HashSet是基于HashMap来实现的，操作很简单，更像是对HashMap做了一次“封装”，而且只使用了HashMap的key来实现各种特性。

1. 类的定义

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();
	/*...*/
}

2.主要操作

 public boolean add(E e) {
        return map.put(e, PRESENT)==null;
    }
	public boolean remove(Object o) {
        return map.remove(o)==PRESENT;
    }
	 public boolean contains(Object o) {
        return map.containsKey(o);
    }