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Taming Tiger, Part 1

An introduction to Java 1.5

By Tarak Modi, JavaWorld.com, 04/26/04

Page 2 of 6

// A list that stores numbers
// Generics will make this obvious...
List numbers = new ArrayList();
numbers.add(89);

Prior to version 1.5, this code would not compile since ArrayList's add method expects an object. To make it work, you must modify the second line as follows:

   numbers.add(new Integer(89));

That seems unnecessary and, with J2SE 1.5, it is. With J2SE 1.5, the compiler automatically adds the code to convert the integer value (such as 89) into the proper class (Integer in our example). That is called boxing. The opposite process of converting an object (such as of type Integer) into a value (such as an int) is called unboxing. Boxing and unboxing eliminate the (frequent and often burdensome) need to explicitly convert data of primitive type to reference and vice versa. Such explicit required conversions are verbose and induce clutter in the program text. The following table shows the rules for boxing and unboxing.