is HashSet is implemented via a HashMap instance

Question

I am wondering since the HashSet is implemented via a HashMap instance , what would be the key that would be used to put data into HashSet.

i gone through the link http://www.coderanch.com/t/251832/Programmer-Certification-SCJP/certification/Difference-HashMap-HashSet...

which i dint understood properly.. Can anybody help me to understand it better

Answer 1

From the source:

// 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;
}

Answer 2

the key would be the object that went into the hashset itself since keys of maps are sets.

Answer 3

The idea is to use the object you add to the HashSet as a key of the HashMap. That way the add, remove, and contains run in O(1).

Answer 4

Yes (source code here). HashSet is essentially an interface to a HashMap's keySet.

   /**
    * HashSet is an implementation of a Set. All optional operations (adding and
    * removing) are supported. The elements can be any objects.
    */
   public class HashSet<E> extends AbstractSet<E> implements Set<E>, Cloneable,
           Serializable {

       private static final long serialVersionUID = -5024744406713321676L;

       transient HashMap<E, HashSet<E>> backingMap; // right here!