### 1.2.1 HashMap(JDK 1.8)
#### 构造函数
#### hash(Object key)
```java
static final int hash(Object key) {
int h;
// 判断是否key是否为null
// 若为null 则返回0
// 若不为null 获取key的hashCode 将hashCode的高16位与低16位进行与运算
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
```
#### put(K key,V value)
```java
public V put(K key,V value){
return putVal(hash(key), key, value, false, true);
}
```
```java
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,boolean evict) {
Node<K,V>[] tab; Node<K,V> p; int n, i;
// 先给tab赋值，判断tab是否为null 给n赋值 并判断n是否为0 若tab为null或者n==0
if ((tab = table) == null || (n = tab.length) == 0)
// tab初始化 并给n赋值
n = (tab = resize()).length;
// 先给i赋值 获取tab[i]并赋值给p
// 若p为null 给tab[i]赋值
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
Node<K,V> e; K k;
// 判断p节点的key和hash值是否跟传入的相等
// 如果相等, 则p节点即为要查找的目标节点，将p节点赋值给e节点
if (p.hash == hash && ((k = p.key) == key || (key != null && key.equals(k))))
e = p;
// 若p为红黑树节点 通过putTreeVal方法查找目标节点
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
// 普通节点 调用普通的链表方法进行查找 使用binCount统计链表的节点树
for (int binCount = 0; ; ++binCount) {
// 若下一个节点为null 则代表查找不到目标节点 并新增一个节点插入链表的结尾
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
// 判断节点数超多8 则通过treeifyBin方法转化为红黑树节点
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
// 若e的hash和key的值相同 则e节点为目标节点 跳出循环
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
// 若e不为null 则代表目标节点存在 使用传入的value覆盖该节点的value 并返回oldValue
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
// 如果size大于阈值 调用resize()方法扩容
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
```
#### get(Object key)
```java
public V get(Object key) {
Node<K,V> e;
// 根据hash和key获取节点
return (e = getNode(hash(key), key)) == null ? null : e.value;
}
```
```java
final Node<K,V> getNode(int hash, Object key) {
Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
// first是从tab数组中查找
if ((tab = table) != null && (n = tab.length) > 0 &&
(first = tab[(n - 1) & hash]) != null) {
// first验证成功 则返回first 如果没有查找到 则从链表中查找
// always check first node
if (first.hash == hash &&
((k = first.key) == key || (key != null && key.equals(k))))
return first;
// 若e=first的下一个节点不为空
if ((e = first.next) != null) {
// 如是红黑树 从红黑树查找
if (first instanceof TreeNode)
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
// 如果不是红黑树 则返回元素
do {
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}
```
#### remove(Object key)
```java
public V remove(Object key) {
Node<K,V> e;
return (e = removeNode(hash(key), key, null, false, true)) == null ?
null : e.value;
}
```
```java
final Node<K,V> removeNode(int hash, Object key, Object value,
boolean matchValue, boolean movable) {
Node<K,V>[] tab; Node<K,V> p; int n, index;
// 通过tab数组中查找获取节点 诺查找通过 则node赋值为p 负责从链表中查找
if ((tab = table) != null && (n = tab.length) > 0 &&
(p = tab[index = (n - 1) & hash]) != null) {
Node<K,V> node = null, e; K k; V v;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
node = p;
// 从链表中查找
else if ((e = p.next) != null) {
// 若p是红黑树 则从红黑树中查找
if (p instanceof TreeNode)
node = ((TreeNode<K,V>)p).getTreeNode(hash, key);
else {
do {
if (e.hash == hash &&
((k = e.key) == key ||
(key != null && key.equals(k)))) {
node = e;
break;
}
p = e;
} while ((e = e.next) != null);
}
}
// 匹配节点
if (node != null && (!matchValue || (v = node.value) == value ||
(value != null && value.equals(v)))) {
if (node instanceof TreeNode)
((TreeNode<K,V>)node).removeTreeNode(this, tab, movable);
else if (node == p)
tab[index] = node.next;
else
p.next = node.next;
++modCount;
--size;
afterNodeRemoval(node);
return node;
}
}
return null;
}
```
