JUC-Set

  HashSet构造函数：默认容量为16的HashMap。（底层是HashMap）

  /**
     * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
     * default initial capacity (16) and load factor (0.75).
     */
    public HashSet() {
        map = new HashMap<>();
    }

  HashSet的add方法：只关心键值，value为常量。

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

  CopyOnWriteArraySet解决Set线程不安全性问题，代码如下：

/**
 * @author luffy
 **/
public class HashSetDemo {
    public static void main(String[] args){
        Set<String> set = new CopyOnWriteArraySet<>();
        //Set<String> set = Collections.synchronizedSet(new HashSet<>());
        for(int i =0 ;i< 30;i++){
            new Thread(()->{
                set.add(UUID.randomUUID().toString().substring(0,8));
                System.out.println(set);
            },String.valueOf(i)).start();
        }
    }
}

  CopyOnWriteArraySet的构造方法，底层也是CopyOnWriteArrayList。

public CopyOnWriteArraySet() {
    al = new CopyOnWriteArrayList<E>();
}