HashSet

HashSet源码：

public class HashSet<E>
    extends AbstractSet<E>
    implements Set<E>, Cloneable, java.io.Serializable
{
    static final long serialVersionUID = -5024744406713321676L;

    private transient HashMap<E,Object> map;

    private static final Object PRESENT = new Object();

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

public boolean remove(Object o) {
    return map.remove(o)==PRESENT;
}

实际上HashSet就是封装了一个HashMap对象来存储对象，用Key来保存对象，而value则是保存了一个静态Object对象。并且大部分方法调用map的方法实现。

比如add方法，在调用map的put方法时判断返回的对象是不是null来区分是不是添加进去key. 因为map的put方法返回的是key值对应的旧对象：

/**
 * Associates the specified value with the specified key in this map.
 * If the map previously contained a mapping for the key, the old
 * value is replaced.
 *
 * @param key key with which the specified value is to be associated
 * @param value value to be associated with the specified key
 * @return the previous value associated with <tt>key</tt>, or
 *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
 *         (A <tt>null</tt> return can also indicate that the map
 *         previously associated <tt>null</tt> with <tt>key</tt>.)
 */
public V put(K key, V value) {
    return putVal(hash(key), key, value, false, true);
}