's how to make your program do that.
JDK 1.0.2: Handle Event.WINDOW_DESTROY to do a hide() and dispose() on the
Frame.
JDK 1.1:
a) Listen for WindowEvent and do hide(); dispose(); in windowClosing() -
this really ought to be the "default" behaviour, so was made so for
a Swing JFrame. Or
b) enable AWTEvent.WINDOW_CLOSING and do the hide() and dispose() in
processWindowEvent().
4.0.20How
do I clear the contents of a TextArea?
-
A. Set it to an empty String with this:
4.0.21 What are
those preferredSize() and
minimumSize() methods in Component?
-
A. Those methods allow a LayoutManager to calculate
the preferred and minimum sizes of the Components it is arranging. You can
control the values that the LayoutManager gets by creating subclasses of
the Components you are using and overriding these methods.
4.0.22 How can I
force
a synchronization of the graphics state, e.g. of a cursor change, or an animation
frame to be rendered?
A. This is done by the sync() method in the toolkit.
So just use:
AnyComponent.getToolkit().sync();
4.0.23 How do I
plot a single pixel
to the screen?
A. Use g.drawLine(x1,y1,x1,y1) to draw a line
one pixel in length. If you are drawing a very large number of individual
pixels, consider using a java.awt.MemoryImageSource object and measure whether
this offers better performance.
4.0.24 How can I
tab between components?
A. In JDK 1.0, you have to read the key press,
and program it explicitly. JDK 1.1 supports tab and shift-tab (previous field)
automatically. The tab order is the order that the components were added
to the container.
4.0.25
What is the difference between "low level" and "semantic" events?
-
A. Low-level events are tied to a specific component
(resizing a window, dragging the mouse, striking a key, adding a Component
to a Container, etc). Semantic events are those generated when you frob a
control (move a scrollbar, click on a button, select from a menu, etc), and
the same kind of event can be generated by several different components.
A Button and a List both fire an Action event when they are clicked on.
To the programmer, the important difference is that you can change a low
level event such as the key value in a keypress, and it will display the
new value. You can also consume low level events so that they do not appear
in the widget. You can't do these things with semantic events - they have
already "occurred" to the widget.
Semantic events: Use the method addXListener() to add a listener object which
implement the XListener interface, to get XEvent objects delivered (usually
via the AWTEventMulticaster). Low level events: use the method enableEvents()
and override performX() to grab those events in the object itself.
4.0.26Is
it possible to have a Java window float above all other windows. for example
a tool palette floats in a super-layer always above all the regular document
windows on which you use the palette's tools?
-
A. On MS Windows, a Window object floats above
all other windows, unlike a Frame, which is layered in with ordinary windows.
This behavior yields a "floating" effect. Whether a Window object is really
supposed to float is another question entirely.
On Mac, a Window object is either layered in with other windows, just like
a Frame is, or else it is entirely modal -- depending on which VM you use.
In Java -- there appears to be no easy way to get floating behavior. If anyone
knows otherwise, please send in your comments.
4.0.27How
do I center a dialog box?
-
A. You cannot currently get the applet's absolute
screen coordinates. Its location (0,0) is relative to the browser, not the
screen itself. But you can center something that it pops up or displays centered
on the screen with code like this:
Dimension screen = Toolkit.getDefaultToolkit().getScreenSize();
my_window.move(
( screen.width - window.size().width ) / 2,
( screen.height - window.size().height ) / 2 );
my_window.show().
In a related fashion, you can center something on its parent like this. Note
the intelligent use of APIs like translate() to do the work for you.
void centre(Component parent) {
pack();
Point p = parent.getLocation();
Dimension d = parent.getSize();
Dimension s = getSize();
p.translate((d.width - s.width) / 2, (d.height - s.height) / 2);
setLocation(p);
}
4.0.28 How do I get back to a
normal
echo after I have used TextField.setEchoChar('*');
-
A. TextField.setEchoChar('\0') works on most
Windows based browsers... but for most other platforms (ie: Netscape under
UNIX), it just locks up the textfield.
There is only one good solution, and that is to make two TextFields on top
of each other, one normal, and one with .setEchoChar('*'), and switch between
them.
4.0.29 How can I get the absolute mouse coordinates?
-
A. You mean, if the browser is about 640x480,
you want a y-coord between 0 and 480. If the browser window is about 800x600
you want a y-coord between 0 and 600. This might be needed for a pop-up menu,
where you want to pop up at the absolute mouse position.
The approach is to sum up the event's (x,y) and the locations of the target
and its parents until there is no parent. Though on some browsers, it seems
this is not reliable. [Better suggestions are solicited.]
4.0.30 How can iconify/deiconify a window in
Java?
-
A.There is no way in Java today to write code
to force a window to iconify or deiconify. There is a way (tested on Windows
and UNIX) to achieve this effect that involves creating and destroying peers,
but it is not recommended. You can drop into native code to do it. The "party
line" in JavaSoft is that it is because Java is an application language,
not a window manager. Everyone wishes they'd add it.
4.0.31 How can I write text at an angle?
-
A.Check out
http://www.nyx.net/~jbuzbee/font.html. Jim has some code to do exactly
this. A good way to do it is to draw the text to an offscreen image and write
an ImageFilter to rotate the image.
Also, in JDK 1.2 on, the Java 2D API handles arbitrary shapes, text, and
images and allows all of these to be rotated, scaled, skewed, and otherwise
transformed in a uniform manner.
4.0.32 How do I know which mouse button was pressed,
and how often?
-
A. To handle mouse events you have to implement
the MouseListener interface, or derive from the MouseAdapter class in order
to use one of the mouse handling methods. The MouseEvent argument passed
to the handling methods contains fields that say which button was pressed,
and the click count. Use code like this.
public void mouseClicked(MouseEvent m) {
boolean leftButtonPush =
(m.getModifiers() & java.awt.event.InputEvent.BUTTON1_MASK) != 0;
boolean centerButtonPush =
(m.getModifiers() & java.awt.event.InputEvent.BUTTON2_MASK) != 0;
boolean rightButtonPush =
(m.getModifiers() & java.awt.event.InputEvent.BUTTON3_MASK) != 0;
int click = m.getClickCount(); // might be 1,2,3 clicks or more
You can also call at m.isPopupTrigger(). If it returns a true value, the
user has asked for a pop-up menu. On a lot of window systems, the right mouse
button is the trigger for pop-up menus.
You can overload processMouseEvent of your component.
public void processMouseEvent(MouseEvent e) {
if (e.isPopupTrigger()) {
// do what you want
}
else
super.processMouseEvent(e);
}
The code above applies to JDK 1.1. You can also call
java.awt.swing.SwingUtilities.isRightMouseButton(MouseEvent me).
See also question 15.10.
4.0.33 How do I detect a resize of a Frame?
-
A. Register a ComponentListener on the frame,
like this:
import java.awt.*;
import java.awt.event.*;
class MyFrame extends Frame {
public MyFrame() {
addComponentListener(new CmpAdapter());
}
class CmpAdapter extends ComponentAdapter {
public void componentResized(ComponentEvent evt) {
//doSomething();
}
}
}
Alternatively, the same effect can be achieved like this:
class MyFrame extends Frame implements ComponentListener {
public MyFrame() {
addComponentListener(this);
}
public ComponentHidden(ComponentEvent evt) { }
public ComponentMoved(ComponentEvent evt) { }
public ComponentShown(ComponentEvent evt) { }
public ComponentResized(ComponentEvent evt) {
//doSomething
}
}
4.0.34 How do I get word-wrap in a TextArea?
4.0.35 How can I make an offscreen image with
transparent pixels?
How can I grab the pixel values from an offscreen image?
How can I use AWT drawing primitives (e.g. drawString() or drawOval()) on
an image I created from an ImageProducer?
-
A. None of these things can be done.
Despite the fact that there is only one class called Image in the AWT libraries,
it suffers from a (currently undocumented) severe case of schizophrenia:
there are really two unrelated types of Image. The first type are those created
by the Component.createImage(int, int) call, known as "offscreen" images,
and the second are those created by the Component.createImage(ImageProducer)
call, or by the Toolkit/Applet.getImage() calls, which I will call "produced"
images.
The only common ground between these kinds of Image is the following:
-
you may find their width and height by the methods of the Image class.
-
you may use them as the argument to the various Graphics.drawImage() calls.
The differences between these objects are the following:
-
you may not put transparent pixels into an offscreen - note that all Java
primitives accept Color objects, which all represent completely opaque colours
as if Produced from an int with the upper 8 bits equal to 0xff. (see 8.3)
-
you may not call Image.getGraphics() on a produced image, and hence may not
use any AWT primitives.
-
you may not grab pixels from an offscreen image using PixelGrabber.
-
anyone know of any other limitations?
In these cases, "you may not" generally means "you may not successfully".
Symptoms on attempting these range from Exceptions to garblings of the Image.
Any or all of these restrictions may be removed in Java 1.2, which features
a new 2d API. Wait and see.
There are some relevant bugs shown in the Java Developer Connection: Bug
ID 4098505. Apparently, from the report from the Sun engineer, PixelGrabber
is specified to work with offscreen images, just it is currently buggy, and
invariably gets the wrong colour model. No fix has been scheduled yet.
Bug ID 4077718 reports that setting transparent Colors in offscreen images
has been available since Java 1.2beta1. I am personally unable to verify
this.
There is an incorrect answer from Sun to the third matter, of getGraphics()
on produced images, in article 1501 in Questions&Answers.
4.0.36 How can I grab a pixel from an Image
object?
-
A. This is the purpose of the
java.awt.image.PixelGrabber class. A fragment of code showing its use is:
import java.awt.image.PixelGrabber;
import java.awt.Image;
....
....
public static int pixelValue(Image image, int x, int y) {
// precondition: buffer must not be created from ImageProducer!
// x,y should be inside the image,
// Returns an integer representing the color value of the x,y pixel.
int[] pixel=new int[1];
pixel[0]=0;
// pixel grabber fills the array with zeros if image you are
// trying to grab from is non existent (or throws an exception)
PixelGrabber grabber = new PixelGrabber(image,
x, y, 1, 1, pixel, 0, 0);
try {
grabber.grabPixels();
} catch (Exception e) {System.err.println(e.getMessage());}
return pixel[0];
}
By the way, one issue on working with images is that the Java VM will consume
virtual memory pretty fast if you are loading lots of images without calling
the Image.flush() method when done. The getImage() method probably caches
old images so they aren't garbage collected.
4.0.37 How can I limit a TextField to no more
than N characters, or to only allow numeric input?
-
A. The approach is to look at keystrokes as they
happen, and disallow input that does not meet your criteria.
A neat variation is to extend the basic AWT component, and in your subclass
also include the handler that will look at the keystrokes. This bundles
everything neatly in one place. The code may look like:
import java.awt.*;
import java.awt.event.*;
public class XCTextField extends java.awt.TextField implements
java.awt.event.TextListener {
public XCTextField(int columns) {
super(columns);
enableEvents(AWTEvent.FOCUS_EVENT_MASK);
addTextListener(this);
}
// other constructors may be useful, too
public void textValueChanged(java.awt.event.TextEvent event) {
int col = this.getColumns();
int len = getText().length();
// int caret = getCaretPosition();
if (col > 0 && len > col) {
// or if the char just entered is not numeric etc.
String s = this.getText();
Toolkit.getDefaultToolkit().beep();
this.setText(s.substring(0,col));
this.setCaretPosition(col-1); // caret at end
}
}
public void processFocusEvent(java.awt.event.FocusEvent e) {
// this routine highlights according to focus gain/loss.
super.processFocusEvent(e);
int id = e.getID();
if (id==java.awt.event.FocusEvent.FOCUS_GAINED)
this.selectAll();
else if (id==java.awt.event.FocusEvent.FOCUS_LOST)
this.select(0,0);
}
}
Here is a much briefer example, which very cleverly does the work
in the Listener. Oracle really dislikes the "tick" character in a data
text fields, as it is interpreted as part of an SQL statement. Here is
the code that James Cloughley wrote to suppress "ticks" in a TextField.
import java.awt.*;
import java.awt.event.*;
public class NoTick extends KeyAdapter {
final char tick = '\'';
public void keyPressed( KeyEvent event ) {
TextComponent tc = ( TextComponent )event.getSource();
char C = event.getKeyChar();
if ( c == tick ) { event.consume(); }
}
}
Use it like this:
TextField sometextfield = new TextField();
sometextfield.addKeyListener( new NoTick() );
Brief and clever -- make the event handler consume unwanted
characters.
Finally, check out iDate, iTime, and iNumeric from IBM's alphaworks
javabeans, available free at
http://www.alphaworks.ibm.com/alphaBeans.
These beans do the kind of validation you want.
4.1 APPLETS
4.1.1 What is the
difference
between an application, and applet and a servlet?
A. An application is a standalone program. An
applet is a downloadable program that runs in a web-browser. Typically an
applet has restricted access to the client system for reasons of security.
Other than that it is virtually no different to a regular Java program.
A servlet is a java program whose input comes from a server and whose output
goes to a server. Other than that it is virtually no different to a regular
Java program. Think of a servlet as an application, but one that (like an
applet) requires a context in which to run, namely web-server software. Servlets
are used like CGI, and they allow the server end to be written in Java as
well as the client.
The Web Server starts up a servlet when the URL is referenced, and now your
applets have something that they can talk to (via sockets) on the server
that can write files, open connections to other servers, or whatever.
There is also a software technology from IBM called an "Aglet". An aglet
is a mobile agent that can go from machine to machine, performing tasks,
serializing data collected, and "shipping itself" (code and data) to the
next machine. It's too early to say if aglets are a flash in the pan or or
a dawning technology. Read about aglets at
http://www.trl.ibm.co.jp/aglets/
Finally, there is the ticklet (Tcl/Tk) plugin for your browser (Netscape
or Explorer) available at
http://sunscript.sun.com/plugin/
Don't confuse Sun's JWS "Java Web Server" with JWS "Java Workshop". Java
Web Server supports servlets as does the lightweight
and free server at Acme.com:
4.1.2
My applet
works on my machine, but fails when I put it on our web server. Why?
4.1.3 How do I
use an image
as the background to my applet? How do I set the background color of my applet
the same as the browser?
A. You can simply do a g.drawImage(yourImage,
x,y, this) in the paint() routine of your applet. If the image isn't big
enough to fill the entire background, tile it or scale it There is some code
to tile at:
(under AWT) Alternatively, the AWT can scale your background image to the
size of
the applet. The result quality will depend on the kind of image. Inside an
applet class, you can use:
drawImage(img, 0, 0, size().width, size().height, this);
You can set the background color to match the background color of the browser
by passing the value in as a parameter, like this:
In the HTML applet tag:
param name=BrowserColor value=F1F1F1
(value should be the same hex as the HTML COLOR value).
In the Applet init() method:
String colparam = getParameter("BrowserColor");
int col = Integer.valueOf(colparam,16).intValue();
setBackground( new Color(col) );
An applet cannot override the size imposed by the HTML. If you make the applet
larger, the browser will still clip to the origional size. If you need more
room, open up a new Frame, Window or Dialog to show your output.
4.1.4 How do you make the
applet's background
transparent?
4.1.5 How do you do
file I/O from an applet?
A. See answer to question 6.8.
4.1.6 How do you get a
MenuBar/Menu in an applet?
Container parent = getParent();
while (! (parent instanceof Frame) )
parent = parent.getParent();
Frame theFrame = (Frame) parent;
This second suggestion definitely doesn't work in the appletviewer, and probably
won't work on Macs (where would the menubar go?) or in some browsers. In
JDK 1.1, just use a popup menu, which isn't attached to a Frame.
4.1.7
Can
I get rid of the message "Warning:Applet Window" along the bottom of my popup
windows in my Applet?
-
A. This is a security feature that prevents the
applet programmer from popping up a window that looks like it came from the
native OS and asking for passwords or credit card info (etc). Users must
always be aware of when they are talking to an unsigned applet. You can get
rid of it by signing the applet, if the user accepts signed applets from
you. See the Java Signing FAQ at
http://www.fastlane.net/~tlandry/javafaq.txt
In Netscape (only), using the Capabilities API to make the call
PrivilegeManager.enablePrivilege("UniversalTopLevelWindow");
before creation of the Frame eliminates the message, if the security manager
passes it.
4.1.8When
I subclass Applet, why should I put setup code in the init() method? Why
not just a constructor for my class?
-
A. The browser invokes your constructor, then
setStub, then init(). Hence when your constructor is invoked, the AppletStub
(and through it the AppletContext) is not yet available. Although in principle
you can do things in the constructor that don't rely (even indirectly) on
the AppletStub or AppletContext, it is less error-prone to simply defer all
setup to the init method. That way you know that anything that needs the
stub/context will have it
available.
4.1.9How
do I pull a non-class file, such as a .gif, out of a jar file?
A. In your class, you should be able to do something
like this:
String imageFileName = "foo.jpg"
URL imageURL = getClass().getResource(imageFileName);
Toolkit tk = Toolkit.getDefaultToolkit();
Image img = null;
try {
java.awt.image.ImageProducer I_P;
I_P = (java.awt.image.ImageProducer)imageURL.getContent();
img = tk.createImage(I_P);
Or equivalently, and possibly simpler, this:
String imageFileName = "foo.jpg";
InputStream jpgStream
= getClass().getResourceAsStream(imageFileName);
Toolkit tk = Toolkit.getDefaultToolkit();
Image img = null;
try {
byte imageBytes[]=new byte[jpgStream.available()];
jpgStream.read(imageBytes);
img = tk.createImage(imageBytes);
(Like anything involving Jar files, this is from JDK 1.1 on).
getResource(String) does not work in applets in Netscape due to Netscape
security issues. You should use getResourceAsStream(String) there instead.
See
http://developer.netscape.com/software/jdk/relnotes.htm
Note that getResourceAsStream() is also good for reading a text (or other
kind of) file from a JAR file.
public void init() {
InputStream myStream = getClass().getResourceAsStream("myFile");
// ...
}
Once you have it as an InputStream you can read it how you like.
4.1.10 I want to know about {applets,applications}
but the
lousy book I got just talks about {applications,applets}. What can I do?
A. The truth is that 95% of the material is the same, whichever your
book chooses to focus on. Some people write their apps to work completely
in a Panel, then depending on whether they're running stand-alone or in a
browser the Panel is either added to a Frame or an Applet. The trick is that
you need to add a listener to the application's Frame to handle the
WINDOW_CLOSING (previously WINDOW_DESTROY) event yourself.
If you fail to do this, when running as an application, the window won't
close. See question 15.7 for a sample of the right handler.
In this scenario the following code will tell you which environment you're
running in:
public boolean isRunningInBrowser() {
Component p = getParent();
while(p != null && !(p instanceof Frame)) {
p = p.getParent();
}
return (p == null);
}
4.1.11 How do I print a page with an
applet?
-
A. Browsers are starting to introduce support for this. Until they
all have it, your best bet is to print a screendump. Using the browser
to print the page may leave a blank where the applet is. Putting print
support in the applet will print the applet only, not the rest of the browser
page. See also Q5.2.
4.1.12 How can I position my dialogs
centered(not top
left)?
4.1.13 How can I get two applets on the same
page to communicate with each other?
-
A. In your HTML page, give a NAME in the APPLET tag for the applet
receiving message, say <APPLET ... NAME=someName ...>. In the Java
code of the other applet do
Applet anotherApplet = getAppletContext.getApplet("someName");
Cast anotherApplet to the correct applet subclass, and you can call any methods
in the applet subclass. Don't forget to use appropriate synchronization when
two threads tweak variables.
You can walk through all the applets on an HTML page using code like
that below. However this appears to be broken in Communicator 4.04 on Win95.
Applet otherApplet;
AppletContext ac =getAppletContext;
for (Enumeration applets=ac.getApplets();
applets.hasMoreElements(); ) {
otherApplet=(Applet)applets.nextElement();
if (otherApplet!=this) break;
// do something with otherApplet, e.g.
// if (otherApplet instanceof FooApplet) ...
}
Some people suggest using the static members of a common class to communicate
information between the applets. This is not recommended as it relies on
class-loading behavior that may change in future.
Inter-applet communication sometimes arises when you have a multi-screen
type program and you don't want to force the user into downloading everything
at once. One alternative is to make them into one applet with two GUIs. Try
to avoid the need for applets to talk to each other. Also check the URL
http://java.sun.com:81/products/hotjava/1.1/applet_environment.html which
explains how it can be done in HotJava 1.1. Recommendation: stay far away
from code which is browser-specific.
4.1.14 How can I resize an applet?
-
A. If you want resizing behavior from an applet, you should launch
an external frame that can be resized independently.
One programmer suggests using percentages for the height/width parameters
in an applet tag, like this:
<APPLET CODE = "lewinsky.class" WIDTH="100%" HEIGHT="100%" >
You can't resize the applet directly, but it does get resized when you resize
the browser window (tested with Netscape 3.04 and 4.04, but does not work
with appletviewer). If you have nothing else on your HTML page and use 100%
for your width and height, the browser window looks almost like a real
application.
For the extremely tricky: have the browser reload the page with the applet
when the browser resizes using new values for width and height (probably
not what you want most of the time). You would need javascript to generate
a page dynamically using document.write("...") when the browser resizes.
Not recommended. Another possibility is to use the new SplitPane class in
JFC.
4.1.15 How do I read a text file stored
in a JAR?
-
A. The best way to do this is to use the method
Class.getResourceAsStream() which will give you an input stream from which
you can read the text file in the JAR. This technique can be used to get
parameter info from a text file for an applet. See www.uq.net.au/~zzcmumme
for an example.
4.2BROWSERS
-
4.2.1When
will my favorite browser support Java 1.1?
-
A: All popular browsers now have JDK 1.1 support. Netscape Communicator
4.04 plus patch "J" fully supports the features of Java 1.1. It was released
in December 1997, and is only missing the JavaSoft support for applet signing
(Netscape has gone its own way on this).
http://developer.netscape.com/support/faqs/champions/java.html#21
Sun's HotJava browser fully supports the JDK 1.1 features.
People who are obliged to use a browser without standard Java support should
use Java Plug-In. Java Plug-In substitutes a standard Java virtual machine
for that shipped with the browser. It allows you to use RMI, JavaBeans
components, and Java Foundation Classes in Internet Explorer 3.02, 4.0, and
4.01. Java Plug-In also works flawlessly with Netscape browsers. You can download
Java Plug-In from
http://java.sun.com/products/.
4.2.2Is
it possible to set and retrieve cookies from Java, in a manner that is compatible
with all browsers supporting cookies?
4.2.3 I am developing an applet and testing it
in Netscape
Navigator.I
find that after I recompile, I press reload, clear the caches, retype the
URL of the HTML wrapper, and I still have the old version. Why is this?
-
A. It is because Netscape has completely failed
to improve the defective code that does this monstrously wrong thing. It
has been like this for many successive releases.
Flushing the network cache will make no difference; that isn't where the
caching is taking place. Although applets are sometimes "pruned" and their
ClassLoaders garbage-collected, this doesn't happen predictably, so restarting
Netscape is the only reliable work-around at the moment.
A related question is "how do I make the browser reload from a URLConnection
instead of just getting the content from the local cache?" The answer is
to use
java.net.URLConnection.setUseCaches(false)
Browsers seem to vary in their conformance to this programmatic request.
Netscape caching varies depending on whether a proxy server is in use, and
which thread in the applet made the get request.
4.2.4
So,why
can't Netscape reload the applet when you press the Reload button?
4.2.5 Should I use Microsoft
CAB files or Java
JAR files?
-
A. The question contains its own answer.
CAB format is a Microsoft-only format. So do not use it as it destroys software
portability.
JAR format is the Java standard format, based on PKZIP format including data
compression. JARs were introduced with JDK 1.1
See
http://www.ibm.com/java/community/viewarchive4.html
for more information.
You should use the Java standard format JAR (Java Archive) files, not a
vendor-specific format. JAR files are not just a Java standard, they are
in industry-standard PKZIP format. One reader comments that both formats
can be used with this tag:
-
<APPLET NAME=myapplet
ARCHIVE="myzip.zip"
CODE="com/nnnnn/nnnn/cccccccc.class"
WIDTH=n
HEIGHT=n>
<PARAM NAME="cabbase" VALUE="mycab.cab">
</APPLET>
IE3 does not support JAR
IE4 supports compressed and uncompressed JAR, but not signed JAR
4.3 SWING
-
4.3.1 What is Swing?
-
A: Swing is a new GUI toolkit bundled with JDK 1.2, and available
as an add-on extension library for JDK 1.1. Swing is the first GUI toolkit
that lets developers create lightweight components with a pluggable look-and-feel
design. From an architectural standpoint, the Swing component set extends
-- but does not totally replace -- the Abstract Windowing Toolkit (AWT).
Swing has
many components that can be used in place of components in the AWT (e.g.
JFrame instead of Frame, JButton instead of Button,
JApplet instead of Applet, JPanel instead of Panel).
It also has many components that don't exist in the AWT (e.g. tool tips,
toolbars, and progress bars). However Swing relies on the underlying AWT
being there.
The Swing toolkit allows the creation of GUI's that are every bit as
sophisticated as native code toolkits like MFC -- with the Java advantage
that they run on every platform. The pluggable look and feel means that they
can have the exact same appearance on every platform too, if the user chooses
it.
With Swing, native window behavior is confined to external window frames
(and their borders) and a few other things such as fonts and the buffers
used to hold window contents. The composition, layout, and
drawing of controls
is now all handled by Java code. So identical code is executed to create
and manage your user interface on every platform.
Swing provides a much greater
consistency of behavior across different platforms.
Swing works with JDK 1.1 if you download the swing.jar file and
add it to your path. Swing is built-in to JDK 1.2, and Javasoft has
just changed its 1.2 swing package naming strategy. It is leaving
it as com.sun.java.swing to be compatible with JDK 1.1.
4.3.2 Why
is my menu showing up behind other components when I use Swing?
-
A: The answer relates to lightweight and heavyweight (peer-based)
components. There is a good article about it at
http://www.javasoft.com/products/jfc/swingdoc-current/mixing.html
For those who want the quick fix, and will read the article later, adding
the line:
com.sun.java.swing.JPopupMenu.setDefaultLightWeightPopupEnabled(false);
before you create any menus will probably fix it (even if you're using menus
other than JPopupMenu).
The summary answer is that a Lightweight component will not appear over a
heavyweight component by default.
4.3.3 Why does
my JFrame go away after I chose system close on the window?
A: Assume that you have a Swing JFrame component, and you handle the
windowClosing event, but do nothing in the handler. You will see that the
JFrame disappears anyway.
The reason is that JFrame's have default handling of the system close operation,
separate from the windowClosing event. You have to override that by calling:
setDefaultCloseOperation(DO_NOTHING_ON_CLOSE);
on your JFrame.
4.3.4
Should I use Swing or AWT to build my GUIs?
A:
Use Swing to build your apps now instead of AWT components, wherever you
have a choice.
4.3.5
Where can I find a Swing tutorial?
4.3.5
How can I run Swing code in a browser?
A:
Most browsers have to be specifically set up to run Swing applets.
Read the article at
http://www.javasoft.com/products/jfc/swingdoc-current/applets.html
for information about this. The article also contains a simple swing
example applet, so you can confirm that that's your problem.
Another approach is to use the Java plug-in, which automatically gives a
Swing compatible Java in the browser.
5.CORE LIBRARIES
-
5.1I
can't seem to change the value of an Integer object once created.
-
A. Correct. Integer (Float, Double, etc) are
intended as an object
wrapper for a specific value of a number, not as a general purpose
way of shipping a primitive variable around as an Object. If you need
that it's easy enough to create: class General { public int i; }
5.2
How do I print from a Java program?
A. Use the Toolkit.getPrintJob() method
Component c = this.getParent();
while (c!=null && !(c instanceof Frame))
c=c.getParent();
PrintJob pj = getToolkit().getPrintJob((Frame) c, "test", null);
Graphics pg = pj.getGraphics();
printAll(pg);
pg.dispose();
pj.end();
This feature was introduced with JDK 1.1. A common place to put this is in
the code that handles a button press. Printing from an untrusted applet is
subject to a check from the SecurityManager.
The JDK 1.1 printing API is more a screen hardcopy facility than
a full blown publishing and illustration hardcopy API.
JDK 1.2 offers a more full-featured printing API.
If you simply want to print text, then write it to a file and
print the file. Or open a filename that corresponds to the
printer. On Windows, that is "LPT1" and the code looks like:
try {
FileOutputStream fos = new FileOutputStream("LPT1");
PrintStream ps = new PrintStream(fos);
ps.print("Your string goes here");
ps.print("\f");
ps.close();
} catch (Exception e) {
System.out.println("Exception occurred: " + e);
}
The final formfeed is needed by windows to start the printjob.
5.3
What
are the properties that can be used in a PrintJob?
A. The properties are
awt.print.destination - can be "printer" or "file"
awt.print.printer - printer name
awt.print.fileName - name of the file to print
awt.print.numCopies - obvious
awt.print.options - options to pass to the print command
awt.print.orientation - can be "portrait" or "landscape"
awt.print.paperSize - can be "letter","legal","executive" or "a4"
The defaults are destination=printer, orientation=portrait,
paperSize=letter, and numCopies=1
You can search for info like this by joining the Java Developer Connection
(it's free)
http://developer.javasoft.com/developer/index.html
Then do a search for "PrintJob".
5.4 Is there any
package in Java to
handle HTML?
A. See the answer to Question 8.14
5.5Why
don't Dialogs work the way I want them to?
A. Modal dialogs (dialog windows that stay up
until you click on them) are buggy in many browsers and in the 1.0.2 JDK.
One bug is that the dialog is not necessarily put on top when it is displayed.
Most of the modal dialog bugs are fixed in JDK 1.1.
5.6 Where can I
find
information about the sun.* classes in the JDK?
A. You're not supposed to. Those classes are
only to support functions in the java.* hierarchy. They are not part of the
API, and won't be present in Java systems from non-Sun vendors. Some people
have reverse engineered the code and published an API for these classes but
you use it at your own risk, and it may change without warning.
Worst of all, those programs will not have the portability of true Java but
will only run on Sun JDKs. For the same reason you shouldn't use classes
outside the java.* packages when using JDKs from other vendors.
If you still insist on going ahead, check these URLs:
http://java.sun.com/products/api-overview/index.html
http://www.parmly.luc.edu/javaudio/
http://www.users.interport.net/~mash/javamidi.html
5.7 How do you
read
environment variables from with a Java program?
A. Environment variables are not used in Java,
as they are not platform portable. The Mac doesn't have environment variables
for example. A Windows 95 application not started from a DOS window does
not have environment variables. Use properties instead. It was a design error
in JDK 1.0 that programmers had to set the CLASSPATH environment variable.
This should have been set in a property file. JDK 1.1 removes the need to
set the CLASSPATH environment variable, and (as the documentation explains)
it's generally a mistake if you do.
Create your own properties file (see java.util.Properties) or specify them
with the -D option when you invoke the interpreter or JRE. Additionally,
on some systems you can set a property from the command invocation line like
this:
java -Dfoo=$foo MyClass (Unix)
or
java -Dfoo=%foo% MyClass (Win95/NT)
This sets the "foo" property to the value of the environment variable foo,
and makes it available in the System properties. Make sure you do not leave
any spaces after the -D or around the = sign. Inside the program you get
the value with:
String env = System.getProperty("foo");
More simply, just put the environment variable on the commandline
and read it as arg[0]
-
java MyClass %FOO% ; Win32
java MyClass $FOO ; Unix
Finally, you could execute a Runtime process to get the environment variables
if you are on a platform that has them. This is a poor approach as it builds
platform-specific knowledge into the program. See Q10.5 for more details.
On Unix, the command that prints environment variables is "/usr/bin/env".
On Windows95, it is "set"
5.8
How do you use the Date class to display the current time in my
timezone?
-
A. There is more than one bug involving timezones,
so some people have it working OK, but others trying something very similar
are hitting other date format and timezone bugs. Before diving into the details,
let's review the date-related classes.
-
class Date represents a specific instant in time, with millisecond precision.
The design of this class is a very bad joke - a sobering example of how even
good programmers screw up.
-
class Calendar is an abstract class for converting between a Date object
and a set of integer fields such as year, month, day, and hour.
-
class GregorianCalendar is the only subclass of Calendar in the JDK. It does
the Date-to-fields conversions for the calendar system in common use. Sun
licensed this overengineered junk from Taligent - a sobering example of how
average programmers screw up.
-
class DateFormat is an abstract class that lets you convert a Date to a printable
string with fields in the way you want (e.g. dd/mm/yy or dd.MMM.yyyy or
whatever). DateFormat and SimpleDateFormat classes use the default timezone
for your JVM. This is the JVM's "user.timezone" property (a more sensible
choice would be to attempt to use the tz of the underlying OS). If the
"user.timezone" property is not set, it now defaults to GMT (not PST as it
used to). A very small improvement. To associate the tz of your choosing
with a date format, use:
DateFormat df = DateFormat.getDateInstance();
df.setTimeZone(TimeZone.getDefault());
-
class TimeZone is an abstract class that represents a time zone offset, and
also figures out daylight savings. GregorianCalendar tries to default to
the timezone (if any) set in your Java system properties file. This timezone
will be something like, e.g. "PST", "EST", "CET" etc.
If there is no preset timezone, it used to default (prior to JDK 1.1.2 or
so) to PST - Pacific Standard Time as used in California. Since 1.1.2 the
code now defaults to GMT when no TZ is in the properties file.
The default TZ that Java starts with may well be wrong for
your purposes. You are frequently better off if you explicitly
set the default TZ (not much of a default, is it?) with, e.g.
TimeZone tsz = TimeZone("ECT"); // use ECT timezone or whatever.
TimeZone.setDefault(tsz);
It is also possible to set a timezone not with a string, but
according to a number of hours and minutes offset from GMT.
Actually, the default timezone
-
class SimpleTimeZone is the only subclass of TimeZone in the JDK. It represents
a time zone for use with a Gregorian calendar. This simple class does not
handle historical changes, and has limited rules. When you ask for the default
timezone, you'll actually get an object of this subclass back.
If you want to see the properly formated version of your Date, you shouldn't
use myDate.getTime().toString(). A Date object now only represents an instant
in time, resolved to the millisecond. All other uses (such as extracting
individual fields or formatting) have been deprecated. To display the current
date declare a GregorianCalendar and pull the fields out of that, as in this
example:
import java.util.*;
...
GregorianCalendar g = new GregorianCalendar();
int year = g.get(Calendar.YEAR);
int mon = g.get(Calendar.MONTH);
int date = g.get(Calendar.DATE);
mon++; // in class Calendar & GregCal, months run 0-11 ;-(
System.out.println(mon +"/" +date +"/" +year);
If you want fancy formats for printing/expressing the date, you should use
the new internationalization routines. You will need to use the DateFormat
and SimpeDataFormat classes in the java.text package. Here is an example:
import java.util.*;
import java.text.*;
Locale.setDefault( Locale.ENGLISH );
System.out.println(
DateFormat.getDateInstance(DateFormat.SHORT).format(new Date())
);
This will print (on April 7, 1998):
4/7/98
There is some sample code in the Java API documentation. As an example of
how the new method should work jdk1.1/src/java/util/Date.java contains the
method:
DateFormat formatter = new SimpleDateFormat (
"d MMM yyyy HH:mm:ss 'GMT'", Locale.US);
formatter.setTimeZone(TimeZone.getTimeZone("GMT"));
System.out.println( formatter.format(new Date()) );
The last line results in output of: "7 Apr 1998 21:52:06 GMT"
You can even use:
formatter.setTimeZone(TimeZone.getDefault());
It should be reasonably straight forward to adapt this code for your preferred
format (e.g. change string to "hh:mm" or other) and timezone, e.g. change
"GMT" to "ECT", "JST" or other timezone.
5.9How
are dates represented in Java?
-
A. java.util.Date stores dates as long integers
representing the number of milliseconds since 00:00:00 UTC Jan 1, 1970 (the
birth of Unix, a date known as the "Epoch"). Dates earlier than the Epoch
are represented as negative numbers, counting away from 1/1/1970.
The scheme is sufficient to represent dates from 292,269,053 B.C. to 292,272,993
A.D. (64 bits covers -9223372036854775808 to +9223372036854775807 millesecs
so divide by 1000 to get secs, divide that by 3600*24*365.25 to get years,
and you get -2.9227E8 to 2.9227E8 centered on the Epoch i.e. about 292,269,053
BC to 292,272,993 AD).
In JDK 1.1, Date was augmented by Calendar. Things didn't get any better.
Instead of being ill-conceived and simple, it is now ill-conceived and
complicated. The code was all licensed from Taligent. It didn't really improve
matters. Dates are the lemon of Java, as Roedy Green truly notes.
You could consider using the BigDate class written by Roedy, and available
at http://oberon.ark.com/~roedy
5.10
That may be,
but how do I extract day, month, year from a Date?
Then construct a Calendar to do the translation:
Calendar mycal = Calendar.getInstance();
mycal.setTime(now);
// Now read the Calendar's fields using get().
System.out.println("Year = " + mycal.get(Calendar.YEAR));
We hope (perhaps in vain) all this junk will be fixed once and for all in
a future release.
5.11 How do I create
my own Time Zone to apply to dates?
-
A. The Calendar, DateFormat, and even TimeZone
classes will give you the timezone of your system (wherever you are). You
can create a TimeZone object with the GMT offset of your choice like this:
bst_tz = new SimpleTimeZone( 6 * 60 * 60 * 1000, // +6 hour offset from GMT
"BST");
TimeZone.setDefault(bst_tz);
You can then apply that TimeZone to create a Calendar object like so:
GregorianCalendar BritishSummerTime =
new GregorianCalendar(bst_tz);
5.12 How do I create
the BST timezone specifically?
-
A. First, note the pitfall that as well as being
the Bangkok Standard Time timezone 6 hours east of GMT, "BST" also means
"British Summer Time".
In most or all other timezones, daylight saving time is handled automatically
and internally. But GMT is the reference for all other timezones, and so
does not have the summertime update applied automatically. So here is the
code to create a British Summer Time timezone that is offset 1 hour from
GMT between two dates:
import java.util.*;
import java.text.*;
// BST runs from last Sun in March to 4th (not last) Sun in Oct
// create a BST timezone (code courtesy of Tony Dahlman).
SimpleTimeZone bst_tz =
new SimpleTimeZone( 0, // no offset from GMT
"BST", // individualized tz id
Calendar.MARCH, -1, Calendar.SUNDAY,2*60*60*1000, // first Sun Apr 2AM
Calendar.OCTOBER, 4, Calendar.SUNDAY,2*60*60*1000 // 4th Sun Oct 2AM
);
// set the BST timezone as the default
SimpleTimeZone.setDefault(bst_tz);
// apply that TimeZone to create a Calendar object like so:
GregorianCalendar BritishSummerTime =
new GregorianCalendar(bst_tz,Locale.UK);
and here is how you would print out values using BST:
// create a template for printing the date
DateFormat df = DateFormat.getTimeInstance(
DateFormat.LONG,Locale.UK);
// tell the template to use BST tz.
df.setTimeZone(TimeZone.getDefault());
System.out.println("Using British Summer Time "
+"the time is: "
+ df.format( BritishSummerTime.getTime() ) );
// Now get and compare with current time in GMT
df.setTimeZone(TimeZone.getTimeZone("GMT") );
System.out.println("\nCurrent time in GMT is: "
+ df.format(BritishSummerTime.getTime() ) );
In the winter, this BST zone is aligned with GMT; in the summer it is one
hour later (4am GMT is 5am BST).
You can look at a list of timezone names and offsets in the file
$JAVAHOME/src/java/util/TimeZone.java
5.13
What kind of different date formats are allowed for the Date constructor with
the string parameter?
-
A.
That constructor is now deprecated, so you should not use it.
Instead, you should convert a date/time String into a Date like this:
// don't forget to set the default TZ if needed.
DateFormat HHmm = new SimpleDateFormat( "HH:mm" );
HHmm.setTimeZone( TimeZone.getDefault() );
Date lunch = HHmm.parse( "12:30" );
System.out.println( lunch );
The above code would result in (when in the MST timezone),
Thu Jan 01 12:30:00 MST 1970
To parse other date and time fields, refer to
the SimpleDateFormat documentation.
5.14
How do I get Java talking to a Microsoft Access database?
A. Use the JDBC-ODBC bridge. It is not especially
challenging to set up, but it does require painstaking attention to detail.
There is a step-by-step example in the van der Linden text "Just Java 1.1"
mentioned in the sponsorship section of this document.
Note that the Microsoft version of the Java kit does not support JDBC-ODBC
access because it uses a non-standard native code interface. Also check the
JDBC FAQ listed at the end of this document.
5.15I
can't seem to change the current working directory.
-
A. Correct. This missing functionality is an
oversight that we hope will be corrected in future. Changing the user.dir
property merely changes the text property, not the underlying reality that
it is supposed to reflect.
5.16How
do I create a Vector of ints?
-
A. ints are primitive types and hence can't be
stored by the Vector class which stores objects. You'll need to wrap the
ints. Try this:
5.17
I have several worker threads. I want my main thread to wait for
any of them to complete, and take action as soon as any of them completes.
I don't know which will complete soonest, so I can't just call Thread.join
on that one. How do I do it?
-
A. You need to use the wait/notify mechanism
to allow any of the worker threads to wake up your main thread when the worker
has completed.
5.18How
do I get random numbers?
-
A. If you just need a quick random double between
0.0 and just less than 1.0
The notation "[0,1)" is common math notation for "zero to .9999999 etc" The
Sun documents say this returns 0.0 to 1.0, but inspection of the source shows
they are wrong. However, due to the inherent inaccuracies of floating point
arithmetic, multiplying N by 0.999999 etc can result in an answer of N, not
N * .999999 . So watch out when N is big.
Where things get even trickier is in the case where you want an int within
a certain range, say 1 to 6 to simulate the throw of a die or 1 to 52 to
represent a playing card. Class Random has a nextInt method that will return
any integer:
However, that has an (almost) 50% chance of being negative, and it doesn't
come from the right range. So you just take the abs() value and then mod
it into the right range:
Except, the abs() method fails gracelessly in the presence of the
Integer.MIN_VALUE (it returns the same, negative, result!). So it is better
to AND to get the non-negative value: In general, to get a random int between
high and low limits (inclusive):
The sentence above states "(almost) 50% chance" because there is one more
value in the negative integers than in the positive integers in two's complement
arithmetic as used by Java. For most purposes, the bias introduced will be
insignificant, but we "and" the nextInt() to convert it to zero. Sure, it's
unlikely to occur, but you don't want the reactor going critical just because
you missed this case while testing.
A worse problem is that with the algorithm used, the low order bits are
significantly less random than the higher order bits. And the low order bits
are precisely the ones you get when you do a (mod 2^n) operation.
5.19
What does "deprecated" mean? I got this in a compiler error message.
5.20
Where/why
should I use the Enumeration interface?
-
A. It's a very convenient way to step through
some of the library data structures, such as HashTable, Vector, and ZipFile.
It is thread safe. If you're looking at an element in one thread while another
thread is trying to delete it, it won't half vanish.
Here's how you might look at every file in ZIP file:
ZipFile z = new ZipFile("foo.zip");
for (Enumeration e=z.entries(); e.hasMoreElements(); ) {
ZipEntry ze = (ZipEntry)e.nextElement();
System.out.println("got " + ze.getName() );
}
And here's how you might look at all the tokens in a String:
StringTokenizer tokens =
new StringTokenizer (SomeArbitraryString, "\n", false);
Enumeration enum = tokens.elements();
while enum.hasMoreElements()
System.out.println(enum.nextElement());
You should look for opportunities in your own data structures to implement
Enumeration, anywhere where the structure has repeated elements.
5.21 Which
version of WinZip is compatible with java.util.zip?
-
A. You need WinZip version 6.2 or later. Version
6.1 or earlier is not good enough. WinZip can be downloaded from
http://www.winzip.com/download.cgi. The pkzip software works fine.
5.22
What is the Model/View/Controller paradigm?
5.23 How can
Java access other fonts on my system?
6. I/O
//class that opens the printer as a file and writes "Hello World" to it
import java.io.*;
public class lpt {
public static void main (String[] argv) {
try {
FileOutputStream os = new FileOutputStream("LPT1");
//wrap stream in "friendly" PrintStream
PrintStream ps = new PrintStream(os);
//print text here
ps.println("Hello world!");
//form feed -- this is important
//Without the form feed, the text
//will simply sit in the print
//buffer until something else
//gets printed.
ps.print("\f");
//flush buffer and close
ps.close();
} catch (Exception e) {
System.out.println("Exception occurred: " + e);
}
}
}
-
-
If you wish to change the characteristics of the port (e.g. baud rate, parity,
etc), not just read/write data, Java currently offers no portable way to
do this. You will need to use one of the packages mentioned above or some
native code or a system command.
6.4How
do I append to a file?
-
A. There are two ways. JDK 1.1 introduced new
constructors for two of the output classes, that allowed you to set a boolean
flag:
Another way is to do this:
Then write using fd. Note that the latter method does not take advantage
of the "append" mode present in many operating systems (such as all Unixes).
Such a difference may make a difference with multiple processes or threads
appending to the same output file. This can happen frequently, even if not
intended by the programmer, e.g. with logfiles in multitasking environments.
With the lack of file-locking mechanisms in Java the issue becomes even more
significant.
6.5Is
it possible to lock a file using Java ?
-
A. Java does not feature an API to lock a file
or regions within a file. Code that needs to do this must take one of four
approaches:
-
1. implement an advisory locking scheme using features that Java does have
(synchronized methods). This allows you to lock files against use by other
Java code running in the same JVM.
2. Use an atomic operation like file "renameTo()" and have all processes
(Java and non-Java) follow the same protocol: if the operation succeeds,
you have the lock, and you change the file back to give up the lock. The
FAQ previously recommended delete() or mkdir() as the primitive, but there
has been a report that these operations are idempotent (can be repeated without
error) in some JVMs. The suggestion of renameTo has been proven to work in
practice. See
http://www.camb.opengroup.org/~sanfilip/FileLock/ for example locking
class docs and source.
3. make calls to native code to issue the locking ioctls. This approach is
not portable, but gives you a shot at having your locks respected by other
programs using standard locking ioctls outside Java.
4. Push the work to a central server. Since socket connection requests arrive
in a single queue on the server, this can be used to serialize lock requests.
There might be some merit in copying the NFS lockd protocol for a general
approach. Rolling your own simple version for a specific application is pretty
easy. A database would be better off locking records or fields, not byte
offsets. In theory, the server socket approach would make it easier to perform
automatic cleanup of a lock on abrupt VM process failure, e.g. by asking
"are you still alive?" to the lock holder occasionally.
6.6How
do I make the keyboard beep in Java?
6.7
How do I execute a command from Java? How do I do I/O redirection in Java
using exec() ?
-
A. See answer to question 10.5
6.8
How do you do file I/O from an applet?
-
A. For security reasons, untrusted
applets accessed across the network are restricted from doing
certain operations, including I/O. This prevents rogue applets
from sending out your private data, or deleting it. A
trusted (signed) applet can perform these operations (JDK 1.1 on).
The simplest approach for server-side I/O is to use the Linlyn
class available from
http://www.afu.com. This is free software under the GNU
license, and uses FTP to move files between an applet and the
server. It is suitable for low-volume non-critical use like
keeping a high-score file. The Linlyn class has a very simple
application programmer interface.
The following suggestion is for server-side input.
- You can read a file on the server if you can create a URL referencing the
file. Then open a stream, then use any of the stream-based methods to read.
This allows reading but not writing. It requires an http demon running on
the server, which will usually be the case.
try{
URL url = new URL("http://somewhere.com/test.txt");
// or URL url = new URL( getDocumentBase(), filename);
// JDK 1.0.2: DataInputStream in = new DataInputStream(url.openStream());
BufferedReader in = new BufferedReader( // JDK 1.1 and later
new InputStreamReader( url.openStream() ) );
String s = in.readLine(); //read till you get a null line.
} catch(MalformedURLException e){System.out.println("URLException:"+e);
} catch(IOException e){System.out.println("IOException:"+e);}
}
You cannot write a file on the server this way.
The following suggestions are for server-side output.
It absolutely requires the cooperation of the server to allow an applet to
write a file to the server. This cooperation may take any of several forms:
-
ftp server
-
file server (webnfs or custom written)
-
listening on a socket for data from applets
-
CGI script
-
Java RMI (remote method invocation)
-
JDBC process
In particular:
1. FTP code. Use the Linlyn class mentioned above.
2. WebNFS. This is an evolution of the NFS (Network File System) to make
file resources visible in browsers. More information at
http://www.sun.com/webnfs
3. Open a socket back to the server and read/write the data. Have a process
on the server that listens for socket connections from applets and does the
requisite I/O. This does I/O on the server.
4. Or use a CGI script or servlet on the server to write when browsed.
There
is some source at
ftp://ftp.oyster.co.uk/pub/java/fileIO/
The following suggestions are for client-side I/O.
3. Use a trusted applet (see section on security). This will permit local
I/O without any of the restraints mentioned above. In this regard, the
appletviewer and many other browsers regard applets loaded from a local
filesystem (rather than across the net) as being more trustworthy, and
perhaps even allowed to do I/O.
4. Or use a browser that has a security policy that is configured to allow
file I/O (such as Sun's appletviewer).
6.9
I used a C program to write a binary file. When I instantiate a DataInputStream
on the file in Java, and try to readInt, I do not get the correct numbers.
Why is this?
public int swap(int i) {
int byte0 = i & 0xff;
int byte1 = (i>>8) & 0xff;
int byte2 = (i>>16) & 0xff;
int byte3 = (i>>24) & 0xff;
// swap the byte order
return (byte0<<24) | (byte1<<16) | (byte2<<8) | byte3;
}
Alternatively, the following code assembles bytes from a byte array
in big-endian order (as used by Java) into an int:
byte[] bytes = ... // whatever
int start_index = ... // wherever
int value = 0;
for ( int i = start_index; i < start_index+4; ++i ) {
value = ( value << 8 ) | ( bytes[i] & 0xFF );
}
If the bytes are in little-endian order, just change the "for"
for ( int i = start_index+3; i >= start_index; --i )
And this code will assemble a double that has been written in
reverse byte order:
byte[] gnol = byte[8];
stream.read(gnol);
long l = ( (gnol[7]&0xff)<<56)
| ((gnol[6]&0xff)<<48)
| ((gnol[5]&0xff)<<36) | ... // and the rest
double d = Double.longBitsToDouble(l).doubleValue();
6.10How
do I make I/O faster? My file copy program is slow.
A. This is the purpose of BufferedInputStream.
It is a flaw in Java that buffered I/O is not the default, with a flag or
different constructor to turn it off. I/O is the second worst designed
package in Java, after the Date class.
6.11
How do I do formatted I/O of floating point numbers?
6.12How
do I read numbers in exponential format in Java?
A: The program below (written by Steve Chapel)
uses StreamTokenizer to read data from the standard input and recognizes
doubles in exponential format (e.g. -1.23e-45).
import java.io.*;
public class ReadExponential {
public static void main(String argv[]) {
DataInputStream in = new DataInputStream(System.in);
StreamTokenizer st = new StreamTokenizer(in);
try {
while (st.nextToken() != StreamTokenizer.TT_EOF) {
switch (st.ttype) {
case StreamTokenizer.TT_NUMBER:
double num = st.nval;
int exp = 0;
st.ordinaryChars('\0', ' ');
st.nextToken();
st.whitespaceChars('\0', ' ');
if (st.ttype == StreamTokenizer.TT_WORD &&
Character.toUpperCase(st.sval.charAt(0)) == 'E') {
try {
exp = Integer.parseInt(st.sval.substring(1));
} catch (NumberFormatException e) {
st.pushBack();
}
} else if (st.ttype < 0 || st.ttype > ' ')
st.pushBack();
System.out.println("Num " + num * Math.pow(10, exp));
break;
case StreamTokenizer.TT_WORD:
System.out.println("Word " + st.sval);
break;
default:
System.out.println("Char '" + (char) st.ttype + "'");
break;
} // end switch
} // end while
} catch (IOException e) { System.out.println("IOException: " + e); }
} // end main
}
6.13
I'm trying to read in a character from a text file using the DataInputStream's
readChar() method. However, when I print it out, I get ?'s.
A. Remember that Java characters are 16-bit Unicode
characters, while many hosts systems store characters as 8-bit ASCII characters.
Therefore, to read individual chacters from a text file, you need to ensure
the proper conversion. The proper way to do this is to use an InputStreamReader,
which converts from 8 to 16 bit streams:
The less-favored way (because it is not so portable, as the encodings translation
is not done) is just to read a byte and cast it into a character:
6.14How
do I delete a directory in Java?
6.15
How do I tell how much disk space is free in Java?
-
A. There currently aren't any good Java APIs
for system introspection. There is no Java way to control processes, or look
at system resources. You can use Runtime.getRuntime().exec() to do "df" on
unix or "dir" on Windows right now.
Alternatively, check out JConfig:
http://www.tolstoy.com/samizdat/jconfig.html
JConfig is a cross-platform library that fills in many of the gaps in the
core Java API, and makes it possible to work with files, processes, file
types, video monitors, etc. in a much more Windows and Mac friendly manner.
6.16
How do I get a directory listing of the root directory C:\ on a PC?
6.17
What is the
difference between
the various ZIP formats: ZIP, GZIP, and PKZIP?
-
A. GZIP files (the Gnu format) are not archives
and compress only one file. GZIP is essentially a one file subset of the
Zip format. Zip is an archive file format, popularized on PCs, that
contains multiple compressed files. PKZIP is a set of
commercially-available programs that create Zip files. All three PKZIP, GZIP
and Zip use the deflate compression format, which is based on the LZ77 algorithm.
This compression is also used by the ZLIB library and hence the PNG graphics
file format (which uses ZLIB). PNG - Portable Network Graphics - provides
a patent-free replacement for GIF and TIFF. If you save a GIF, don't forget
to pay the royalty to Unisys -- see Unisys's web page at
http://www.unisys.com/LeadStory/lzwfaq.html. That patent is why GIFs
should not be used.
The PNG format is specified in RFCs
1950,
1951, and
1952, and is unencumbered
by licenses or patents.
An alternative compression technology, LZW compression, is encumbered by
Unisys's patent. LZW is used in GIF files and by the Unix compress command.
Luckily, as well as being free from patent restrictions, LZ77 also gives
better compression than LZW. LZW is the initial letters of the last names
of the three computer scientists who developed the algorithm (Lempel, Ziv,
Welch).
The basic classes (all in java.util.zip) that read LZ77 Zip format are Deflater
and Inflater. These are used by the stream classes DeflaterOutputStream and
InflaterInputStream. The java.util.zip classes GZIPInputStream and ZipInputStream
inherit from InflaterInputStream.
PKZIP is a commercial program for DOS, Windows, and OS/2, sold by PKWARE
Their FAQ
(http://www.pkware.com/zipgfaq.html)
specifically says
-
"Because PKWARE has dedicated the .ZIP file format to the public domain,
it is possible for other people to write programs which can read .ZIP files.
NOTE THAT THE PKZIP, PKUNZIP, PKSFX PROGRAMS AND THEIR ASSOCIATED SOURCE
CODE AND SUPPORT PROGRAMS ARE THE EXCLUSIVE PROPERTY OF PKWARE INC. AND ARE
NOT PUBLIC DOMAIN SOFTWARE PROGRAMS.
The "other people" PKZIP's FAQ refers to is the InfoZIP project, a group
of public-minded programmers spread over the world producing free software
that works on most ANSI C compilers and platforms. See
Jar files are in ZIP format, but are not as complete as a full filesystem
archive format since file permissions are not saved. Some versions of WinZip
are known to be inadequate for processing the full PKZIP format. Use InfoZIP
instead.
6.18 How can I use characters other than ASCII
in Java?
6.19 I did a read from a Buffered stream, and
I got fewer bytes than I specified.
-
A. This is the way that BufferedInputStream
works up to and including the current release. The behavior is so unintuitive
that it really represents a bug. Javasoft has "resolved" the bug by writing
the comments in the program so that the behavior is not disallowed.
When you instantiate a buffered input stream, you can specify the size of
buffer it should use. Let's call this the internal buffer. When you call
read() you can say how many bytes to read. Let's call this the request. If
the request is smaller than the internal buffer and not a multiple of the
internal buffer, then the last read returns only the odd bytes left in the
internal buffer! The more reasonable and intuitive behavior would be for
the internal buffer to be refilled, so that the whole request can be granted.
For example, if you create a BufferedInputStream with an internal buffer
of 1000 bytes, and try to read 512 byte chunks, your first read will return
512 bytes, but your second read will only return (1000-512), or 488, bytes.
(Assuming that the file has at least that many bytes in it). The following
code illustrates the problem.
// troubleshooting by Tov Are Jacobsen
import java.io.*;
class filebug {
public static void main(String args[]) throws
FileNotFoundException,IOException {
BufferedInputStream bis =
new BufferedInputStream( new FileInputStream("test.txt"),
1000 );
byte[] buf = new byte[2000];
int numread;
System.out.println( "Available: "+bis.available() );
while (true) {
numread = bis.read(buf,0,512);
if (numread<0) break;
System.out.println( "got "+numread+
", avail:"+ bis.available());
}
}
}
Of course, a valid reason for getting less than you asked for is that you
asked for more data than is actually available in the Stream, e.g. you
requested 512 bytes from a file that only contains 40 bytes. In
general, there are no guarantees about how much data is returned for
a given buffered input stream read request. To avoid this problem, push
a DataInputStream on top of your buffered stream. Then
you can call readFully() which will do what you want.
A similar "got less than I asked for" occurs when reading a socket.
Network protocols frequently packetize data and send it across in bursts.
Nothing is lost of
course, and you are always told how many bytes you actually got. You will
get the remaining bytes on a subsequent read. This happens regardless of
the language used.
Be sure to check the "amount of data returned"
when using the read(byte[], int, int) method of BufferedInputStream,
or when reading from a socket.
Another problem with java.io.InputStream.read(byte[], int, int) is
that it catches and ignores IOExceptions. Instead, these
exceptions should be passed on to the caller.
Ace programmer Jef Poskanzer, jef@acme.com, has a version to
do this at http://www.acme.com/jef/.
6.20
How do I redirect
the System.err stream to a file?
FileOutputStream err = new FileOutputStream("stderr.log");
PrintStream errPrintStream = new PrintStream(err);
System.setErr(errPrintStream);
This was introduced
with JDK 1.1. There is also a corresponding setIn() for redirecting standard
in, and a setOut() for standard out.
Note that you will get a compiler warning about a deprecated construct
when you do this. PrintStreams are deprecated in favor of PrintWriters
which handle Unicode properly. The PrintStream is marked as deprecated by
marking all its constructors as deprecated. There is no way to create the
PrintStream needed to redirect System.err or System.out without triggering
the deprecated warning.
7.NETWORKING
& DISTRIBUTED OBJECTS
7.1Should
I use CORBA in preference to RMI? Or DCOM? Or what?
A. If your distributed programs are all in Java,
then RMI provides a simpler mechanism that allows the transfer of code,
pass-by-value of real Java objects, and automatic garbage collection of remote
objects.
If you need to connect to C++ (or other language) systems or you need
CORBA-specific services, then CORBA is your choice.
has a good intro to CORBA in the Java world.
In July 1997, Sun announced that it was aligning RMI to work more closely
with CORBA. Sun is simply adding an IIOP transport layer to RMI to support
interoperability with CORBA. Java programs can then access CORBA-based objects
through IIOP, the OMG's CORBA-based protocol. This is very good news for
those building heterogenous Enterprise systems, although it will take some
additions to IIOP to support the pieces that RMI uses.
Microsoft spokespeople have tried to promote DCOM by spreading misinformation
that RMI is changing or being dropped. That is totally wrong. The RMI API
continues unchanged in its current form. Using DCOM would restrict your code
to only ever run on Microsoft platforms using Intel hardware, and negates
the "write once, run anywhere" Java philosophy. Non-portable, single vendor
code should be avoided.
7.2
Why does <my java debugger/IDE/other> hang for a
couple of minutes if my Windows PC is not dialed up to the Internet?
-
A. Java has networking support built in. When
the Java program starts the Winsock dll automatically gets loaded. The first
thing this does is to try to resolve the fully qualified domain name for
your machine under the name "localhost". If your system doesn't have this
name mapped, it will try to query a nameserver on the internet, which is
typically (on a PC) your dialup ISP. So it either prompts you to connect
to the ISP, or waits till the attempt times out.
You can sometimes minimize the Win95 problem by giving your system another
way to resolve DNS names. Edit the hosts file for your system so that localhost
and the full domain name are both mentioned. On Windows 95 systems the hosts
file is: %windir%\HOSTS (for example, C:\WINDOWS\HOSTS)
On Windows NT systems the hosts file is: %windir%\System32\DRIVERS\ETC\HOSTS
(for example, C:\WINNT\System32\DRIVERS\ETC\HOSTS)
One gotcha under Win95 is that if the last entry in the hosts file
is not concluded with a carriage-return/line-feed then the hosts
file will not be read at all.
So if my system is called goober.best.com change the hosts file from
to
127.0.0.1 goober.best.com localhost
Showing more of the file:
# Hosts file
127.0.0.1 localhost
129.146.77.177 goober
Another alternative is to dial up with a PPP connection to your ISP whenever
you want to run networking programs. Fundamentally the experience of some
people has been that networking is not completely satisfactory on Windows95,
and is subject to sporadic unexplained failures. If this occurs to you, there
is little choice but to reboot and start again. Microsoft has several
network-related patches at its site
http://www.microsoft.com
7.3 If I call the InetAddress.getByName() method
with an IP-address-string argument, like
"192.168.0.1",I
get an UnknownHostException on some platforms, but not others. Code like
triggers the exception. Why?
-
A. This is a platform difference that arises
out of different semantics in the underlying network libraries, and is [said
to be, but subject to confirmation] fixed in JDK 1.1. On Solaris and Windows
NT, the IP address string only works for IP addresses that have an associated
hostname. On Linux and Windows 95, the IP address string works in all cases.
http://www.cdt.luth.se/~peppar/java/InetAddress/ has a workaround.
When InetAddress is instantiated with an IP address, a reverse DNS lookup
is done. If the IP address is not associated with a valid hostname, the
instantiation will fail. This is part of anti DNS-spoofing, and in JDK 1.1
works because the reverse lookup will not occur until the hostname is asked
for. So in JDK 1.1,
[Note: this info is still to be confirmed. Net gurus?]
7.4 I am using JDK 1.1.1 on Windows95, and when
I start jdb I get "Uncaught exception:
java.lang.UnsatisfiedLinkErrorno
winawt in shared library path"
The same program works OK using jdk1.1
-
A. It *sounds* like your java\bin directory is
not on your PATH and so the system can't find the winawt DLL.
But actually the problem is the version of Microsoft's Visual C++ that was
used to build the product. VC++ 4.2 incorrectly generates code that depends
on MSCVRT.DLL or in the case of java_g, MSVCRTD.DLL. These DLLs are not present
in (some versions of) Win95. To make things even more interesting, some versions
of Win95 (yes, there are at least four different ones...) ship with a broken
MSVCRT.DLL (and MSVCRTD.DLL?) that seems to work, only it doesn't, and after
a while it dies.
Sun linked the winawt_g.dll with VC++ 4.2, which wrongly brought in MSVCRTD.DLL,
the debug version of the VC++ runtime. You have to get that library from
somewhere (like, say, VC++)
in order to get jdb to run.
You'll hit this problem any time you try to debug 1.1.1 code with jdb on
a win95 system that doesn't have VC++ (or the MSVCRTD.DLL library from some
other source) installed. At least this is a problem you can solve without
waiting for the next release.
Some people say that the missing library has been seen at
http://cag-www.lcs.mit.edu/curl/Binaries/PC/
Others say you need to buy VC++ to get it.
7.5I
want to pass a class file to willing recipients who are using my applet.
Any ideas how?
-
A. You could use a trick: put your .class file(s)
in a .zip archive and use showDocument() on the URL. A person accessing this
will get a dialog box put up asking them about saving the file to their local
hard disk. You can see this in action and try it out yourself at:
7.6
How do you succeed with new URL(someURL) from behind a proxy server?
A. Tell the run time system what you are trying
to do, like this:
-
java -DproxySet=true -DproxyHost=proxy_host -DproxyPort=proxy_port
\MyJavaProgram
note proxyPort is optional and it defaults to 80.
Without this, you will see an exception like:
java.net.UnknownHostException
7.7What
is "swizzle", as in "Swizzle this object?"
-
A. It means serialize. To swizzle an object is
to recursively serialize or flatten composed objects.
7.8I
have been using the Serializing capabilities in 1.1 to save some objects
to disk. I added a new field to one of my objects that get serialized and
now deserializing my old data no longer works. I get this exception:
-
java.io.InvalidClassException: MacroData; Local class not compatible
A. You need to add a declaration such as
in the modified class. The actual value of this long is supplied by the
"serialver" utilitity suppied with the JDK. Any versions of a class other
than the first version require this static to be defined
in the class. This is how versioning is achieved.
7.9My
socket code looks good, but is broken!
7.10
How do I map
between IP address and hostname?
-
A. In Java 1.1 (earlier releases were buggy)
use:
7.11How
do I embed an anchor in a URL? Just putting it as part of the string in the
constructor doesn't work.
7.12RMI
seems to have stopped working for me in JDK 1.1. Why is this?
7.13
After
a number of RMI client to server connections (55 on my system), subsequent
RMI clients trying to connect fail. Why?
A. You are hitting the default limit of 64 open
file descriptors. Try increasing the limit in your OS.
In addition there is currently a practical RMI connection limit imposed by
the scalability of the VM and the performance of object serialization. In
JDK 1.2 this is addressed. The actual number of active clients you will be
able to support will depend on the workload mix you have (i.e. the number
of clients, how often they talk to the server, and how much work must be
done per call).
7.14 How do I POST to a CGI script from an applet?
A. Let's start by noting that this is more
troublesome than it might seem at first, and that GET is preferred. For an
untrusted applet, the CGI script can only be on the server that served the
applet. Then use code like this:
try { sock = new Socket(host, 80);
dock = new DataOutputStream(sock.getOutputStream());
dock.writeBytes("POST "+cgiloc+" HTTP/1.0\n");
dock.writeBytes("Content-type: text/html\n");
dock.writeBytes("Content-length: " +my_string.length() + "\n\n");
dock.writeBytes(my_string+"\n");
dock.close();
sock.close();
uresp = new URL(getDocumentBase(),"respond.html");
getAppletContext().showDocument(uresp); }
The my_string contains the data you want to POST to the CGI script. The string
should be encoded in the special way CGI expects. The class method
java.net.URLEncoder.encode(my_string) will do it. The CGI has to write its
output to respond.html so that it can be displayed by the browser. Even this
won't really work, because respond.html could be overwritten by a subsequent
request to the same CGI before the results of the first POST are read back.
To get an acceptable solution takes quite a lot of effort. There's a pretty
good explanation at
http://www.javaworld.com/javaworld/javatips/jw-javatip41.html In general
you should prefer GET to POST for CGI access from Java. As it says on the
Javaworld page, the answers to the question are really: you can't, don't
POST (use GET), use a bean, or cheat.
Finally, if you request a url via the URLConnection/HttpURLConnection, the
server sets the content type, and your applet can use
URLConnection.getContentType() to get the type. Alternatively, use
setRequestProperty to set it, like this:
url = new URL(cgiUrl);
urlc = url.openConnection();
urlc.setRequestProperty("Content-type", "application/x-www-form-urlencoded");
7.15 How do I get a URLConnection to work through
proxy firewalls?
A. The following code should do what you want.
// set up to use proxy
System.getProperties().put("proxySet", "true");
System.getProperties().put("proxyHost", "proxy.server.name");
System.getProperties().put("proxyPort", "80");
Q. But how do I know the name of the proxy server?
A. This code just tells you how you can get a URL connection to the outside.
Since it is your proxy server, you are expected to know the name of it. There
isn't any code that you can write that will allow arbitrary URL connections
to be initiated from outside the firewall. Think about it! If there were,
the firewall would not be doing its job.
7.16 How can I write CGI programs in Java?
A. CGI (the Common Gateway Interface for web
servers) is an API for writing programs that use the web as its user interface.
By far, the most popular language for this task is Perl, because of its powerful
text handling capabilities, and excellent resources available for making
the jobs of CGI programmers easier. CGI programs can be written in any language,
including Java.
Unfortunately, the interface between the web server and the CGI program uses
environment variables extensively. Use of environment variables has
always been deprecated in Java,
because of portability issues (not all systems have environment
variables). The way to get around this is to write a "wrapper" for the Java
program in a language that supports environment variables, and can then invoke
the Java program with the appropriate environment data passed in as Java
properties.
Because the Java runtime environment is not a lightweight process, it might
take a moment for the CGI program to get started before anything happens.
This is particularly true on operating systems, like NT, that have a large
overhead to spawning new processes.
In preference to using Java for CGI on the server, you might consider using
the Java servlet API in Netscape's Enterprise Server. This allows you to
develop server-side programs in Java without suffering the same performance
restrictions and other limitations of the CGI API. See
http://search.netscape.com/comprod/server_central/query/eval_guide/enterprise/advantage.html
for more details.
7.17 How can I write the "ping" program in Java?
A. You can't. Quoting from the Java Networking
FAQ, which you can find at
http://www.io.com/~maus/JavaNetworkingFAQ.html
Ping requires ICMP packets. These packets can only be created via a socket
of the SOCK_RAW type. Currently, Java only allows SOCK_STREAM (TCP) and
SOCK_DGRAM (UDP) sockets. It seems unlikely that this will be added very
soon, since many Unix versions only allow SOCK_RAW sockets to be created
by root, and winsock does not address ICMP packets (win32 includes an unsupported
and undocumented ICMP.DLL).
8.MULTI-MEDIA
8.1
Why won't my audio file play?
A. Java 1.1 and earlier releases use one audio
format exclusively. The audio file must be in .au format, recorded at 8 KHz,
mono, in mu-law encoding. If your audio clip is in a different format (e.g.,
.wav) or a different frequency it must be converted to the exact specifications
above before Java can play it. Support for .wav and other formats is part
of the Java Media Framework coming in JDK 1.2.
Search at www.yahoo.com for GoldWave for Win 95, sox for Unix and similar
conversion utilities for other systems. One conversion utility in Java is
at
http://saturn.math.uaa.alaska.edu/~hursha
The source of a Java class to play linear PCM .WAV files is at:
http://www.shef.ac.uk/~cs1mjp/Java/WhiteBoard/WavePlayer.html
It can be used in any Java application or applet.
8.2
Does Java support Animated GIFs?
A. Java 1.0.2 and earlier releases use GIF and
JPEG formats, and do not use the GIF89 animated GIF format. (An animated
GIF is one that contains successive frames of an image, so when they are
displayed in quick sequence the image appears to contain movement). When
you display an animated GIF in Java 1.0.2, you will just get the first frame.
There doesn't appear to be any easy way to get other frames from the image.
The advantage of an animated GIF file is that there is only one file to download,
and it is simple to do simple animations. The advantage of programmatic control
over individual frames is that you control the rate and order of displaying
them.
Here's a surprise: JDK 1.1 supports the animated display of animated GIFs.
For simple animations animated GIFs are a lot easier and lighter-weight than
coding an animation explicitly.
8.2.0
How do I create animated GIFs?
A. Use GIFanimator from ULead (said to be the
best) http://www.ulead.com, or GIF
Construction Set from Alchemy Mindworks
8.2.1How
do I prevent animated GIFs from flashing while displaying?
A. The problem is most likely that in your paint
method you have
g.drawImage(img, ix, iy, this);
You should change this to
g.drawImage(img, ix, iy, getBackground(), this);
This will change all the transparent regions of the image to the background
color before painting to the screen. If you paint transparent images directly
to the screen they flicker.
If that does not solve it then check that:
public boolean imageUpdate(Image img, int flags, int x,
int y, int width, int height) {
if ((flags & (FRAMEBITS|ALLBITS))!= 0) repaint();
return (flags & (ALLBITS|ABORT)) == 0;
}
2. update is
public void update(Graphics g) { paint(g); }
If you have a background Image behind the partly transparent animated GIF
you will have to double buffer. You can crop the backgound image so you won't
have to double buffer the full app and waste too much memory.
8.3
Does Java support transparent GIFs?
A. GIF89a images with a transparent background
show up as transparent without further filtering. This has been supported
from 1.0 on. Java correctly displays both animated GIFs and transparent GIFs.
Even better, you can fill the transparent pixels with a color (so they appear
non-transparent in Java). Just pass the fill color explicitly:
drawImage(img, x, y, w, h, fillcolor, this);
Further, you can filter the pixels of an Image to turn any bits you wish
transparent. However, the most you can do is reveal what is underneath the
image. You cannot reveal what is underneath the applet (i.e. on the browser
itself). By default applets have a plain grey background.
8.4How
do I play video in Java?
-
A. Use the Java Media Framework Player API
The spec can be found at
Intel has released a SDK for the Java Media Framework Player API. The SDK
is for Windows 95 and Windows NT For more information, see
SGI has released an implementation of JMF for IRIX:
8.5How
can I play *.au files from an application?
-
A. A new static method was introduced in JDK1.2
in the class Applet:
public final static AudioClip newAudioClip(URL url) {
The method turns a URL (pointing to an audio file) into an AudioClip. Since
the method is static it can be used without any objects of class Applet needing
to exist, e.g. in an application. Once you have an AudioClip, you can play
it with
MyAudioClip.play();
The Java Media Framework provides a richer API for playing sounds in an
application.
For code prior to JDK 1.2, you can use the AudioClip or AudioPlayer class
in sun.audio
http://www.javaworld.com/javaworld/javatips/jw-javatip24.html If you
do this your code is no longer 100% pure Java, as it relies on a vendor library.
b. Use the new Java Media Framework API, allowing a wide range of
video and audio formats to be played back. See previous question for
implementations of this.
8.6How
do I read in an image file, in an application?
8.7 When I initialize a
component,I
call MyComponent.getImage() to get its image. createImage() returns null!
I know the image works elsewhere. What's wrong?
-
A. A peer component needs to exist for your component
before you can get its image. This is done by the method addNotify() (surely
one of the most poorly named methods in all Java -- it doesn't mean "add
a Notify". It means "Notify that the Component has been added to a Container".
It tells the system, "you need to create the peer for this Component now").
addNotify will be called for you when you add your component to a container.
Javasoft recommends against explicit calling (and, presumably overriding
also) of addNotify() by applications programmers - this is
especially important
in these days of Lightweight components that do not have peers. If you have
any code that requires peer resources, move it into a thread that is
started from a conditional branch
of the paint() or update() method.
A common reason for seeming to require
peer resources in a constructor, or alternatively in the getPreferredSize()
method, (which is also usually before the peer is created) is to measure
the area required for your window, in terms of font and image sizes.
Font sizes may be obtained by applying
Toolkit.getDefaultToolkit().getFontMetrics() to the required font.
This does
not require a peer. Image sizes may be obtained by waiting for the relevant
Image to load from the ImageProducer by using an ImageObserver, or a
MediaTracker
(see 8.15), also without requiring a peer. See 15.4 for more discussion of
this problem.
If you override addNotify(), don't forget to call super.addNotify() in your
overriding version.
8.8 How can
Iforce
a reload a fresh version of an image into my applet? My image file is changed
periodically, and I want the applet to go and retrieve it, not cache it.
-
A. You need to turn off caching for the URL.
Note: some programmers have reported that it caches anyway, even if that
do this. That is a browser bug.
One programmer reported that even after turning off caching and calling
image.flush() before getImage(..), he was still seeing the same picture even
though it had been changed on the server.
He worked out a solution: access the image via a cgi script that returned
a URL. This redirects the browser, and he put in an Expires: header as well
to force the reload. Painful and laborious, but it got the result.
8.9 How can
Isave an Image
file to disk in jpg or gif format?
-
A. A number of people have written utilities
to do that. One of them was available at the same place as this FAQ, but
has now been removed since it was pointed out that Unisys asserts that
it has license control even over freeware GIF writers.
You may find the code on other sites, in the meantime use other image
formats like JPEG that are not encumbered.
Jef Poskanzer has written an abstract ImageEncoder class and implemented
it for GIFs and PPMs. Those are at
http://www.acme.com/java/software/
A non-java solution is to use the standard IJG 'cjpeg' utility. It supports
GIF, PPM, BMP, PNG and Targa input files.
Hong Shi wrote a PPM to JPEG convertor.
http://www.ctr.columbia.edu/~hshi/report6880.htm
Sean Breslin has written a program that compresses a Java Image into a JPEG
file. http://www.afu.com/jpeg.txt
Florian Raemy has written a program that encodes a JPEG then decodes it again.
http://lcavwww.epfl.ch/demos/jpeg.html This is a proof of concept, not
industrial strength code that meets the JFIF (JPEG File Interchange Format)
standard.
8.10 What causes this problem?
-
$ appletviewer m.html
Premature end of JPEG file
sun.awt.image.ImageFormatException: JPEG datastream contains no image
at sun.awt.image.JPEGImageDecoder.produceImage(JPEGImageDecoder.java:133)
at sun.awt.image.InputStreamImageSource.doFetch(InputStreamImageSource.java:215)
at sun.awt.image.ImageFetcher.run(ImageFetcher.java:98)
A. There's a known bug in early releases of the
JDK which can cause the above failure when reading a JPEG across a slow
connection. The failure only occurs if the JPEG contains a large application
data block (APPn marker) --- the problem is that the JPEG decoder is trying
to skip over the APPn and failing if not all of the APPn has been received
yet. The quoted error message is only one of several possible complaints,
but they all stem from the same root.
Photoshop is the most common source of JPEGs containing oversize APPn blocks.
In particular, if you allow Photoshop 4 to save a thumbnail (preview) in
a JPEG, the thumbnail plumps up Photoshop's private APPn marker to several
K, which is usually enough to cause this problem.
There are several possible workarounds:
-
1. Get a newer JDK --- this problem is said to be fixed in 1.1. (If you are
putting images up on the Web, this isn't much of a solution, because you
can't assume visitors to your site have an up-to-date Java installation.)
2. When making JPEGs for Web use from Photoshop, make sure you have turned
off the "save thumbnails" preference. (This is a good idea quite aside from
bug workarounds, because the thumbnail is just a waste of download time as
far as a Web browser is
concerned.) You might still have a problem if you've got verbose comments
or lots of paths being saved into the file, but 99% of the time, getting
rid of the thumbnail will make Photoshop's APPn small enough to not trigger
the Java bug.
3. Use a tool such as 'jpegtran' (from the Independent JPEG Group) to strip
out the Photoshop APPn entirely without any loss of image quality. Recommended
answer for the compulsive byte-trimmer.
4. (Last resort) Load and resave the image in a different image editor that
won't insert any APPn or other overhead data. This implies a JPEG generational
loss, so I don't recommend it if you are picky about image quality.
Any large overhead marker will cause the same problem; 4K of comment text,
say, in a COM marker. So Photoshop is not the only source of tickling this
bug.
8.11How
can I convert between GIF and JPEG formats?
-
A. In a word: don't.
There's hardly any overlap between the set of images that JPEG works well
on and the set that GIF works well on. Sometimes, with enough care, you can
get an acceptable conversion ... but most of the time gif<->jpeg conversion
will just turn your image to mush. It's better to pick the right format in
the first place.
If you're determined to convert formats anyway, try the GBM (Generalized
Bitmap Module). The package is GNU licensed, in C and is very good. Find
it at
http://www.interalpha.net/customer/nyangau/
GBM does a good job converting to JPG, and 'lossiness' is adjustable to 0%.
It also converts to/from about 20 other formats, does cropping, sizing, color
mapping, gamma correction, halftoning, everything you could want. GBM source
doesn't support jpeg directly, but utilizes jpg source from IJG called jpeg-6a
and found at ftp://sun2.urz.uni-heidelberg.de/pub/simtel/graphics/jpegsr6a.zip
For more info see the JPEG FAQ at
http://www.faqs.org/faqs/jpeg-faq/
8.12If
you have an InputStream (rather than a file) that contains an Image, how
can you display it?
-
A: Using this method, and some adroit shuffling.
-
Toolkit.getImage(URL url)
Create a thread that pretends to be an http server. Make it listen to some
port (8765 for example) for incoming requests. When the thread gets a request,
it should simply whisk up the appropriate http headers and follow it by the
InputStream. Thus the component that has the input stream and wants to do
the getImage(url) can now invoke:
Toolkit.getImage("localhost:8765/")
The thread will act as a stream-to-url adapter, and send back the data It
saves you from having to read 200K of JPEG data before you can begin drawing
anything.
8.13 How can I record sounds in Java?
-
A. The Java Media Framework will eventually support
this, but it does not yet. JMF 1.0.1 only supports playback.
JMF 1.0.1 is bundled with JDK 1.2, and available as a separate download for
JDK 1.1 and Netscape Communicator 4 with Java 1.1. In the meantime, there
is a package for Win95/NT available at
http://www.scrawl.com/store/
It supports 8, 16-bit, stereo, mono, 11025, 22050, 44100 Hz record/play,
load/save .WAV files. You could also interface to native code for your platform.
8.14 Does Java have any built-in support for
displaying HTML?
-
A. JDK 1.1 supports rendering HTML using the
unbundled JFC 1.1 package known as Swing. The Swing package is bundled in
JDK 1.2. It has an elementary (graphics, tables, text) HTML bean that is
good enough for simple rendering (help files, email, etc). Other alternatives
include:
8.15 I loaded an Image file from a JPEG/GIF file
using the Toolkit/Applet.createImage(URL/String) method, and (the height
and width are -1 / it will not draw to the screen). What is wrong?
-
A. The behaviour of the AWT on creating images
in this way is to
-
Do nothing at all.
-
When the image is first drawn using Component.drawImage(), or its size is
requested, the image begins to load in a different Thread.
-
As the image loads, the ImageObserver specified in the drawImage()/getHeight()
call is periodically notified of the loading status of the image, by calls
to its imageUpdate() method.
-
In the case of Component.drawImage() call, the default behaviour of
Component.imageUpdate() is to schedule *another* repaint() call when the
image has fully loaded.
This means that, in particular the following
code will not work:
class MyComponent extends Component {
...
public void paint(Graphics g) {
ImageFilter cropper=new CropImageFilter(0,0,16,16);
Image cropped_image=createImage(new
FilteredImageSource(image.getSource(),cropper));
g.drawImage(image,10,400,this); // this line works
g.drawImage(cropped_image,400,15,this); // this line doesn't -
}
}
since the cropped_image will not be created in time to be painted, and when
it is finally created, another call will be scheduled to paint, which will
try to draw another one, etc.
(Note also that creating objects like this in paint() methods is generally
a very poor idea in Java, since they are called very frequently, and you
will strongly offend the garbage collector. See 15.4 for examples of how
to reuse objects.)
In order to get round this problem, you may i) add all such Images to a
MediaTracker, and call the waitForAll() method. ii) implement your own
ImageObserver interface, and wait for the imageUpdate() method to be called
with the ALLBITS/FRAMEBITS value. i) is easier, but ii) is recommended, since
there are reports of MediaTracker not working in some environments. See also
8.12.
9.SECURITY
-
9.1What
is a "trusted applet"?
9.2 What is the story
with
Java and viruses? What is the blackwidow virus?
-
A. Java was designed with security in mind. The
security features make it very difficult, probably impossible, to attach
a virus (self- copying code) to a Java applet. As far as is known, there
has never been a Java virus.
There has been mention of a "Java virus" called "BlackWidow" in the media
(it was mentioned in Unigram in late 1996, and obliquely on the RISKS newsletter
in February 1997). A request to the editor of Unigram for more information
brought the answer that there was no more information, it was just a report
of a rumor. As far as is known, this story exists *only* as rumors reported
on by the press. There is no actual Java virus or blackwidow virus (there
was a legitimate commercial product of that name, since renamed).
In spring 1998 there were press reports of a "Java applet" called
White Ghost.
It turns out that it relies on security flaws in ActiveX, and
the only susceptible systems are Microsoft's Active Desktop. If
anyone has a URL for a copy of this code, and an analysis of it,
please contact the FAQ author.
If anyone
has more concrete information about a virus that can attack a Java applet
(again, this is thought to be impossible), please would they contact the
FAQ author.
9.3 Why do I get the warning string at the bottom
of popup
windows"Unsigned Java
Applet Window" in my applets?
A. This is a security feature, to make certain
that users can always tell that a window asking for their password and credit
card details (or whatever) is from an applet. There should be no way for
an untrusted applet to work around this message. See also the answer to 4.1.7.
9.4Where
can I find crypto libraries for Java?
-
A. Cryptographic libraries are not part of the
Java release because US Government policy classifies strong cryptography
under the same rules as munitions. Its export is regulated under the
International Traffic in Arms Regulations. Many people regard this as a
Kafka-esque (and futile) attempt to stem the use of cryptography inside the
US.
A comprehensive and free crypto library (called Cryptix) is at
Another crypto library for Java is at
Blowfish, CRC16, CRC32, DES, DES3, IDEA, RC4, ROT13 (can they really call
that "crypto"?), and more.
One commercial Java encryption source (from Ireland) is
A complete crypto API for Java (with HTML documentation) at:
The library provides comprehensive and complete range of crypto library and
functions covering DES, 3DES, IDEA, Blowfish ...and RSA, DH, DSA and PGP
access to Java programmers. The crypto functions are based on the C cryptlib,
by Peter Gutmann. It would be illegal to export this under current US government
rules, but the author of the code is outside the US, and not subject to US
export regulations. Download it today before it becomes illegal.
Also, early access to Sun's Java Cryptography Extension (JCE) is available
for JDK 1.1 at:
(This may not be exported outside the USA and Canada).
An actual port of PGP v2.6.3i to Java is at
http://tassun.math.nsc.ru
9.5How
do I find out what these terms mean?
A. Read Bruce Schneier's excellent book "Applied
Cryptography 2nd Ed." for more info on what these terms mean. Read David
Kahn's excellent (if exhaustive) book "The Codebreakers" for more info on
the history and background of encryption.
9.6
Where is Javasofts Security FAQ?
10.Java IDIOMS
10.1
How do I convert a String to an int?
A. There are several ways. The most straightforward
is:
String mystring = numString.trim();
int i = Integer.parseInt(myString);
long l = Long.parseLong(myString)
or
String mystring = numString.trim();
i = Integer.parseInt(myString,myIntRadix);
Note 1:
there is a gotcha with parseInt: it will throw a NumberFormatException
for String values in the range "80000000" to "ffffffff". You might
expect it to interpret them as negative, but it does not.
The values have to be "-80000000" .. "-ffffffff" to be properly recognized
as negative values. This is true for all radixes. According to Bug Parade
bug report 4068580, the proper way to generate negative-valued hex Strings
for eventual use by parseInt() is with Integer.toString( i, 16 ) ."
Once that
high "sign bit" is on, without the accompanying - character, parseInt()
says "too big".
Note 2: there are similar methods for Byte, Short, and Long.
Use myString.trim() to get rid of unwanted spaces before the conversion.
Some of the parse methods can cope with spaces, others can't. They were
written by two different people.
int i = Integer.valueOf(my_str).intValue();
also works but involves the creation of an extra object. Note: use this for
floating point values, as there are no parseDouble or parseFloat methods.
float f = Float.valueOf(my_str).floatValue();
double d = Double.valueOf(my_str).doubleValue();
10.2
How do I convert an int to a string?
A. String s = String.valueOf(i);
String s = Integer.toString(i);
or
String s = Integer.toString(i, radix);
or
String s = "" + i; // briefer but may result in extra object allocation.
Note: there are similar classes for Double, Float, Long, etc.
10.3
How do I print the hex value of an int?
A.You can print the hex equivalent of an int with:
int i = 0xf1;
System.out.println("i is hex " + Integer.toHexString(i) );
10.4 How can
you send a function
pointer as an argument?
A. Simple answer: use a "callback". Make the
parameter an interface and pass an argument instance that implements that
interface.
public interface CallShow { public void Show( ); }
public class ShowOff implements CallShow {
public void Show( ) { .... }
public class ShowMe implements CallShow {
public void Show( ) { .... }
public class UseShow { CallShow callthis;
UseShow( CallShow withthis ) { callthis = withthis; }
void ReadyToShow( ) { callthis.Show( ); }
// in some other class that uses all this stuff:
UseShow use_1 = new UseShow( new ShowOff() );
UseShow use_2 = new UseShow( new ShowMe() );
and then the ReadyToShow() method on use_1 or use_2 will call the appropriate
method, as if you had stored a pointer to the method.
10.5How
do I execute a command from Java?
A. Use Runtime.getRuntime().exec( myCommandString
) Where myCommandString is something like "/full/pathname/command"
10.6
How do I do I/O redirection in Java using exec() ?
A. This solution works on Unix platforms using
either JDK 1.0.2, or JDK 1.1. The trick is to use an array of Strings for
the command line:
String[] command = {"/bin/sh", "-c", "/bin/ls > out.dat"};
If you don't do this, and simply use a single string, the shell will see
the -c and /bin/ls and ignore everything else after that. It only expects
a single argument after the -c.
import java.io.*;
import java.util.*;
class IoRedirect {
public static void main(String Argv[]) {
try {
String[] command = {"/bin/sh", "-c", "/bin/ls > out.dat"};
Process p = Runtime.getRuntime().exec(command);
p.waitFor();
System.out.println("return code: "+ p.exitValue());
} catch (IOException e) {
System.err.println("IO error: " + e);
} catch (InterruptedException e1) {
System.err.println("Exception: " + e1.getMessage());
}
}
}
10.7 So why can't I exec common DOS commands
this way (as in 10.6)?
10.8
OK,how
do I read the output of a command?
A. As above (10.5, 10.6), adjusted like this:
BufferedReader pOut= new BufferedReader(
new InputStreamReader(p.getInputStream()));
try {
String s = pOut.readLine();
while (s != null) {
System.out.println(s);
s = pOut.readLine();
}
} catch (IOException e) { }
Another possibility is to read chunks of whatever length as they come in:
...
p = r.exec(cmd);
InputStream is = p.getInputStream();
int len;
byte buf[] = new byte[1000];
try {
while( (len = is.read(buf)) != -1 ) {
String str = new String(buf,0,0,len);
System.out.println( "Process out: " + str );
}
} catch( java.io.EOFException eof ) { ...
} catch( java.io.IOException ioe ) { ... }
10.9 How do
Icompile code
which has a cyclic dependency, i.e class pkg1.X contains a reference to class
pkg2.Y ?
A. You throw both classes at the compiler at
the same time.
javac pkg1/X.java pkg2/Y.java
10.10 How can
I store
the errors from the javac compiler in a DOS file? javac foo.java > errorfile
doesn't work.
A. javac writes errors to stderr, so on NT use:
javac myfile.java 2> errors.dat
On Win95, this doesn't work (as command.com is very poor software), so you
have to use the javac error redirection mechanism:
javac -J-Djavac.pipe.output=true myfile.java > errors.txt
You typically use this when a compilation produces a lot of error
messages, and they scroll off the DOS window before you can read them.
Alternatively, you can get a scollbar to appear on a DOS window by
changing the properties with the "Layout" tab. Change the Screen
Buffer Size Height: to some multiple > 1 of the Window Size Height.
E.g. use a buffer height of 100 and screen height of 25 (the default).
This will give you three buffers of scroll "history."
10.11How
can I pretty-print Java source?
10.12
What
is the point of creating the temporary reference to this.layoutMgr? This
code is from the 1.0 AWT, and the programmer was probably pretty skilled.
public synchronized void layout() {
LayoutManager layoutMgr = this.layoutMgr;
if (layoutMgr != null) {
layoutMgr.layoutContainer(this);
}
}
A.
The code makes a local copy of a global variable
for one or both of two reasons.
The first reason is that accessing local variables can be faster than accessing
(non final) member variables It's good for loops or where there are many
references in the source.
The second reason is so that even if other threads update the global,
this.layoutMgr = someOtherLayoutMgr;
this method will still have a pointer to the original layoutMgr.
If the local variable were omitted, and another thread used the setLayout()
method to change layoutMgr to null between when the layout method checked
for null and when it invoked layoutMgr's layoutContainer method, a
NullPointerException would result.
Note that the synchronized keyword on the layout method doesn't help any,
since setLayout (which could make such a dire change) isn't synchronized.
Synchronized methods only lock out other synchronized methods on this object.
(The unhelpful synchronized keyword on the layout method is gone in JDK 1.1.)
There are two alternative solutions. One would be to make setLayout synchronized
and make layoutMgr private, so that it can't be set other ways. This provides
a stronger form of thread serialization, in that you would never be able
to see an old layout manager being used after it had been replaced. However,
it is slower. Another option that provides no increase in thread serialization
over the original would be to catch the NullPointerException.
Threads programming is hard! This idiom was probably put in place by someone
who got really bitten by this in the past.
10.13What
is the difference between "a & b" and "a && b" ?
-
A. "a & b" takes two boolean operands, or
two integer operands. It always evaluates both operands. For booleans, it
ANDs both operands together producing a boolean result. For integer types,
it bitwise ANDs both operands together, producing a result that is the promoted
type of the operands (i.e. long, or int). "|" is the corresponding bitwise
OR operation. "^" is the corresponding bitwise XOR operation.
"a && b" is a "conditional AND" which only takes boolean operands.
It always avoids evaluating its second operand if possible. If a is evaluated
to false, the AND result must be "false" and the b operand is not evaluated.
This is sometimes called "short-circuited" evaluation. "||" is the corresponding
short-circuited OR operation.
Possible mnemonic: The longer operators "&&" or "||" try to shorten
themselves by not evaluating the second operator if they can.
10.14If
I create a thread, and then null out the reference to it, what happens to
the thread? Does it get interrupted or what?
A. The code looks like this:
Thread t = new Thread( my_runnable_obj );
t.start();
...
t = null; // what happens to the thread?
The answer is that you may no longer have a reference to the thread, but
the JVM still does. Once a thread is started, and as long as it keeps running,
it is a root object. Root objects are the starting points for "things in
use" that the garbage collector uses.
10.15
Howdo
I calculate the number of days between two dates?
A. There is no API for this (there should be),
but you can calculate it by hand, like this:
Calendar earlierDate = new GregorianCalendar();
Calendar laterDate = new GregorianCalendar();
earlierDate.set(1997, 1, 1, 0, 0, 0); // Jan 01, 1997
laterDate.set(1998, 1, 1, 0, 0, 0); // Jan 01, 1998
// the first getTime() returns a Date, the second takes
// that Date object and returns millesecs since 1/1/70.
// The API has misleading and horrible naming here, sorry.
long duration = laterDate.getTime().getTime()
- earlierDate.getTime().getTime();
long nDays = duration / (24 * 60 * 60 * 1000);
System.out.println("difference in days: " + nDays);
10.16
How can
a Java program determine the level of JDK support given by the underlying
VM? I.e. is it running in a JDK 1.0.2 or 1.1 VM?
A. Look at the java.version system property with:
String ver = System.getProperty("java.version");
There isn't a lot of standardization on the string contents however. Another
possibility is to try { ... } a library method that didn't exist in JDK 1.0.2,
such as SomeComponent.setSize(), and catch the exception. If you got an
exception, it's a 1.0 implementation. In 1.0 setSize() was known as resize().
10.17
How can I set a system property?
System.setProperty( "property", "new value" );
Until then, you can get all the properties, and set just the
one you want with code like this:
System.getProperties().put("property", "new value" );
10.18
How can I clone using serialization?
A.
Look at the code below, submitted by expert programmer John Dumas.
It uses serialization to write an object into a byte array, and
reads it back to reconstitute a fresh copy. This is a clever hack!
import java.io.ByteArrayOutputStream;
import java.io.ByteArrayInputStream;
import java.io.ObjectOutputStream;
import java.io.ObjectInputStream;
public class Cloner
{
private Cloner() {}
public static Object cloneObject(Object o) throws Exception
{
ByteArrayOutputStream bOut = new ByteArrayOutputStream();
ObjectOutputStream out = new ObjectOutputStream(bOut);
out.writeObject(o);
ByteArrayInputStream bIn = new ByteArrayInputStream(bOut.toByteArray());
ObjectInputStream in = new ObjectInputStream(bIn);
return(in.readObject());
}
public static void main(String args[]) throws Exception
{
java.util.Vector v = new java.util.Vector();
v.addElement(new StringBuffer("Hello"));
java.util.Vector vClone = (java.util.Vector)Cloner.cloneObject(v);
// Changing the StringBuffer int the cloned vector has no effect
// on the original StringBuffer object -- demonstrating that we have
// indeed done a deep copy
((StringBuffer)vClone.elementAt(0)).append(" world");
StringBuffer sb = (StringBuffer)v.elementAt(0);
System.out.println(sb.toString());
sb = (StringBuffer)vClone.elementAt(0);
System.out.println(sb.toString());
int array[] = { 1, 2, 3, 4, 5 };
int arrayClone[] = (int [])Cloner.cloneObject(array);
// Again, changes to an element in the cloned array do not have any
// effect on the original
arrayClone[0]++;
System.out.println(array[0]);
System.out.println(arrayClone[0]);
}
}
The main() routine is just a driver. All the cleverness is in the
very brief cloneObject(). It does a "deep" clone, which is what
you usually want (though Java gives you a "shallow" clone by default).
11.FOR
C and C++ AFFICIONADOS
-
11.1How
do I translate C/C++ into Java or vice-versa?
A. In general it is not simple to translate C/C++ into Java, as Java
lacks the arbitrary pointer arithmetic of those languages. If your C code
does not use pointer arithmetic, automatic translation gets a lot simpler.
Try these URLs:
Going the other way there are currently three freely-available tools to translate
Java into C. It seems that these have been done for hacking value, rather
than practical purposes.
-
j2c from Japan,
-
Toba from the Sumatra research project, translates 1.0.2 .class files into
.c source code
-
JCC from Nik Shaylor.
None of them support the AWT yet, and both j2c and JCC have additional
restrictions.
There's a product to convert Visual Basic to Java. Details at
This program dumps info about the class file:
Chuck McManis was one of Sun's original Java implementors.
11.2
How are finalizers different from C++ destructors?
A. Java objects are not explicitly deleted and
do not have destructors. Instead they are implicitly garbage collected when
the JVM realises your program can no longer access them. Typically this
technology is _not_ based on reference counting and _will_ cope with circular
references.
Every object has a routine called finalize() which will be called before
the object is collected. This is Java's nearest equivalent to C++'s destructor.
However, it is not a good idea to rely on finalization for the timely freeing
of resources.
This is because garbage collection and hence finalization may be arbitrarily
delayed, and may never happen at all if the program terminates before it
runs out of memory. You should instead provide your objects with methods
similar to Graphics.dispose() to free resources, and call the
dispose() method explicitly when you have finished using them - typically
within the "finally" clause of a "try/catch" block. You may then call your
dispose() method from within your finalize() method as a last-ditch attempt
to free the resource if someone forgets.
Alas, all this means the C++ idiom of "object construction is resource
aquisition" does not translate well to Java. However, note that 90% of
destructors in C++ are there to free memory, and the GC means you don't need
to do that in Java. As well as fixing an important source of bugs, the GC
is essential to Java's security model; without it you could forge object
references by preserving the reference after the object has been deleted.
If your program appears to be crashing due to running out of some system
resource (like File, Window or Graphics handles), it probably because the
system is running out of handles before it has run out of memory. Check that
you have called the dispose() method (or equivalent) on every object that
uses system resources. You can help the GC a little bit more by explicitly
NULLing out references that you've finished with.
11.3What's
the Java equivalent of sizeof()?
-
A: There isn't one. sizeof() in C and C++ is
used in three main places:
-
1) To check on the size of a primitive type. In Java, the sizes of primitive
types are fixed in the language specification (a short is _always_ 16 bits;
an int is _always_ 32 bits, etc), so this is no longer necessary.
2) In memory allocation (i.e. malloc (32 * (sizeof(int));) In Java you always
allocate a specific type of object, rather than a block of raw memory that
you will fill as you like. The system always knows the size of the kind of
objects you are allocating. So sizeof is not needed.
3) in pointer arithmetic (i.e. p += sizeof (int)) Pointer arithmetic of this
type is not allowed in Java, so this isn't necessary, either.
For all these reasons, there is no need for a Java sizeof() operator. Some
people have suggested that you can find out the size of an object by having
the object serialize itself to a ByteArrayOutputStream, and looking at the
bytearray.length.
That won't work because a lot of additional data is written when an object
is serialized. The additional data includes a description of the class, any
objects referenced by the serialized object, null references (written as
a single byte), etc. If you write another instance of the same class, the
amount of data written can differ dramatically. And if you serialize the
same object again, it isn't written at all -- even if its data fields have
changed! Instead, a one byte token and a four byte "sequence number" that
refer to the first writing are output. Using Object Serialization to determine
the size of an object does not (except by coincidence) give the right answer.
11.4Does
Java have the equivalent of "const" arguments in C and C++?
11.5Are
there any hacks around this?
11.6How
can I write C/C++ style assertions in Java?
-
A. The two classes shown below provide an assertion
facility in Java. Set Assert.enabled to true to enable the assertions, and
to false to disable assertions in production code. The AssertionException
is not meant to be caught--instead, let it print a trace. Since the exception
is not meant to be caught, we just extend Error instead of RuntimeException.
As with RuntimeException, a method does not need to declare that it throws
Error. In addition programmers are less likely to write "catch(Error) ..."
than "catch(RuntimeException)".
With a good optimizing compiler there will be no run time overhead for many
uses of these assertions when Assert.enabled is set to false. However, if
the condition in the assertion may have side effects, the condition code
cannot be optimized away. For example, in the assertion
the call to size() cannot be optimized away unless the compiler can see that
the call has no side effects. C and C++ use the preprocessor to guarantee
that assertions will never cause overhead in production code. Without a
preprocessor, it seems the best we can do in Java is to write
In this case, when Assert.enabled is false, the method call can always be
optimized away, even if it has side effects. However, an opposing view holds
that even this might not work in the face of java's late binding. If the
Assert class is in a package, and if the computer the applet/application
is running on has a different version with Assert.enabled being true, then
it's possible that assertions *should* be enabled in that case. Therefore,
the compiler can't assume that enabled is *false* at runtime just because
it *is* false in the compilation environment. This may be thought perverse,
but it could happen.
If a language lawyer wants to weigh in on this theory, please go ahead.
11.7How
do I do stuff like scanf and sscanf in C/C++? And how do I do stuff like
sprintf, e.g.
-
float x = 12345.6789;
printf("%6.3f/n", x);
A. You can break a string like "5 loaves 2 fishes"
into its parts by using java.util.StringTokenizer. This is the Java equivalent
of sscanf().
StreamTokenizer does a similar thing on a file or any stream (i.e, what scanf()
and fscanf() do in C).
To do formatted character output, create a format string, and then use that
to format your binary value, e.g.
gives:
There are lots of different characters you can feed to the DecimalFormat
constructor, not just "0".
See $JAVAHOME/src/java/text/DecimalFormat.java source
for details.
11.8
What is the Java equivalent of C++ friend"?
A: The keyword "friend" in C++ is a hack to allow
a piece of code to access the private member declarations of another class.
In Java, you would do this by labelling, not the friend, but the private
members. Instead of making them private, make them either
protected or package (no keyword) or public.
The four different Java protection levels are: private, package,
protected, and public.
- private members can only be accessed by the containing class
and internal classes.
- package (specified by omitting other keywords) is the default level
of protection; members are accessible from any class within the
package of the containing class.
- protected is package-level-access plus access to sub-classes of
the containing class. So "protected" is less protected than the
default.
- public fields in public classes are accessible from all classes.
11.9Does
anything like the C++ Standard Template Library exist for Java ?
-
A: Yes, only it's better and simpler to use in
Java. It's called the Java Generic Library. This library (JGL) is freely
downloadable from
http://www.objectspace.com/
It includes about a dozen nice data structures (including sets and bags)
and algorithms like unions, searching, and sorting.
[Some Java vendors are bundling it with their next release]
11.10 to be continued
12.FURTHER
RESOURCES
-
12.1 Useful URLS
12.2Newsgroups
-
These are the Java newsgroups since the reorganization that I
arranged in April 1997
comp.lang.java.help
simple programming and setup questions
comp.lang.java.announce
(moderated) announcements
comp.lang.java.advocacy
for arguments: no it isn't, yes it is
comp.lang.java.programmer
programming in Java
comp.lang.java.security
security issues
comp.lang.java.machine
JVM and native interfaces
comp.lang.java.databases
JDBC,ODBC, java access to DBs.
comp.lang.java.softwaretools
IDES, editors, compilers, tools, etc
comp.lang.java.gui
AWT, JFC, AFC, WFC, etc etc
comp.lang.java.beans
Software components in Java
comp.lang.java.corba
(newsgroup added Dec 3 1997) interaction between Java and CORBA.
Please make an effort to post only to the single most appropriate group.
As with the other language groups on Usenet (comp.lang.c, comp.lang.c++,
etc) questions about products from specific vendors that only work on one
specific platform are best posted to other newsgroups. For example, questions
about ActiveX belong in comp.os.ms-windows.programmer.ole, not the Java groups.
For questions about J/Direct try the group microsoft.public.java.visualj++
or microsoft.public.java.sdk. These are available from Microsoft's news server
msnews.microsoft.com. Many IPSs don't provide direct access to these groups
(or those of other companies) but you can access them by setting the NNTPSERVER
environment variable to point to the newserver.
12.3Are
there any commercial/shareware/free Java libraries?
-
A. Take a look at the Java Collection Framework,
a group of classes that are part of Java 1.2. These classes implement
general-purpose data structures, and they will become widely used.
The documentation for JDK 1.2 explains that the Collection Framework defines
three kinds of thing:
-
Standard interfaces representing data structures of various kinds for you
to implement. Since these are interfaces, you can use them in your code before
you have implemented them.
-
Partial implementations of those interfaces, saving you some work.
-
Complete implementations, ready to use for data in your programs.
The standard interfaces are Collection, Set, List and Map, plus the more
specialised SortedSet and SortedMap. Lists have duplicate elements whereas
Sets do not. Finer distinctions such as immutability are defined in the
implementor classes, enforced by throwing runtime exceptions. See the JDK1.2
documentation for a full discussion. Also see
http://byrden.com/java/Tree/index.shtml for a description of extending
the collection framework, and a freeware class implementing trees.
For more about sorting prior to JDK 1.2, look at the class SortDemo in the
demo directory of the JDK. Alternatively, use one of the several classic
sorts available from Roedy Green. They are supplied free with heavily commented
Java source code.
See "QuickSort",
"HeapSort" and
"RadixSort" in the
Java glossary at
http://oberon.ark.com/~roedy/index.html .
Also, try the Java Generic Library. This library (JGL) is freely downloadable
from http://www.objectspace.com/
Also Visual Engineering has JChart at:
http://www.ve.com. No licensing fees.
Visual Numerics has its Java Numeric Library available for download at
http://www.vni.com/products/wpd/jnl/jnl_1_0.html. They offer the JNL as a
proposed standard library for numerical functions missing from Java.
12.4 Why doesn't
somebody write a shell in Java? Then they could use it on all platforms!
12.5
Are there any URLs for other libraries?
12.6
Are there any URLs for regular expression handlers in Java?
A.
http://www.oroinc.com/products/OROMatcher.html
This comes from ORO Inc, is free, and handles Perl5 style exprs.
And don't forget to check out Lava -- a set of Java classes designed to support
programmers who develop console-mode applications and/or C programmers who
are converting to Java. The first release of Lava has printf and other text
formatting, encryption, parsing and miscellaneous I/O. Lava can be downloaded
from
http://www.newbie.net/sharky/lava/
Also consider the Java version of the Unix find command. It offers
Regex filename matching, mindepth, maxdepth,
symlink follow / no follow, file type matching all cross-platform.
The package is at
http://www.geocities.com/~shecter/java.html
12.7
Are there any installers for Java? Preferably platform-neutral ones.
A.
There are several possibilities.
13. MOST COMMON JAVA
NOVICE PROBLEMS
13.1 I can't seem to
get my classpath environment variable set correctly.
A. Classpath was a hack from the early releases of Java. The CLASSPATH
tells the Java Virtual Machine and other Java applications where to find
the class libraries, such as the runtime library classes.zip file.
Don't set your CLASSPATH at all when starting out. If you have it set from
a previous installation of JDK 1.0.2, unset it. The system knows where to
look for libraries and your Java class files. By default, it searches your
current directory. If you are creating your own packages, you might want
to set CLASSPATH so it looks in the right places to find them.
13.2 How do
I do keyboard (interactive) I/O in Java?
-
A. Interactive I/O in Java is very poorly supported. Programmers must
piece together several library classes in non-obvious ways to get the required
functionality. See the answer to question 6.1.
13.3 How do I
do file I/O in an applet?
-
A. In general, an applet can read files on the server, but not write
them, and has no access to the client. This is for reasons of security. It
would be very unsafe to let any old applet that you downloaded from an unknown
origin on the Internet read/write your files. It would be as unwise as allowing
this kind of access to an ActiveX control (which is one reason ActiveX is
dead on the Internet). There are several different ways to relax the rules.
See the answer to question 6.8.
13.4 How
do I do I/O to the serial port on my computer?
-
A. Java 1.0 and 1.1 do not have a serial port API. There are several
commercially-available libraries that supply the needed functionality. JDK
1.2 introduces access to the serial and parallel ports as an extension (optional
extra) library. See also the answer to question 6.3.
13.5 How do
I do formatted I/O like printf and scanf in C/C++?
-
A. The java.text package introduced with Java 1.1 supports formatted
I/O. See also the answer to questions 6.11, 6.12, and 12.3.
13.6 I have spent more
debugging time finding case (upper vs lower) typos than everything else put
together and squared!
-
A. Do not forget that your remark must be phrased in the form of a
question to win on FAQ Jeopardy. In any event, it is worth reminding those
new to Java that letter case really matters in Java, and that the names of
public classes should exactly match (including case) the names of the files
they live in. See also the answer to question 4.1.2
13.7 Why do I get a
compiler error with this statement: "double y = sin(90);"?
-
A. You need to write it this way:
double cvtDegToRad = Math.PI/180;
double x = 90*cvtDegToRad;
double y = Math.sin(x);
You need to use the "Math" classname, e.g. Math.sin instead of plain sin,
because you have to say what class or object these methods belong to. These
are static methods of the Math class, so you give the name of the class in
invoking them.
Also recall that the Math package works in radians, not degrees. 360 degrees
= 2 pi radians. So use a conversion factor as shown if you are working with
degrees.
13.8
Why do I get this compiler
error: "Can't make static reference to method..."?
-
A. Your code probably looks something like this:
class myclass {
public static void main(String args[]) {
myMethod();
}
public void myMethod() { //some code
}
}
The issue is this: a static method means it belongs to the class, not each
individual object. If you leave the static keyword off (the usual case) as
is done here with the method "myMethod()", it means that method can only
be invoked on an object. But your call from main() has not told myMethod()
which object it is to be invoked on. Inside a non-static method, you don't
have to provide this information, as it assumes you mean the same object
on which it was invoked. But when calling from a static method, you must
provide the information, and you haven't - hence the error message.
A common fix is to instantiate a member of the class, on which to invoke
myMethod(), like this:
public static void main(String args[]) {
myclass m = new myclass();
m.myMethod();
}
This problem is especially common when you are writing code that you want
to run as an applet and as an application. Naturally, you call init() and
start() from main. What you really need to do is:
public static void main(String[] args) {
Applet ma = new myApplet();
ma.init();
ma.start();
}
13.9 How do I close
a Java window by using the icon in the upper right hand corner of a window?
-
A. Create an event handler class to extend WindowAdapter. Then override
WindowAdapter's windowClosing() to do the actions you want when a window's
"close" action is clicked. Then add that to the listeners for that window.
import java.awt.*;
import java.awt.event.*;
public class MyFrame extends Frame {
public MyFrame(String s) {super(s);}
public class WL extends WindowAdapter {
public void windowClosing(WindowEvent e) {System.exit(0);}
}
// do your other Frame stuff
}
Somewhere in your initialization code, put:
f1.addWindowListener( f1. new WL() );
This last syntax is not commonly known to many people yet, it's another whacky
artifact of inner classes.
Alternatively, combining the inner class and setting the handler in one go,
you could do this:
MyFrame f1 = new f("wave");
f1.addWindowListener( new WindowAdapter() {
public void windowClosing(WindowEvent e) {
// and/or setVisible(false) and/or dispose()
System.exit(0); }
});
See also the answer to Q.4.0.19, 4.0.30, and 15.7.
13.10 Why is b+=100;
OK, but b = b+100; fails to compile?
-
A. You have code like this
byte b = 0;
Incompatible type for =. Explicit cast needed to convert int to byte.
b = b + 100; // compiler error message
^
b += 100; // works OK
The reason is "promotion". Arithmetic expressions are promoted to the type
of the longest, floatiest operand in them, or at least to int. The first
statement involves the assignment of an expression. The expression is promoted
to 32 bits, and must be cast back down to 8 bits, like this "b = (byte)
(b+100);". The second is an operator assignment, and the cast back to byte
takes place automatically. The Java Specification says:
-
"A compound assignment expression of the form E1 op= E2 is equivalent to
E1 = (typecast)((E1) op (E2)), where "typecast" is the type of E1, except
that E1 is evaluated only once" [JLS 15.25.2]
The compile-time narrowing of constants means that code such as:
byte theAnswer = 42;
is allowed, with no cast necessary. [JLS 5.2]
13.11 How do I add two Float
objects together?
-
A. You want to write code like this
Float One;
Float Two;
Float Hard = One + Two;
but the compiler does not allow it.
Java has two separate ways of representing a 32 bit floating point number,
Float and float. Float is a class, that whose sole purpose is to "wrap" a
floating point number so it can be treated as an object. The class does not
support floating point arithmetic, because the performance would be too slow.
float is a primitive type (like int) that is used for floating point arithmetic.
You choose one or the other depending on your predominant use. If all you
need of your floating point numbers is arithmetic, declare them to be "float".
If you need to use them as objects, for example to place them in a Vector,
declare them as "Float".
If you need both, tough. You have to declare them one way and convert whenever
you need the capabilities of the other. Your specific code can be written
as:
Float One = new Float(1.0);
Float Two = new Float(2.0);
Float Hard = new Float(One.floatValue() + Two.floatValue());
See also 3.8, 5.1 and 10.1.
13.12 How can I put
all my classes and resources into one file and have java run it?
A. Use a JAR file. Put all the files in a JAR,
then use the jre to run the app, like:
jre -cp app.jar Main
assuming the jar is called app.jar and it has a class called Main that has
the main() method for the application.
13.13
How can I see line numbers in a stack trace using JDK 1.1.6?
See also "The 10 Most Common Java Programming Mistakes":
http://webreview.com/wr/pub/97/12/19/feature/sidebar.html
14. FOR WINDOWS USERS
-
14.1 Is there a Java
implementation for Windows 3.1?
14.2 I'm using Win95,
and my DOS window won't accept filenames longer than 8.3.
14.3 I'm using notepad
to edit my files, and how can I save them with the extension ".java"?
Also, in notepad
some source files have all the characters on one line. Why is that?
A. First answer: put the entire filename in quotes in the
save dialog.
Second answer: notepad expects to see a "carriage return/line feed" pair
at the end of each line, rather than just the "newline" (line-feed) commonly
used on Unix. Use this program to expand all newlines,
/*
* Usage: jre crlf file1.java file2.java ... fileN.java
*/
import java.io.*;
class crlf {
public static void main(String s[]){
byte b[]; byte p;
FileInputStream is;
BufferedOutputStream os;
File f;
for (int i=0; i < s.length;i++){
try{
f=new File(s[i]);
b=new byte[(int)f.length()];
is = new FileInputStream(f);
is.read(b); is.close();
os = new BufferedOutputStream(
new FileOutputStream(s[i]),b.length);
p='?';
for(int j=0; j < b.length; j++){
if((p!='\r')&&(b[j]=='\n')) os.write('\r');
p=b[j]; os.write(p);
}
os.flush(); os.close();
}catch(IOException e){
System.err.println(e.toString());
}
}
}
}
The source code is to show new users a way to make a simple
program which can read a file and write it out buffered.
compile with "javac crlf.java" and run with
java crlf outfile.txt
or just use wordpad instead of notepad. Wordpad is under
Start->Programs->Accessories->WordPad
14.4 What's J++6?
A. It is an abbreviation for "Microsoft J++ version 6". Microsoft
launched J++ 1.0 in 1996 and it was compatible with Java 1.0. When Java 1.1
came out, Microsoft decided not to support key parts of it, such as JNI,
JFC, RMI, and Java Beans. Nevertheless, Microsoft moved the version number
of J++ to 1.1, which confused people who expected it to support JDK 1.1
Microsoft has continued its attempt to fragment and undermine support for
Java with J++6. First, it bumped the version number up, skipping over releases
1.2 , 3, 4, and 5. This aligned the number with other MS products, but falsely
conveys the impression that J++6 is an equally stable and mature product.
(A similar thing ocurred with NT, which was introduced at version 3.1, and
with Microsoft's CIFS - Common Internet Filesystem, which is neither common
nor an internet standard. It was actually renamed to CIFS as part of Microsoft's
wishful thinking to make this NT remote file protocol be adopted all over
the web). Then Microsoft introduced two new keywords ("delegate" and
"multicast"), as well as the complexity of conditional compilation and
conditional methods.
Microsoft's SDK has been moved further and further from standard Java with
each successive release. J++ combined with the visual editor generates calls
to the Windows-only WFC library. Anyone writing Java code that uses VJ++6
is needlessly restricting their programs to only ever run on Microsoft
platforms. Microsoft has been cut off from future Java technology, so they
do not have JNI, JFC, RMI, Java Beans, IIOP/CORBA hooks, Swing, PLAF &
Accessibility, the Security API, Drag N Drop, among other features. If you
like Java, you should encourage Microsoft to be compatible by not using their
divergent tools.
The entire purpose of VJ++ is to get people producing near-Java code that
only runs on Windows. The strategy is being challenged in the law courts,
giving VJ++ an uncertain future.
So please use one of the other free or inexpensive standard
IDEs for Windows instead.
See Q1.14 for a list of these IDEs.
You may see some programmers refer to J--. By this they mean Microsoft's
incompatible J++ SDK.
14.5 How do I fix the
message about "out of environment variable space"?
A. This occurs under Windows when you have long CLASSPATH names.
You need to increase the enviroment space. Put this in your config.sys:
shell=command /e:4096
15. Java GOTCHA'S
-
15.1 What is a "GOTCHA"
(for non-English native speakers)?
A. It is an abbreviation of "Got you!" It is the triumphant exclamation
that a bug or programming idiom makes as it traps the unwary programmer.
This section details some of the popular "gotcha's" of Java.
15.2 I changed a final
value, and recompiled just the file that it was in, and the entire rest of
the program used the old value!
A. This is the "expected" behavior. If you have this in one file
class Flags { final static boolean debug = true; }
and you change it to, and recompile just this file:
class Flags { final static boolean debug = false; }
Then the rest of your Java .class files will still see it as "true".
When you declare a "static final int" (or any other primitive), the compiler
turns that into a compile time constant whose value can be substituted wherever
it is used in your program. If you update the value in the source file, you'll
need to recompile every class that uses it.
See Java Language Specification, section 13.4.8 "final Fields and Constants":
"We call a field that is static, final, and initialized with a compile-time
constant expression a primitive constant. If a field is a primitive constant,
then deleting the keyword final or changing its value will not break
compatibility with pre-existing binaries by causing them not to run, but
they will not see any new value for the constant unless they are recompiled."
15.3 What is
the "substring trap"?
A. The "substring trap" is the name for a mistake
that is all too easy to make when using the substring() method of class String.
The method signature is:
public String substring(int beginIndex, int endIndex)
The name "endIndex" suggests that is the index where the Substring ends.
But in fact, the substring extends only to the character at position
(endIndex-1)! It seems to be done this way so that s.substring(0,s.length())
is equal to s. If so, the name of the second parameter should be something
like endInxLessOne or Length. But not the confusing and misleading endIndex.
Beware of the substring trap.
15.4 Why does
getGraphics() return null on my offscreen image?
class MyFrame extends Frame {
MyFrame() {
Image offscreen = createImage(100,100);
Graphics offg = offscreen.getGraphics();
}
...
}
will usually not work, since the peer will not exist at this time.
Without the peer for the Frame, you cannot succeed in creating offscreen
Images. (There's no problem creating Produced Images without a peer. Trying
to draw them, of course, is another matter).
One "standard" form of offscreen code looks like this: (note the reuse of
the Graphics and Image objects for as long as possible)
class Gumble extends java.awt.Something {
private Image offi;
private Graphics offg;
public void update(Graphics g) {
if (g == null) return; // Paranoia
Dimension size = size();
if ( offi == null
|| offi.getWidth()!=size.width
|| offi.getHeight()!=size.height ) {
if (offg!=null) offg.dispose();
offi = createImage(size.width, size.height);
offg = offi.getGraphics();
// Regenerate offi here...
}
// If you use getClipBounds() here,
// check that for being null, too!
// several implementations have been known to pass them....
g.drawImage(offi);
}
public void paint(Graphics g) {
update(g);
}
}
See also 8.7;
15.5 The
dynamic type of a method argument doesn't seem to be used to choose an
overridden method at runtime.
A.Correct. Generally, if you invoke a method
on an object, the object's actual runtime type, not the type of the reference
that you used to reference it, determines which method is invoked. This is
regular polymorphism.
It's not the same for object parameters: the compiler decides at compile
time, depending on the types of the parameter expressions, which method
signature
to use, and this is "hardwired" into the bytecode. The compiler does not
look at the object argument at runtime and say "ah, this is a derived type,
so I will choose the method that takes the derived type as an argument."
This is best seen in a code example:
class Base { }
class Derived extends Base { }
public class foo {
public static void method(Base b) {
System.out.println("In the base method...");
}
public static void method(Derived d) {
System.out.println("In the derived method...");
}
public static void test(Base b) {
if (b instanceof Derived)
System.out.print("Derived: ");
else
System.out.print("Base: ");
method(b); // which method? method(base) or method(derived)?
}
public static void main(String args[]) {
Base b = new Base();
Derived d = new Derived();
System.out.println("test calls.");
test(b);
test(d);
}
}
Running the program gives an output of
test calls.
Base: In the base method...
Derived: In the base method...
See JLS section 15.11.4.4 and 15.11.3:
"If class S contains a declaration for a method named m with the
same descriptor (same number of parameters, the same parameter types,
and the same return type) required by the method invocation
as determined at compile time then this is the method to be
invoked."
15.6 Why did I
lose my updates when I changed data fields in a graph that I was serializing?
A. Quoting from the object serialization
specification at:
http://www.javasoft.com/products/jdk/1.1/docs/guide/serialization/spec/serial-arch.doc.html#4176
The writeObject method serializes the specified object and traverses its
references to other objects in the object graph recursively to create a complete
serialized representation of the graph.
Within a stream, the first reference to any object results in the object
being serialized or externalized and the assignment of a handle for that
object. Subsequent references to that object are encoded as the handle.
In other words, changing an object and then writing it again does not really
write it twice. Instead it just writes a reference back to the first occurrence,
losing any fields that have changed in the meantime.
There are three ways around this: (1) (inefficient) Reset (or close and reopen)
the stream, and start again by writing the new value of the object. This
is drastic -- you are throwing away all the serialization that you have already
done.
(2) (kludgey) Create a new object and write that.
(3) (could be a lot of work) Write your own protocol for object serialization.
Have something like a data stream where the contents of an object are marked
by special identifiers. Each "end" of the stream can decide whether it will
use a new object each time or reuse an existing object.
15.7 When I click
on a Java window frame, it doesn't close!
Frame mf = new Frame("binky");
mf.addWindowListener( new WindowAdapter() {
public void windowClosing(WindowEvent we) {
System.exit(0); // or setVisible(false); etc.
} });
This really should be the default behavior of an AWT Frame. So you'll be
delighted to hear that JavaSoft has "made it so" for the JFrame Swing component.
That leads to a slightly different problem. See 4.3.3. See also 13.9.
15.8 What's the deal with
"super"? How far back into parent classes can I go?
A. The keyword "super" is best thought of a compiler
metaname that refers only to the immediate superclass. The call "super()"
invokes a constructor in the superclass, and the reference "super.someField"
lets you access other fields in the immediate superclass.
There is no way to "chain" several together, and reach back higher into the
parent class hierarchy. You can reference your immediate parent class and
that's it. Looking at the generated byte code, if you have
class Parent { }
class Child extends Parent { }
then, in Child "super.someParentMethod();" means
"invokespecial X.someParentMethod()"
in the JVM, not "invokevirtual". Invokevirtual means "call the right
method for whatever object this is". Invokespecial means "call the exact
method I am telling you."
15.9 When I change some
component (e.g. a new label on a button) I don't see the change on the screen
immediately even if I repaint().
A. You need to add these calls, instead of the
repaint():
invalidate();
validate();
They cause the component hierarchy to be marked as needing to be laid out
again, and the validate causes that to be done. It may be expensive, but
is always the most reliable way of getting the peers to recalculate size
and to do what is needed to bring the display up to date.
15.10 Why aren't popup menus
working cross-platform for me?
A. On Windows, the pop-up trigger is a mouse
release (except in certain programs like Netscape Communicator). On Unix,
the pop-up trigger is a mouse press.
Therefore you need to ask the question isPopupTrigger() in both the
mousePressed() and mouseReleased() methods when implementing the MouseListener
interface. Alternatively override Component's processMouseEvent as a central
place for handling mouse input.
15.11 Why aren't newlines
working cross-platform for me?
if (c == '\n')
fin = true;
is not cross-platform. On Unix the line terminator is "\n", on Windows, it
is frequently "\r\n", on the Mac it is "\r".
The call System.getProperty("line.separator") will return a string containing
the platform-specific line separator character(s), and you then need to compare
it according to how your data is formatted (e.g. compare 2 characters or
one). There is also a property for the separator character in file pathnames,
and other values too.
15.12 Why did my polygon
come out the wrong shape?
-
A. This question and answer comes directly off
comp.lang.java.programmer, and deserves to be immortalized for posterity.
When I use fillPolygon with the following points I get two inverted triangles
instead of a rectangle. Why?
int xPoints[] = {71, 78, 71, 78};
int yPoints[] = {147, 147, 130, 130};
g.fillPolygon(xPoints, ypoints, xPoints.length);
Developer Felix Pahl supplied the answer in limerick form:
-
A developer (for details bored her)
didn't follow the polygon's border
so instead of right angles
she got two triangles
'cause the endpoints were in the wrong order!
You must put the points in the order you would encounter them in if you went
round the polygon's border. The filling algorithm is doing the right thing!
Try drawing the points on paper to see:
71,130 78,130
O--------O
| |
| |
| |
o--------o
71,147 78,147
Under JDK1.1, the two endpoints are connected automatically and you would
order the array elements as:
int xPoints[] = { 71, 78, 78, 71};
int yPoints[] = {130, 130, 147, 147};
Under JDK1.0.2, you have to explicitly connect the two endpoints, and you
would write the array elements as:
int xPoints[] = { 71, 78, 78, 71, 71};
int yPoints[] = {130, 130, 147, 147, 130};
15.13 Why can't I see
all the components I added to a Frame?
-
A. If you have code like:
Frame myframe = new Frame("Child Frame");
myframe.resize(512,384);
myframe.add(new Label("Child"));
myframe.show();
you may find that the Label does not show up in the Frame. Or it may show
up in appletviewer, but not in a browser.
The default layout manager for Frame is BorderLayout. Components positioned
with a BorderLayout must include a positioning constant to be correct.
So, change the add to
myframe.add("Center", new Label("Child"));
and all will be well.
15.14
Why do I get the wrong results when I compare two Strings together?
if (s1 == s2)
is giving me funny results.
A. The comparison using "==" on objects, like Strings, is asking the
question "do these two objects have the same reference?". That is, do they
have the same address, and hence are not two object but one? What you most
probably meant is "do these two Strings have the same contents?" which you
can express this way:
if ( s1.equals(s2) )
This is a very, very easy mistake to make and impossible to spot until you
have had it explained to you.
People talk about "interning" a String. That means calling the intern() method
on a String. This places the String in the runtime constant pool
if it was not already
there. The compiler is required to intern() all literal Strings.
If you intern() all your Strings as well then all duplicates are shared and
comparisons can be done by the (much faster) address comparison
rather than content comparison. It's a performance optimization. See also
Q3.22.
Note that this comparison error also occurs with other objects, not
just Strings. The code:
if (getBackground() == Color.black)
is a test for object identity, rather than content identity. It will
work if you originally setBackground(Color.black). To avoid difficult
debugging in future, you almost certainly want to say
if (getBackground().equals( Color.black ) )
or even (in this visual case) compare the darkness of the RGB values
of the pixels.
15.15 Why doesn't
final prevent my object from changing?
A. You have code like this
final StringBuffer s = new StringBuffer("don't change me");
// ...
s.append(", but I did");
System.out.println(s);
And the new value of s is "don't change me, but I did". The reason is that
the "final" modifier makes the reference variable (here, s) final,
not the object that s points to. It means
that the reference variable cannot be changed to point to some other
StringBuffer. The state of the StringBuffer can still be modified by calling
methods on it or directly assigning to its public fields.
The right way to think about final is that it prevents you assigning to that
particular variable. The only way to make the fields of an object constant
(unchanging) is to make all its data fields private, and not provide any
set methods for them, only get methods. Even that won't stop other objects
of the same class adjusting it.
15.16
Why can't the compiler find my package?
A. When trying to compile a file in a package you get
a compiler error like:
DBTest.java:10: Class database.Table not found in type declaration.
The file Table.java and DBTest.java are in the same directory.
They both have "package database;" at the top of the file.
The current directory is included in the classpath.
The reason is that when compiling packages, you have to be at
the 'top' of the directory/package hierarchy. So to compile both
Table.java and DBTest.java, you have to be in the directory that
contains the database directory (i.e. where the package hierarchy
starts), and just:
javac database/Table.java
javac database/DBTest.java
and it should all compile fine.
15.17
I have a program with keyboard input and a button. When the
user hits the space bar, the button gets pressed as it is in focus!
A. The VM sets the focus on the first traversible object
in the UI. If you want the button not to be assigned focus by
default, you must subclass the button and override the
isFocusTraversable() method to return false.
Another approach is to manually set the focus on some other component
(say the Frame) when you show the window. To do so you have to jump
through hoops to outsmart the VM that is trying to set it on the
button. One approach is to listen for the windowActivated event
and set a Swing Timer to do a requestFocus() on the frame about
0.1 seconds after the activated message.
This seems convoluted, but it is the only thing found that
consistently works cross platform.
Another reader suggests
that if the frame normally gets the focus first, you can override
its gotFocus() event and set the focus to the component you want.
Don't forget to return true!
15.18 What's the
hidden size limitation of String serialization?
A.
If you wish to Serialize a
string, be alert to the restriction that:
The size of the String, when UTF-encoded, must be < 64Kb
So for robust code you have to examine the String once to ensure that
it will be less than 64Kb after encoding, and then have the JVM effectively
repeat that work in the process of encoding, when you write it to
an ObjectOutputStream.
A possible workaround that is to
strip the string down to "byte[]" and pass it around in RMI that way.
The code with this restriction is in DataOutputStream
public final void writeUTF(String str)
...
[perform the size-after-conversion-to-UTF computation]
...
if (utflen > 65535)
throw new UTFDataFormatException();
...
RMI relies on serialization, so RMI has the same String size limitation.
15.19
What happens to post-increment when an exception is thrown?
array[i++] = foo();
and foo() throws an exception, i will be incremented anyway. This can cause
problems if sometimes foo() throws an exception and you don't want i
incremented in cases when it does.
This is a consequence of JLS 15.25.1 and 15.6.1 "the left-hand operand of a
binary operator appears to be fully evaluated before any part of the right-hand
operand is evaluated." (assignment is taken as a binary operator).
Note that this is not how C++ behaves.
Acknowledgements
FAQ copyright 1997, 1998 by Peter van der Linden. Contributions and help
from:
Matt Kennel, Patric Jonsson, Brad Van Tighem, Tony Hursh , Glenn L Vanderburg,
Peter Jones, John McDowall, Jim Driscoll, Uday, Dave Harris, Bill Wilkinson,
Tom Valesky, Dan Drake, Giles Thomas, Mitch Baltuch, Guy Ruth Hammond, Gordon
Keith, Jason Brome, Shani Kerr, Steve Chapel, Timothy Wolters, Robert Lynch,
Jake Cormier, Sean C Sullivan, Joseph A. Millar, Jim Frost, Jim Balter, Jeff
Bauer, John Kochmar, Carl Burke, William Stubbs, Mark Smith, Volker Turau,
Real Gagnon, Russell Gold, Max Hailperin, Bill Tschumy, Marco Nijdam, Marc
Pawlowsky, Laurence Vanhelsuwe,Ian Macgregor, Mike Faulkner, Rich Koch, Will
Clark, Govind Seshadri, Rich Simkin, Ian Stiles, Kieren, Darren Christie,
Tom Lane, Michael Jungmann, Rob Mayoff, George Ruban, Tom McCann, David Hopwood,
Thomas Phan, Kai Stuke, Rolf Howarth, Derek Snider, David Boydston, Andy
Godwin, John F. Dumas, Doug Bell, David J. Biesack, Tiger Quimpo, Martin
Hugh Rogers, Brian Krahmer, Ian Burrell, Nikki Locke, Bin Li, Jackson Thompson,
Steve Odendahl, Greg Smith, Jeffrey C. Ollie, Mark Halvin, Jeremy Cook, Lak
Ming Lam, Peter S. Morris, Mark Halvin, Juergen Keil, Alex Stewart, Mike
Abney, Rodney Stephenson Mark Gritter, Satish Talim, Tamminen Eero, Alexander
Gridnev, Eric Hodges, Jamey Graham, Will Lockhart, Scott Plante, Tom Sanfilippo,
Jan Newmarch, Sean Breslin, Stuart D. Gathman, rhino@wwdc.com, C Matthew
Curtin, Tor Iver Wilhelmsen, A.N.Pryke, Phil Race, David Holmes, David Rodal,
Dominique Plante, Trent Jarvi, Ingrid Biery, Gopal Unni Krishnan, Grant Lewis,
Tov Are Jacobsen, Gary McGath, Marty Hall, Will Forster, Colin Mummery, Darin
McBride, Mayank Shah, Jens Alfke, Glen Stampoultzis, Philip Brown, Peter
Steiner, Kurt Spaugh, Rasmus Ekman, Jonathan Revusky, Ken Kalish, Dave Sanders,
Bill Hyden, James Cloughley, Philip "diodes" Gustafson, Paul Kinnucan, Juan
Valdéz, Antranig Basman, Felix Pahl, David N. Still, Simon Arthur,
Mark Hammond, Dan Kegel, Thomas Weidenfeller, Pavel Shvartsman, Christen
Monberg, George Reese, Ian Macgregor, John Sublett, David Zimmerman, Tony
Dahlman, Druid, Chris Kelly, Patricia Shanahan, Paul Hill, Lyne Lamoureux,
Don Kennedy, Alec Muffett, Andrew Mickish, Pavel Shvartsman, Neil of
Parkway Consultants, Chris Thiessen, David Michaels, Bob Sutherland,
Michael Allen Latta
I am maintaining a FAQ list to address specifically programming issues
(not a general tutorial on Java). Please mail suggested FAQ entries including
answer to me at pvdl on the site best.com.
Question with answer gets you a credit in the FAQ.
Peter van der Linden, Sun Certified Java Programmer.
-- end --