HashSet集合保证元素唯一性源码分析
//创建集合对象
HashSet<String>  hs = new HashSet<String>();
//添加元素
hs.add("hello");
hs.add("world");
hs.add("java");
hs.add("world");
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public boolean add(E e) {
    return map.put(e, PRESENT)==null;
}

static final int hash(Object key) {
    int h;
    return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}

public V put(K key, V value) {
    return putVal(hash(key), key, value, false, true);
}

//hash值和元素的HashCode()相关
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
               boolean evict) {
    Node<K,V>[] tab; Node<K,V> p; int n, i;
    //如果哈希表未初始化，就对其进行初始化
    if ((tab = table) == null || (n = tab.length) == 0)
        n = (tab = resize()).length;
    //根据对象的哈希值计算对象的存储位置，如果该位置没有元素，就存储元素
    if ((p = tab[i = (n - 1) & hash]) == null)
        tab[i] = newNode(hash, key, value, null);
    else {
        Node<K,V> e; K k;
        /*
            存入的元素和以前的元素比较哈希值
            如果哈希值不同，会继续向下执行，把元素添加到集合
            如果哈希值相同，会调用对象的equals()方法比较
               如果返回false,会继续向下执行，把元素添加到集合
               如果返回true,说明元素重复，不存储

        */
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            e = p;
        else if (p instanceof TreeNode)
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        else {
            for (int binCount = 0; ; ++binCount) {
                if ((e = p.next) == null) {
                    p.next = newNode(hash, key, value, null);
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                        treeifyBin(tab, hash);
                    break;
                }
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    break;
                p = e;
            }
        }
        if (e != null) { // existing mapping for key
            V oldValue = e.value;
            if (!onlyIfAbsent || oldValue == null)
                e.value = value;
            afterNodeAccess(e);
            return oldValue;
        }
    }
    ++modCount;
    if (++size > threshold)
        resize();
    afterNodeInsertion(evict);
    return null;