java Reference counting

The most straightforward garbage collection strategy is reference counting. Reference counting is simple, but requires significant assistance from the compiler and imposes overhead on the mutator (the term for the user program, from the perspective of the garbage collector). Each object has an associated reference count -- the number of active references to that object. If an object's reference count is zero, it is garbage (unreachable from the user program) and can be recycled. Every time a pointer reference is modified, such as through an assignment statement, or when a reference goes out of scope, the compiler must generate code to update the referenced object's reference count. If an object's reference count goes to zero, the runtime can reclaim the block immediately (and decrement the reference counts of any blocks that the reclaimed block references), or place it on a queue for deferred collection.

Many ANSI C++ library classes, such as string, employ reference counting to provide the appearance of garbage collection. By overloading the assignment operator and exploiting the deterministic finalization provided by C++ scoping, C++ programs can use the string class as if it were garbage collected. Reference counting is simple, lends itself well to incremental collection, and the collection process tends to have good locality of reference, but it is rarely used in production garbage collectors for a number of reasons, such as its inability to reclaim unreachable cyclic structures (objects that reference each other directly or indirectly, like a circularly linked list or a tree that contains back-pointers to the parent node).