HashSet

HashSet的结构

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

    private transient HashMap map;

    // Dummy value to associate with an Object in the backing Map
    private static final Object PRESENT = new Object();
}

    HastSet的底层存储结构是一个名字叫做map的HashMap，因为HashSet是单值的，非键值对，把HashSet的值用作了HashMap的Key，Value就是上面代码中的PRESENT。

构造函数

public HashSet() {
        map = new HashMap<>();
    }

 public HashSet(Collection c) {
        map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
        addAll(c);
    }

    public HashSet(int initialCapacity, float loadFactor) {
        map = new HashMap<>(initialCapacity, loadFactor);
    }

    public HashSet(int initialCapacity) {
        map = new HashMap<>(initialCapacity);
    }

常用API

    如果元素e不存在Set当中，那么加入元素e，并返回true，否则返回false

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

    如果此集合包含指定的元素则返回true，否则返回false

  public boolean contains(Object o) {
        return map.containsKey(o);
    }

    在Set中删除元素o，如果删除成功返回true，否则返回false

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