What is a WeakHashMap and when to use it?

checkout Effective Java, Edition 2, page 26.

Another common source of memory leaks is caches. Once you put an object reference into a cache, it’s easy to forget that it’s there and leave it in the cache long after it becomes irrelevant. There are several solutions to this problem. If you’re lucky enough to implement a cache for which an entry is relevant exactly so long as there are references to its key outside of the cache, represent the cache as a WeakHashMap; entries will be removed automatically after they become obsolete. Remember that WeakHashMap is useful only if the desired lifetime of cache entries is determined by external references to the key, not the value.

As others have already pointed out, a weak reference provides a means for using an object as a key without creating a strong reference to it. This is useful in situations where you don't want to impair the JVM's ability to garbage collect the object, but yet still want the ability to track some aspect of the object, which makes a weak reference ideal for caching or storing metadata about the object.

I'd suggest reading "Understanding Weak References" (Oracle blog article), about strong vs. weak references in Java. Without an understanding of the difference, the data structure itself makes little sense.

Elements in a weak hashmap can be reclaimed by the garbage collector if there are no other strong references to the key object, this makes them useful for caches/lookup storage.

Weak reference are not restricted to these hash tables, you can use WeakReference for single objects. They are useful to save resource, you can keep a reference to something but allow it to be collected when nothing else references it. (BTW, a strong reference is a normal java reference). There are also weak references which tend not to be as readily collected as soft references (which don't tend to hang about for long after the last strong reference disappears)