HashMap实现原理

   HashMap()
   HashMap(int initialCapacity)
   HashMap(int initialCapacity,float loadFactor)
   HashMap(Map<? extends K, ? extends V> m)

   DEFAULT_INITIAL_CAPACITY = 1 << 4  // 初始化容量 16
   MAXIMUM_CAPACITY = 1 << 30           //  最大容量 1 073 741 824
   DEFAULT_LOAD_FACTOR = 0.75f    // 负载因子， 超过 ‘容量*负载因子’，开始扩容 *2
   TREEIFY_THRESHOLD = 8          // 一个桶树化的阀值，当key.hashcode,相同超过8，会转成红黑树
   UNTREEIFY_THRESHOLD = 6        //一个桶链表化的阀值，当key.hashcode,相同低于6，会转成链表
   MIN_TREEIFY_CAPACITY = 64      //哈希表最小树形化容量，<64,即使key.hashcode超过8也不会树化

public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        //1、初始化hash表，由此可见 hashMap的初始化是在调用时，不是在调用构造函数时
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        //2、计算key存放下标：当前数组大小-1 & 当前hash值。如果此下标下没有元素，直接放入桶中
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else {
            Node<K,V> e; K k;
            //3、如果节点存在直接替换旧值
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            //4、红黑树
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            //5、链表
            else {
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        //链表长度大于8转为红黑树处理
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);
                        break;
                    }
                    //key 存在直接覆盖旧值
                    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;
                //allbacks to allow LinkedHashMap post-actions
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
       //6、超过容量，就扩容 
        if (++size > threshold)
            resize();
        //allbacks to allow LinkedHashMap post-actions 为linkedHashMap服务的，输入和输出有序
        afterNodeInsertion(evict);
        return null;
    }

public V get(Object key) {
        Node<K,V> e;
        return (e = getNode(hash(key), key)) == null ? null : e.value;
    }
 final Node<K,V> getNode(int hash, Object key) {
        Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
         
        if ((tab = table) != null && (n = tab.length) > 0 &&
            (first = tab[(n - 1) & hash]) != null) {
               //先判断头节点，算法和插入时一致 (n-1)&hash
            if (first.hash == hash && // always check first node
                ((k = first.key) == key || (key != null && key.equals(k))))
                return first;
            if ((e = first.next) != null) {
                //如果头节点是树节点
                if (first instanceof TreeNode)
                    return ((TreeNode<K,V>)first).getTreeNode(hash, key);
                //递归查找与key相同桶的节点
                do {
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        return e;
                } while ((e = e.next) != null);
            }
        }
        return null;
    }