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Language: Java Language
Expertise: Beginner

May 6, 1997

Question:
What happens to dangling pointers in Java? Supposedly there is garbage collection, but when is it performed? From what I have read, it seems really easy to have persistent dangling pointers (pointers to objects that no one is using that aren't being cleaned up by garbage collection).

Let's say I have a collection of objects, and one of the functions that manipulates the collection is called removeObject(int i), which removes the ith object from the collection and returns a pointer to the removed object.

Sometimes I need to get the removed object to salvage it for reuse; other times I simply want to remove the object. Is the returned pointer left dangling if I don't assign it when I make the function call? Should it be left to the function-caller to dispose of unused objects, or should the programmer make two functions, deleteObject() and removeObject()?

Answer:
By definition, a dangling reference is an active binding between a name and a de-allocated segment of the heap. By definition, garbage is a segment of the heap that is no longer bound to a name by an active binding. Only garbage is de-allocated by the garbage collector; hence, a dangling reference is not possible. This, and no memory leaks, are the principle advantages of garbage collection.

If I understand your example, you are asking what the difference is between

removeObject(i);

and

x = removeObject(i);

Assuming there are no other active references to the i-th object of the collection, this object becomes garbage after the first call. A pointer to the object was returned, but was not bound to a name, hence there is no dangling reference.

By contrast, the returned pointer was bound to x in the second call, so it can still be accessed through the name x, and is not garbage.