Reviews : Java Books :
Beginning Java Objects : Chapters 2 and 3

Buy this book Title: Beginning Java Objects ISBN: 1861004176 US Price: $ 39.99 Canadian Price: C$ 59.95 UK Price: £ 28.99 Publication Date: November 2000 Pages: 800 © Wrox Press Limited, US and UK.

If there comes a time when all references holding onto a particular object have let go of the object's handle, then as we discussed earlier the object is no longer accessible to the program, like a helium balloon that has been let loose. Continuing with our previous example:

// We instantiate our first Student object.
Student x = new Student();
// We declare a second reference, but do
// not instantiate a second object.
Student y;
// We pass y a 'handle' on the same object that
// x is holding
// (x continues to hold onto it, too).   We now,
// in essence,
// have TWO 'strings' tied to the same 'balloon'.
y = x;
// We now declare a third reference and instantiate
// a second Student object.
Student z = new Student();

// y now lets go of the first Student object
// and grabs onto the second.
y = z;

// Finally, x lets go of the first
// Student object, and grabs onto
// the second, as well; the first
// Student object is now lost to
// the program because we no longer
// have any reference variables
// maintaining a 'handle' on it!
x = z;

As it turns out, if all of an object's handles are lost, it might seem as though the memory that the object occupies is permanently 'wasted'. In a language like C++, this is indeed the case, and programmers have to take care to 'recycle' the memory of an object that is no longer needed before all of its handles are dropped. In Java, on the other hand, there is a utility called the garbage collector built into the JVM which automatically recycles 'lost' objects' memory for us. We'll revisit this topic in Chapter 13.

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