HashMap源碼分析
- 主要方法源碼解讀
-
- put方法
- resize方法
- get方法
- Hash方法
主要方法源碼解讀
put方法
下面展示一些
内聯代碼片
。
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
Node<K,V>[] tab; Node<K,V> p; int n, i;
//确認table是否為空或者table的長度是否為0（隻有在第一次put的時候才會初始化數組）
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
//p = tab[i = (n - 1) & hash] 這塊兒判斷是否有hash沖突，沒有的話直接建立新的node
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
Node<K,V> e; K k;
//判斷是否是重複資料插入，将重複e設定為重複Node
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
//判斷對應的元素是否是紅黑樹
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
//将hash沖突的元素下挂至連結清單
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
//如果該連結清單長度大于等于7，調用 treeifyBin(tab, hash) 方法去将連結清單轉換成紅黑樹
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
//判斷連結清單中是否有重複元素
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;
//如果key已經存在或之前的value為空，更新value的值，傳回老value的值
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
//modCount++，++size如果超過了數組長度*擴容因子，進行擴容
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
resize方法
final Node<K,V>[] resize() {
Node<K,V>[] oldTab = table;
//擷取舊數組的長度與擴容觸發長度
int oldCap = (oldTab == null) ? 0 : oldTab.length;
int oldThr = threshold;
int newCap, newThr = 0;
if (oldCap > 0) {
//如果舊數組的長度超過了2^30，直接将擴容觸發長度增加至Integer.MAX_VALUE，傳回舊數組
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
//設定新數組的長度為舊數組的2倍，如果舊數組的長度*2小于2^30且大于預設長度16，新數組的擴容觸發長度設定為舊數組擴容觸發長度的2倍
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY && oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // double threshold
}
else if (oldThr > 0) // initial capacity was placed in threshold
newCap = oldThr;
else { // zero initial threshold signifies using defaults
newCap = DEFAULT_INITIAL_CAPACITY;
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}
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;
if (oldTab != null) {
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
if ((e = oldTab[j]) != null) {
oldTab[j] = null;
//如果該下标沒有連結清單
if (e.next == null)
newTab[e.hash & (newCap - 1)] = e;
//如果該下标元素屬于樹
else if (e instanceof TreeNode)
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
else { // preserve order
Node<K,V> loHead = null, loTail = null;
Node<K,V> hiHead = null, hiTail = null;
Node<K,V> next;
do {
next = e.next;
//根據e.hash & oldCap判斷是否将原下标的連結清單分割
if ((e.hash & oldCap) == 0) {
if (loTail == null)
loHead = e;
else
loTail.next = e;
loTail = e;
}
else {
if (hiTail == null)
hiHead = e;
else
hiTail.next = e;
hiTail = e;
}
} while ((e = next) != null);
//将e.hash & oldCap == 0的連結清單放在原下标下
if (loTail != null) {
loTail.next = null;
newTab[j] = loHead;
}
//将e.hash & oldCap ！= 0的連結清單放在（原下标+舊數組元素長度）
if (hiTail != null) {
hiTail.next = null;
newTab[j + oldCap] = hiHead;
}
}
}
}
}
return newTab;
}
get方法
final Node<K,V> getNode(int hash, Object key) {
Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
if ((tab = table) != null && (n = tab.length) > 0 &&
(first = tab[(n - 1) & hash]) != null) {
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k))))
return first;
if ((e = first.next) != null) {
if (first instanceof TreeNode)
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
do {
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}
感覺get方法沒啥可看的，都是流水的代碼，唯一覺得NB的就是(n - 1) & hash直接定位了元素下标
Hash方法
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
這塊兒我是看這個部落客的了解了下标定位的部分
https://blog.csdn.net/weixin_42340670/article/details/80574965
稍微有點流水賬，我也隻是捋了捋邏輯，很多方法還沒有深入
建議自己設計一個類繼承Object類，然後重寫HashCode方法，試着debug進去自己跟着源碼走一遍，會清晰很多
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