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Subsections
Reading and Writing Files
Files and Directories
File Dialog Boxes

Section 11.2

Files

The data and programs in a computer's main memory survive only as long as the power is on. For more permanent storage, computers use files, which are collections of data stored on a hard disk, on a USB memory stick, on a CD-ROM, or on some other type of storage device. Files are organized into directories (sometimes called folders). A directory can hold other directories, as well as files. Both directories and files have names that are used to identify them.

Programs can read data from existing files. They can create new files and can write data to files. In Java, such input and output can be done using streams. Human-readable character data is read from a file using an object belonging to the class FileReader, which is a subclass of Reader. Similarly, data is written to a file in human-readable format through an object of type FileWriter, a subclass of Writer. For files that store data in machine format, the appropriate I/O classes are FileInputStream and FileOutputStream. In this section, I will only discuss character-oriented file I/O using the FileReader and FileWriter classes. However, FileInputStream and FileOutputStream are used in an exactly parallel fashion. All these classes are defined in the java.io package.

It's worth noting right at the start that applets which are downloaded over a network connection are not allowed to access files (unless you have made a very foolish change to your web browser's configuration). This is a security consideration. You can download and run an applet just by visiting a Web page with your browser. If downloaded applets had access to the files on your computer, it would be easy to write an applet that would destroy all the data on a computer that downloads it. To prevent such possibilities, there are a number of things that downloaded applets are not allowed to do. Accessing files is one of those forbidden things. Standalone programs written in Java, however, have the same access to your files as any other program. When you write a standalone Java application, you can use all the file operations described in this section.

11.2.1  Reading and Writing Files

The FileReader class has a constructor which takes the name of a file as a parameter and creates an input stream that can be used for reading from that file. This constructor will throw an exception of type FileNotFoundException if the file doesn't exist. It requires mandatory exception handling, so you have to call the constructor in a try..catch statement (or inside a routine that is declared to throw the exception). For example, suppose you have a file named "data.txt", and you want your program to read data from that file. You could do the following to create an input stream for the file:

FileReader data;   // (Declare the variable before the
                   //   try statement, or else the variable
                   //   is local to the try block and you won't
                   //   be able to use it later in the program.)
                        
try {
   data = new FileReader("data.txt");  // create the stream
}
catch (FileNotFoundException e) {
   ... // do something to handle the error -- maybe, end the program
}

The FileNotFoundException class is a subclass of IOException, so it would be acceptable to catch IOExceptions in the above try...catch statement. More generally, just about any error that can occur during input/output operations can be caught by a catch clause that handles IOException.

Once you have successfully created a FileReader, you can start reading data from it. But since FileReaders have only the primitive input methods inherited from the basic Reader class, you will probably want to wrap your FileReader in a Scanner, in a TextReader, or in some other wrapper class. (The TextReader class is not a standard part of Java; it is described in the previous section.) To create a TextReader for reading from a file named data.dat, you could say:

TextReader data;

try {
   data = new TextReader( new FileReader("data.dat") );
}
catch (FileNotFoundException e) {
   ... // handle the exception
}

Once you have a TextReader named data, you can read from it using such methods as data.getInt() and data.peek(), exactly as you would from any other TextReader.

Working with output files is no more difficult than this. You simply create an object belonging to the class FileWriter. You will probably want to wrap this output stream in an object of type PrintWriter. For example, suppose you want to write data to a file named "result.dat". Since the constructor for FileWriter can throw an exception of type IOException, you should use a try..catch statement:

PrintWriter result;

try {
   result = new PrintWriter(new FileWriter("result.dat"));
}
catch (IOException e) {
   ... // handle the exception
}

If no file named result.dat exists, a new file will be created. If the file already exists, then the current contents of the file will be erased and replaced with the data that your program writes to the file. This will be done without any warning. To avoid overwriting a file that already exists, you can check whether a file of the same name already exists before trying to create the stream, as discussed later in this section. An IOException might occur in the PrintWriter constructor if, for example, you are trying to create a file on a disk that is "write-protected," meaning that it cannot be modified.

After you are finished using a file, it's a good idea to close the file, to tell the operating system that you are finished using it. You can close a file by calling the close() method of the associated stream. Once a file has been closed, it is no longer possible to read data from it or write data to it, unless you open it again as a new stream. (Note that for most stream classes, the close() method can throw an IOException, which must be handled; however, both PrintWriter and TextReader override this method so that it cannot throw such exceptions.) If you forget to close a file, the file will ordinarily be closed automatically when the program terminates or when the file object is garbage collected, but in the case of an output file, some of the data that has been written to the file might be lost. This can occur because data that is written to a file can be buffered; that is, the data is not sent immediately to the file but is retained in main memory (in a "buffer") until a larger chunk of data is ready to be written. This is done for efficiency. The close() method of an output stream will cause all the data in the buffer to be sent to the file. Every output stream also has a flush() method that can be called to force any data in the buffer to be written to the file without closing the file.

As a complete example, here is a program that will read numbers from a file named data.dat, and will then write out the same numbers in reverse order to another file named result.dat. It is assumed that data.dat contains only one number on each line. Exception-handling is used to check for problems along the way. Although the application is not a particularly useful one, this program demonstrates the basics of working with files. (By the way, at the end of this program, you'll find our first example of a finally clause in a try statement. When the computer executes a try statement, the commands in its finally clause are guaranteed to be executed, no matter what.)

import java.io.*;
import java.util.ArrayList;

/**
 * Reads numbers from a file named data.dat and writes them to a file
 * named result.dat in reverse order.  The input file should contain
 * exactly one real number per line.
 */
public class ReverseFile {

   public static void main(String[] args) {

      TextReader data;     // Character input stream for reading data.
      PrintWriter result;  // Character output stream for writing data.
      
      ArrayList<Double> numbers;  // An ArrayList for holding the data.

      numbers = new ArrayList<Double>();

      try {  // Create the input stream.
         data = new TextReader(new FileReader("data.dat"));
      }
      catch (FileNotFoundException e) {
         System.out.println("Can't find file data.dat!");
         return;  // End the program by returning from main().
      }

      try {  // Create the output stream.
         result = new PrintWriter(new FileWriter("result.dat"));
      }
      catch (IOException e) {
         System.out.println("Can't open file result.dat!");
         System.out.println("Error: " + e);
         data.close();  // Close the input file.
         return;        // End the program.
      }
      
      try {
      
          // Read numbers from the input file, adding them to the ArrayList.
          
          while ( data.eof() == false ) {  // Read until end-of-file.
             double inputNumber = data.getlnDouble();
             numbers.add( inputNumber );
          }
       
          // Output the numbers in reverse order.
          
          for (int i = numbers.size()-1; i >= 0; i--)
             result.println(numbers.get(i));
             
          System.out.println("Done!");

       }
       catch (IOException e) {
             // Some problem reading the data from the input file.
          System.out.println("Input Error: " + e.getMessage());
       }
       finally {
             // Finish by closing the files, whatever else may have happened.
          data.close();
          result.close();
       }
         
   }  // end of main()

} // end of class

11.2.2  Files and Directories

The subject of file names is actually more complicated than I've let on so far. To fully specify a file, you have to give both the name of the file and the name of the directory where that file is located. A simple file name like "data.dat" or "result.dat" is taken to refer to a file in a directory that is called the current directory (also known as the "default directory" or "working directory"). The current directory is not a permanent thing. It can be changed by the user or by a program. Files not in the current directory must be referred to by a path name, which includes both the name of the file and information about the directory where it can be found.

To complicate matters even further, there are two types of path names, absolute path names and relative path names. An absolute path name uniquely identifies one file among all the files available to the computer. It contains full information about which directory the file is in and what the file's name is. A relative path name tells the computer how to locate the file starting from the current directory.

Unfortunately, the syntax for file names and path names varies somewhat from one type of computer to another. Here are some examples:

It's reasonably safe to say, though, that if you stick to using simple file names only, and if the files are stored in the same directory with the program that will use them, then you will be OK. Later in this section, we'll look at a convenient way of letting the user specify a file in a GUI program, which allows you to avoid the issue of path names altogether.

It is possible for a Java program to find out the absolute path names for two important directories, the current directory and the user's home directory. The names of these directories are system properties, and they can be read using the function calls:

To avoid some of the problems caused by differences in path names between platforms, Java has the class java.io.File. An object belonging to this class represents a file. More precisely, an object of type File represents a file name rather than a file as such. The file to which the name refers might or might not exist. Directories are treated in the same way as files, so a File object can represent a directory just as easily as it can represent a file.

A File object has a constructor, new File(String), that creates a File object from a path name. The name can be a simple name, a relative path, or an absolute path. For example, new File("data.dat") creates a File object that refers to a file named data.dat, in the current directory. Another constructor, new File(File,String), has two parameters. The first is a File object that refers to the directory that contains the file. The second can be the name of the file or a relative path from the directory to the file.

File objects contain several useful instance methods. Assuming that file is a variable of type File, here are some of the methods that are available:

Here, for example, is a program that will list the names of all the files in a directory specified by the user. Just for fun, I have used a Scanner (Subsection 11.1.5) to read the user's input:

import java.io.File;
import java.util.Scanner;

/**
 * This program lists the files in a directory specified by
 * the user.  The user is asked to type in a directory name.
 * If the name entered by the user is not a directory, a
 * message is printed and the program ends.
 */
public class DirectoryList {

   
   public static void main(String[] args) {
   
      String directoryName;  // Directory name entered by the user.
      File directory;        // File object referring to the directory.
      String[] files;        // Array of file names in the directory.
      Scanner scanner;       // For reading a line of input from the user.

      scanner = new Scanner(System.in);  // scanner reads from standard input.

      System.out.print("Enter a directory name: ");
      directoryName = scanner.nextLine().trim();
      directory = new File(directoryName);
      
      if (directory.isDirectory() == false) {
          if (directory.exists() == false)
             System.out.println("There is no such directory!");
          else
             System.out.println("That file is not a directory.");
      }
      else {
          files = directory.list();
          System.out.println("Files in directory \"" + directory + "\":");
          for (int i = 0; i < files.length; i++)
             System.out.println("   " + files[i]);
      }
   
   } // end main()

} // end class DirectoryList

All the classes that are used for reading data from files and writing data to files have constructors that take a File object as a parameter. For example, if file is a variable of type File, and you want to read character data from that file, you can create a FileReader to do so by saying new FileReader(file). If you want to use a TextReader to read from the file, you could say:

TextReader data;
 
try {
   data = new TextReader( new FileReader(file) );
}
catch (FileNotFoundException e) {
   ... // handle the exception
}

11.2.3  File Dialog Boxes

In many programs, you want the user to be able to select the file that is going to be used for input or output. If your program lets the user type in the file name, you will just have to assume that the user understands how to work with files and directories. But in a graphical user interface, the user expects to be able to select files using a file dialog box, which is a window that a program can open when it wants the user to select a file for input or output. Swing includes a platform-independent technique for using file dialog boxes in the form of a class called JFileChooser. This class is part of the package javax.swing. We looked at using some basic dialog boxes in Subsection 6.8.2. File dialog boxes are similar to those, but are a little more complicated to use.

A file dialog box shows the user a list of files and sub-directories in some directory, and makes it easy for the user to specify a file in that directory. The user can also navigate easily from one directory to another. The most common constructor for JFileChooser has no parameter and sets the starting directory in the dialog box to be the user's home directory. There are also constructors that specify the starting directory explicitly: 

new JFileChooser( File startDirectory )

new JFileChooser( String pathToStartDirectory )

Constructing a JFileChooser object does not make the dialog box appear on the screen. You have to call a method in the object to do that. There are two different methods that can be used because there are two types of file dialog: An open file dialog allows the user to specify an existing file to be opened for reading data into the program; a save file dialog lets the user specify a file, which might or might not already exist, to be opened for writing data from the program. File dialogs of these two types are opened using the showOpenDialog and showSaveDialog methods. These methods make the dialog box appear on the screen; the methods do not end until the user selects a file or cancels the dialog.

A file dialog box always has a parent, another component which is associated with the dialog box. The parent is specified as a parameter to the showOpenDialog or showSaveDialog methods. The parent is a GUI component, and can often be specified as "this" in practice, since file dialogs are often used in instance methods of GUI component classes. (The parameter can also be null, in which case an invisible component is created to be used as the parent.) Both showOpenDialog and showSaveDialog have a return value, which will be one of the constants JFileChooser.CANCEL_OPTION, JFileChooser.ERROR_OPTION, or JFileChooser.APPROVE_OPTION. If the return value is JFileChooser.APPROVE_OPTION, then the user has selected a file. If the return value is something else, then the user did not select a file. The user might have clicked a "Cancel" button, for example. You should always check the return value, to make sure that the user has, in fact, selected a file. If that is the case, then you can find out which file was selected by calling the JFileChooser's getSelectedFile() method, which returns an object of type File that represents the selected file.

Putting all this together, we can look at a typical subroutine that reads data from a file that is selected using a JFileChooser:

public void readFile() {
   if (fileDialog == null)   // (fileDialog is an instance variable)
      fileDialog = new JFileChooser();
   fileDialog.setDialogTitle("Select File for Reading");
   fileDialog.setSelectedFile(null);  // No file is initially selected.
   int option = fileDialog.showOpenDialog(this);
       // (Using "this" as a parameter to showOpenDialog() assumes that the
       //  readFile() method is an instance method in a GUI component class.)
   if (option != JFileChooser.APPROVE_OPTION)
      return;  // User canceled or clicked the dialog's close box.
   File selectedFile = fileDialog.getSelectedFile();
   TextReader in;  // (or use some other wrapper class)
   try {
      FileReader stream = new FileReader(selectedFile); // (or a FileInputStream)
      in = new TextReader( stream );
   }
   catch (Exception e) {
      JOptionPane.showMessageDialog(this,
          "Sorry, but an error occurred while trying to open the file:\n" + e);
      return;
   }
   try {
      .
      .  // Read and process the data from the input stream, in.
      .
     in.close();
   }
   catch (Exception e) {
      JOptionPane.showMessageDialog(this,
          "Sorry, but an error occurred while trying to read the data:\n" + e);
   }	
}

One fine point here is that the variable fileDialog is an instance variable of type JFileChoser. This allows the file dialog to continue to exist between calls to readFile(). The main effect of this is that the dialog box will keep the same selected directory from one call of readFile() to the next. When the dialog reappears, it will show the same directory that the user selected the previous time it appeared. This is probably what the user expects.

Note that it's common to do some configuration of a JFileChooser before calling showOpenDialog or showSaveDialog. For example, the instance method setDialogTitle(String) is used to specify a title to appear in the title bar of the window. And setSelectedFile(File) is used to set the file that is selected in the dialog box when it appears. This can be used to provide a default file choice for the user. In the readFile() method, above, fileDialog.setSelectedFile(null) specifies that no file is pre-selected when the dialog box appears.

Writing data to a file is similar, but it's a good idea to add a check to determine whether the output file that is selected by the user already exists. In that case, ask the user whether to replace the file. Here is a typical subroutine for writing to a user-selected file:

public void writeFile() {
   if (fileDialog == null)      
      fileDialog = new JFileChooser();  // (fileDialog is an instance variable)
   File selectedFile = new File("(default file name)");
   fileDialog.setSelectedFile(selectedFile);  // Specify a default file name.
   fileDialog.setDialogTitle("Select File for Writing");
   int option = fileDialog.showSaveDialog(this);
   if (option != JFileChooser.APPROVE_OPTION)
      return;  // User canceled or clicked the dialog's close box.
   selectedFile = fileDialog.getSelectedFile();
   if (selectedFile.exists()) {  // Ask the user whether to replace the file.
      int response = JOptionPane.showConfirmDialog( this,
            "The file \"" + selectedFile.getName()
                + "\" already exists.\nDo you want to replace it?", 
            "Confirm Save",
            JOptionPane.YES_NO_OPTION, 
            JOptionPane.WARNING_MESSAGE );
      if (response == JOptionPane.NO_OPTION)
         return;  // User does not want to replace the file.
   }
   PrintWriter out;  // (or use some other wrapper class)
   try {
      FileWriter stream = new FileWriter(selectedFile); // (or FileOutputStream)
      out = new PrintWriter( stream );
   }
   catch (Exception e) {
      JOptionPane.showMessageDialog(this,
          "Sorry, but an error occurred while trying to open the file:\n" + e);
      return;
   }
   try {
      .
      .  // Write data to the output stream, out.
      .
     out.close();
     if (out.checkError())   // (need to check for errors in PrintWriter)
        throw new IOException("Error check failed.");
   }
   catch (Exception e) {
      JOptionPane.showMessageDialog(this,
          "Sorry, but an error occurred while trying to write the data:\n" + e);
   }	
}

The readFile() and writeFile() routines presented here can be used, with just a few changes, when you need to read or write a file in a GUI program. We'll look at some more complete examples of using files and file dialogs in the next section.

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