HashMap-陣列+連結串列集合
阿新 • • 發佈：2019-01-02
field
常量
//預設初始化容量，最好為2的冪
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
//最大容量
static final int MAXIMUM_CAPACITY = 1 << 30;
//預設載入因子
static final float DEFAULT_LOAD_FACTOR = 0.75f;
//由雜湊衝突的連結串列結構轉為平衡二叉樹結構節點數閾值（桶的數量需要大於MIN_TREEIFY_CAPACITY ）
static 變數
//hash節點陣列
transient Node<K,V>[] table;
//元素節點
transient Set<Map.Entry<K,V>> entrySet;
//大小 method
tableSizeFor
//相當機智的演算法，用來固定容量為2的倍數
static final int tableSizeFor(int cap) { //10000
int n = cap - 1; //可能初始化就為2的倍數，則減去1 1111
n |= n >>> 1 hash
//高16位於hashcode的低16位 異或取值，保證高16位和低16位的變化同時影響hash值
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}resize
//擴容操作，將舊hash表資料移到新的hash表
final Node<K,V>[] resize() {
Node<K,V>[] oldTab = table;
int oldCap = (oldTab == null) ? 0 : oldTab.length;
int oldThr = threshold;
int newCap, newThr = 0;
//如果原來已經初始化過，若原有容量不超過極限值，則擴容兩倍
if (oldCap > 0) {
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // double threshold
}
// 初始化的容量可能被替換（入參）
else if (oldThr > 0) // initial capacity was placed in threshold
newCap = oldThr;
//初始化定義
else { // zero initial threshold signifies using defaults
newCap = DEFAULT_INITIAL_CAPACITY;
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}
//計算新的threshold
if (newThr == 0) {
float ft = (float)newCap * loadFactor;
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
(int)ft : Integer.MAX_VALUE);
}
threshold = newThr;
@SuppressWarnings({"rawtypes","unchecked"})
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
table = newTab;
//將舊hash表移到新的hash表
if (oldTab != null) {
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
//如果當前舊節點不為空的情況下
if ((e = oldTab[j]) != null) {
oldTab[j] = null;
//如果該節點沒有hash衝突，是單節點
if (e.next == null)
//直接將hash值與桶的容量與運算求桶的索引位。
newTab[e.hash & (newCap - 1)] = e;
else if (e instanceof TreeNode)
//如果e是平衡樹節點，則新增到平衡樹中
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
//該節點下仍有其它元素，需要全部轉移
else { // preserve order
Node<K,V> loHead = null, loTail = null;
Node<K,V> hiHead = null, hiTail = null;
Node<K,V> next;
//超級超級機智的做法，理解後真是深深敬佩
do {
next = e.next;
//與原來的容量做與運算
//只有兩種結果： 0 或 oldCap（2的冪，這是必然的）
//這是hash本來就小於oldCap的情況
if ((e.hash & oldCap) == 0) {
//將剩下的節點逐個copy
if (loTail == null)
loHead = e;
else
loTail.next = e;
loTail = e;
}
//這是hash本來就大於oldCap的情況
else {
//將剩下的節點逐個copy
if (hiTail == null)
hiHead = e;
else
hiTail.next = e;
hiTail = e;
}
//多執行緒條件下可能反正死迴圈
//迴圈列表
} while ((e = next) != null);
//小於oldCap的索引，如果有節點資料，則保持不變
if (loTail != null) {
loTail.next = null;
newTab[j] = loHead;
}
//大於oldCap的索引則，如果有節點資料，在原基礎上加上oldCap
if (hiTail != null) {
hiTail.next = null;
newTab[j + oldCap] = hiHead;
}
}
}
}
}
return newTab;
}putVal
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tab為空或者長度為0，則初始化
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
//如果桶中計算出的索引無hash衝突，則直接新增
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
//具有hash衝突
Node<K,V> e; K k;
//如果hash值，key值都相同，則覆蓋
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
//紅黑樹的節點
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
for (int binCount = 0; ; ++binCount) {
//如果p的下一個元素為null，則將元素新增到P後
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
//到達節點數閾值，則轉變為紅黑樹
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
//如果有重複key，則覆蓋
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
//e 不為null ，則表示已存在相同的key
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
//超過容量就擴容
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}getNode
final Node<K,V> getNode(int hash, Object key) {
Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
//check 是否含有該元素
if ((tab = table) != null && (n = tab.length) > 0 &&
(first = tab[(n - 1) & hash]) != null) {
//檢查連結串列或者二叉樹第一個元素
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k))))
return first;
//該節點不在第一個，開始迴圈檢查
if ((e = first.next) != null) {
//紅黑樹的情況
if (first instanceof TreeNode)
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
//迴圈連結串列
do {
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}