private transient HashMap<E,Object> map;//HashMap集合
    private static final Object PRESENT = new Object();//調(diào)用HashMap方法，裝入的value值

   public class HashSet<E>
    extends AbstractSet<E>//繼承了AbstractSet
    implements Set<E>,//實現(xiàn)set方法
     Cloneable,//標記本類可以被克隆
      java.io.Serializable//可以被序列化

     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);
    }
    HashSet(int initialCapacity, float loadFactor, boolean dummy) {
        map = new LinkedHashMap<>(initialCapacity, loadFactor);
    }

   public boolean add(E e) {//只能添加一個null值
        return map.put(e, PRESENT)==null; 
		//HashMap的put方法，如果是新添加的結點，會返回null；
		//如果是原有結點，那么會返回value值，這樣返回的就是false這樣表明集合中已經(jīng)有該元素；
    }

    public boolean remove(Object o) {
        return map.remove(o)==PRESENT;
		//HashSet添加進HashMap中所有value值都是PRESENT，
		//如果找到key值，那么返回的value值肯定是PRESENT；
		//沒有找到，返回的是null；
    }

        ArrayList<Integer> arrayList=new ArrayList<>(100000);
        for (int i = 0; i <100000 ; i++) {
            arrayList.add((int) (Math.random()*1000));
        }
        iterator=arrayList.iterator();
        HashSet<Integer> hashSet1=new HashSet<>();
        while(iterator.hasNext()){
            Integer val= iterator.next();
            if(hashSet1.add(val)){//第一次添加才會返回true
                System.out.println(val);
            }
        }