HashSet的結(jié)構(gòu)

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;

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

????HastSet的底層存儲(chǔ)結(jié)構(gòu)是一個(gè)名字叫做map的HashMap，因?yàn)镠ashSet是單值的，非鍵值對(duì)，把HashSet的值用作了HashMap的Key，Value就是上面代碼中的PRESENT。

構(gòu)造函數(shù)

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

 public HashSet(Collection<? extends E> 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當(dāng)中，那么加入元素e，并返回true，否則返回false

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

????如果此集合包含指定的元素則返回true，否則返回false

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

????在Set中刪除元素o，如果刪除成功返回true，否則返回false

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