HashMapPut流程原理

Put方法

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

进入putVal方法

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
               boolean evict) {
    Node<K,V>[] tab; Node<K,V> p; int n, i;
    //如果当前table为空  length为空时，resize()是扩容
    if ((tab = table) == null || (n = tab.length) == 0)
        n = (tab = resize()).length;
        //如果当前Hash值的tab下为空的话直接添加进去    
    if ((p = tab[i = (n - 1) & hash]) == null)
        tab[i] = newNode(hash, key, value, null);
    else {
        Node<K,V> e; K k;
        //如果有当前hash值又是相同的Key 则直接覆盖健值  （相当于修改键值对操作）
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            e = p;
        //p树节点直接添加
        else if (p instanceof TreeNode)
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        else {
        //最后不行的话直接添加到链表最后    
            for (int binCount = 0; ; ++binCount) {
                //节点向后遍历
                if ((e = p.next) == null) {
                    //到链表最后添加节点
                    p.next = newNode(hash, key, value, null);
                    //链表长度大于8红转换为红黑树
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                        treeifyBin(tab, hash);
                    break;
                }
                //查找到相等的key 直接break
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    break;
                 //节点继续向后遍历
               p = e;
            }
        }
        
        //替换e的value
        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;
}

put流程图