HashSet怎样保证元素不重复

HashSet其实就是HashMap套上了Set的马甲

HashSet内部维护了一个HashMap;

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();

    public HashSet() {
        map = new HashMap<>();
    }
    public boolean add(E e) {
        return map.put(e, PRESENT)==null;
    }
    //略....
}

总结:HashMap的key是不能重复的，而HashSet的元素又是作为map的key，所以HashSet的元素也不会重复了。

例子:

public static void main(String[] args) {
		
		HashSet<String> hs = new HashSet<String>();
		hs.add("测");
		hs.add("试");
		hs.add("去");
		hs.add("重");
		hs.add("!");
		hs.add("!");
		hs.add("!");
	
		Iterator<String> it = hs.iterator();
		while(it.hasNext()){
			System.out.println(it.next());
		}
}

打印:

另:HashMap则是依赖HashCode保证key不重复的