What is a WeakHashMap and when to use it? [duplicate]

Answer 1

Weak references are about reachability and letting the garbage collector (GC) do the work for you. I think it is best to understand the problems that the weak reference is trying to solve and see it in action:

• An IBM article about:

  Java theory and practice: Plugging memory leaks with weak references. Memory leaks with global Maps, Identifying memory leaks, Weak references to the rescue, ...

• A blog article (link expired) about when to use WeakHashMap:

  ... If the WeakHashMap is no good for caching, then what is it good for? It is good to implement canonical maps. Lets say you want to associate some extra information to an object that you have a strong reference to. You put an entry in a WeakHashMap with the object as the key, and the extra information as the map value. Then, as long as you keep a strong reference to the object, you will be able to check the map to retrieve the extra information. And once you release the object, the map entry will be cleared and the memory used by the extra information will be released. ...

• The Java documentation about java.lang.ref

Answer 2

From jGuru:

A WeakHashMap is a special Map implementation where the keys of the map are stored in a java.lang.ref.WeakReference. By storing the keys in a weak reference, key-value pairs can dynamically be dropped from the map when the only reference to the key is from the weak reference. This makes the WeakHashMap an excellent implementation for a weakly referenced list, where entries that aren't used elsewhere may be dropped with no side effects. Also, just because a key may be dropped, doesn't mean it will immediately be dropped. If the system has sufficient resources, the weak key reference that isn't externally referenced could stay around for a long time.

More on References: