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.