java基础，集合，HashSet，源码理解

1、hashset 不重复的集合，类似于数学中的集

2、内部结构 基于hashmap，使用key唯一性，value值可以重复的特性；　 

 private transient HashMap map;
　　//value都是一个值
    private static final Object PRESENT = new Object();
    public HashSet()
    {
        map = new HashMap();
    }

　　

//添加一个元素的时候，调用hashmap的添加对象
     public boolean add(Object obj)
    {
        return map.put(obj, PRESENT) == null;
    }

     public Object put(Object obj, Object obj1)
    {
        return putVal(hash(obj), obj, obj1, false, true);
    }

//这个部分是jdk1.8的添加的方法 推荐下面的博客看实现原理
//http://blog.csdn.net/qq_27093465/article/details/70314904

public Object put(Object obj, Object obj1)
    {
        return putVal(hash(obj), obj, obj1, false, true);
    }
    final Object putVal(int i, Object obj, Object obj1, boolean flag, boolean flag1)
    {
        Node anode[];
        int j;
        if((anode = table) == null || (j = anode.length) == 0)
            j = (anode = resize()).length;
        Object obj2;
        int k;
        if((obj2 = anode[k = j - 1 & i]) == null)
        {
            anode[k] = newNode(i, obj, obj1, null);
        } else
        {
            Object obj3;
            Object obj4;
            if(((Node) (obj2)).hash == i && ((obj4 = ((Node) (obj2)).key) == obj || obj != null && obj.equals(obj4)))
                obj3 = obj2;
            else
            if(obj2 instanceof TreeNode)
            {
                obj3 = ((TreeNode)obj2).putTreeVal(this, anode, i, obj, obj1);
            } else
            {
                int l = 0;
                do
                {
                    if((obj3 = ((Node) (obj2)).next) == null)
                    {
                        obj2.next = newNode(i, obj, obj1, null);
                        if(l >= 7)
                            treeifyBin(anode, i);
                        break;
                    }
                    Object obj5;
                    if(((Node) (obj3)).hash == i && ((obj5 = ((Node) (obj3)).key) == obj || obj != null && obj.equals(obj5)))
                        break;
                    obj2 = obj3;
                    l++;
                } while(true);
            }
            if(obj3 != null)
            {
                Object obj6 = ((Node) (obj3)).value;
                if(!flag || obj6 == null)
                    obj3.value = obj1;
                afterNodeAccess(((Node) (obj3)));
                return obj6;
            }
        }
        modCount++;
        if(++size > threshold)
            resize();
        afterNodeInsertion(flag1);
        return null;
    }
// 获得hash的值
 static final int hash(Object obj)
    {
        int i;
        return obj != null ? (i = obj.hashCode()) ^ i >>> 16 : 0;
    }

set集合遍历 也是调用hashmap中key值的方法

    public Iterator iterator()
    {
        return map.keySet().iterator();
    }

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

　　

没有看源码的时都没有想到，hashset内部居然通过hashmap中key实现，