Question

In the Java API, the implementation of HashSet is using an Object as a value for the inside HashMap,

   // Dummy value to associate with an Object in the backing Map
private static final Object PRESENT = new Object();

public boolean add(E e) {
    return map.put(e, PRESENT)==null;
}

but HashMap allows its value is null. I think that’s not necessary to fill the value, so why is this needed?

Answer 1

Because the HashSet contract specifies that remove() return true if the specified object existed and was removed. To do this, it uses the wrapped HashMap#remove() which returns the removed value.

If you were to store null instead of an object, then the call to HashMap#remove() would return null, which would be indistinguishable from the result of attempting to remove a non-existent object, and the contract of HashSet.remove() could not be fulfilled.