Wrappers for the Primitive Types

The primitive types  in Java are boolean, char, short, int, long, float, and double. There is also the ``type'' void which is used in the place of the return type to declare a method that returns nothing. For each primitive type, the Java language defines a class that wraps  the primitive. The wrapper classes are called Boolean, Character, Short, Integer, Long, Float, and Double. There is also a wrapper class for void called Void.

Like all Java classes, the wrapper classes are ultimately derived from the Object class. Therefore, they all provide an equals method to test for equality. However, because the wrapper classes do not implement the Comparable interface given in Program , the comparison methods isLT, isLE, isGT, isGE, isEQ, and isNE are not supported.

To circumvent this shortcoming, we might be tempted to extend the wrapper classes like this:

class ComparableInteger
    extends Integer // Wrong.  Integer is a final class!
    implements Comparable
{
     // ...
}

Consequently, we are forced to implement our own wrapper classes if we want them to implement the Comparable interface. Programs , and  define the four such wrapper classes Chr, Int, and Dbl, which are wrappers for Java primitive types char, int, and double.

  
Program: Chr class.

  
Program: Int class.

  
Program: Dbl class.

Java also provides a String class for dealing with character sequences. The String class is special in that it is closely tied to the definition of the language itself. The Java compiler automatically creates a String object for every string literal , such as "Hello world.\n", in a Java program.

Because the String class does not implement the Comparable interface and because the String class cannot be extend, we are forced to implement a wrapper class. Program  defines the class Str which wraps a String instance and implements the Comparable interface.

  
Program: Str class.

Copyright © 1998 by Bruno R. Preiss, P.Eng. All rights reserved.