### 1.2.1 HashMap(JDK 1.8)
#### 構造函數
#### hash(Object key)
```java
static final int hash(Object key) {
int h;
// 判斷是否key是否為null
// 若為null 則返回0
// 若不為null 獲取key的hashCode 將hashCode的高16位與低16位進行與運算
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;
// 先給tab賦值，判斷tab是否為null 給n賦值 并判斷n是否為0 若tab為null或者n==0
if ((tab = table) == null || (n = tab.length) == 0)
// tab初始化 并給n賦值
n = (tab = resize()).length;
// 先給i賦值 獲取tab[i]并賦值給p
// 若p為null 給tab[i]賦值
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
Node<K,V> e; K k;
// 判斷p節點的key和hash值是否跟傳入的相等
// 如果相等, 則p節點即為要查找的目標節點，將p節點賦值給e節點
if (p.hash == hash && ((k = p.key) == key || (key != null && key.equals(k))))
e = p;
// 若p為紅黑樹節點 通過putTreeVal方法查找目標節點
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
// 普通節點 調用普通的鏈表方法進行查找 使用binCount統計鏈表的節點樹
for (int binCount = 0; ; ++binCount) {
// 若下一個節點為null 則代表查找不到目標節點 并新增一個節點插入鏈表的結尾
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
// 判斷節點數超多8 則通過treeifyBin方法轉化為紅黑樹節點
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
// 若e的hash和key的值相同 則e節點為目標節點 跳出循環
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
// 若e不為null 則代表目標節點存在 使用傳入的value覆蓋該節點的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大于閾值 調用resize()方法擴容
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
```
#### get(Object key)
```java
public V get(Object key) {
Node<K,V> e;
// 根據hash和key獲取節點
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是從tab數組中查找
if ((tab = table) != null && (n = tab.length) > 0 &&
(first = tab[(n - 1) & hash]) != null) {
// first驗證成功 則返回first 如果沒有查找到 則從鏈表中查找
// always check first node
if (first.hash == hash &&
((k = first.key) == key || (key != null && key.equals(k))))
return first;
// 若e=first的下一個節點不為空
if ((e = first.next) != null) {
// 如是紅黑樹 從紅黑樹查找
if (first instanceof TreeNode)
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
// 如果不是紅黑樹 則返回元素
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;
// 通過tab數組中查找獲取節點 諾查找通過 則node賦值為p 負責從鏈表中查找
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;
// 從鏈表中查找
else if ((e = p.next) != null) {
// 若p是紅黑樹 則從紅黑樹中查找
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);
}
}
// 匹配節點
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;
}
```
