Why does exist WeakHashMap, but absent WeakSet?

From J. Bloch

> A ... source of memory leaks is > listeners > ... The best way to ensure that > callbacks are garbage collected > promptly is to store only weak > references to them, for instance, by > storing them only as keys in a > WeakHashMap.

So, why there isn't any WeakSet in the Java Collections framework?

Collections.newSetFromMap

Set<Object> weakHashSet = 
    Collections.newSetFromMap(
        new WeakHashMap<Object, Boolean>()
    );

As seen in Collections.newSetFromMap documentation, passing a WeakHashMap to get a Set.

While you can indeed use Collections.newSetFromMap() to get a WeakSet, it's use cases are actually quite limited.

If you want to implement something like String.intern() you might want to have a look at Guava's Interners.newWeakInterner() functionality instead.