About the implementation of HashSet.add( ) method

Question

Hi guys,
recently, I read the source code of HashSet, about the code of method of add():
public boolean add(E e) {
     return map.put(e, PRESENT)==null;
I wonder why we need put 'PRESENT' to the value ? why not we put NULL to the value , like this:
return map.put(e, NULL)==null;
Please tell me the truth, thx a lot.

Answer 1

YoungWinston wrote:
user11230334 wrote:
I wonder why we need put 'PRESENT' to the value ? why not we put NULL to the value , like this:
return map.put(e, NULL)==null;
Please tell me the truth, thx a lot.As I recall, A HashSet<K> is actually backed by a HashMap<K, Boolean>, the Boolean being set to Boolean.TRUE Nope. <K, Object>
(and I suspect you'll find that PRESENT == Boolean.TRUE) for all entries.Nope:
    private static final Object PRESENT = new Object();
- and furthermore is guaranteed to return null if the key didn't originally existAs EJP (and somebody else, I believe) pointed out, this is probably the heart of the reason to use an Object instead of null for the value.