Implementation of add method in java HashSet.

HashSet

public class HashSet<E>
    extends AbstractSet<E>
    implements Set<E>, Cloneable, java.io.Serializable
{
    
    private transient HashMap<E,Object> map;

    // Dummy value to associate with an Object in the backing Map
    private static final Object PRESENT = new Object();
    
    /**
     * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
     * default initial capacity (16) and load factor (0.75).
     */
    public HashSet() {
    	map = new HashMap<E,Object>();
    }
    
    public boolean add(E e) {
	    return map.put(e, PRESENT)==null;
    }
    
}