Java Glossary : pointer

Last updated 2004-06-28 by Roedy Green ©1996-2004 Canadian Mind Products

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pointer

Strictly speaking, in Java, a pointer is a reference that is guaranteed not to be null. However, when people use the term pointer, they usually mean C++ style direct hardware address pointers.

You may have heard some FUDster claiming that Java sucks roadkill because it has no pointers. How can you write serious code without pointers? In Java are you actually limited to FØRTRAN style problems: no linked lists, no queues etc? Java does not have raw pointers like C or C++. It has something almost as powerful, but many times safer called references (Java refers to them as pointers in one place, the NullPointerException). They are like pointers, except that the dangerous features are removed, namely:

• pointer arithmetic
• random initialisation
• dangling pointers to deleted objects
• mismatched types and unions
• You can't construct references out of thin air, only with new.
• Unlike pointers, references cannot point into the middle of an object.
• References are null or always point to something valid of the type they claim to.
• Pointers always point directly to data. References are usually implemented as handles, not pointers. A handle is a pointer to a pointer. This allows objects to be moved around to compact fragmented RAM without having to find and adjust all the pointers to those objects.

The safety comes with a cost.

• Every downcast has to be checked to ensure the object truly is of the type your cast claims it to be.
• Nearly every time you put an object into an array, the runtime must check the type to be sure it is compatible with the allocated type of the array, (which is distinct from the declared type of the array reference.)
• Implementations of linked structures typically require extra tiny glue objects that point to the objects linked. With raw pointers, typically you manage to embed these pointers directly into your objects.

handle ¤ reference

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