HashMap put 方法

最基础的put方法源码: 调用putVal()方法去存放数据，首先对key进行hash方法取得hash值，

public V put(K key, V value) {

return putVal(hash(key), key, value, false, true);

}

首先对当前HashMap进行判断，如果是空的话，进行扩容操作，因为HashMap初始化的时候并没有对tab进行初始化。

源码：

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,

boolean evict) {

Node<K,V>[] tab; Node<K,V> p; int n, i;

if ((tab = table) == null || (n = tab.length) == 0)

n = (tab = resize()).length;

if ((p = tab[i = (n - 1) & hash]) == null)

tab[i] = newNode(hash, key, value, null);

else {

Node<K,V> e; K k;

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;

}

//这个就是如果key不是null的情况对key的hashcode进行异或运算

static final int hash(Object key) {

int h;

return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);

}