2、put方法简单解析：
public V put(K key, V value) {//调用putVal()方法完成return putVal(hash(key), key, value, false, true);}final V putVal(int hash, K key, V value, boolean onlyIfAbsent,boolean evict) {Node[] tab; Node p; int n, i;//判断table是否初始化，否则初始化操作if ((tab = table) == null || (n = tab.length) == 0)n = (tab = resize()).length;//计算存储的索引位置，如果没有元素，直接赋值if ((p = tab[i = (n - 1) & hash]) == null)tab[i] = newNode(hash, key, value, null);else {Node e; K k;//节点若已经存在，执行赋值操作if (p.hash == hash &&((k = p.key) == key || (key != null && key.equals(k))))e = p;//判断链表是否是红黑树else if (p instanceof TreeNode)//红黑树对象操作e = ((TreeNode)p).putTreeVal(this, tab, hash, key, value);else {//为链表，for (int binCount = 0; ; ++binCount) {if ((e = p.next) == null) {p.next = newNode(hash, key, value, null);//链表长度8，将链表转化为红黑树存储if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1sttreeifyBin(tab, hash);break;}//key存在，直接覆盖if (e.hash == hash &&((k = e.key) == key || (key != null && key.equals(k))))break;p = e;}}if (e != null) { // existing mapping for keyV oldValue = http://www.kingceram.com/post/e.value;if (!onlyIfAbsent || oldValue == null)e.value = value;afterNodeAccess(e);return oldValue;}}//记录修改次数++modCount;//判断是否需要扩容if (++size> threshold)resize();//空操作afterNodeInsertion(evict);return null;}
【源码解析HashMap底层实现和原理】如果存在key节点，返回旧值，如果不存在则返回Null 。
