目錄:
1.數(shù)據(jù)結(jié)構(gòu)
2.put插入
3.get查找
4.resize擴(kuò)容
5.HashMap節(jié)點(diǎn)紅黑樹(shù)存儲(chǔ)
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如上圖所示,JDK7之前hashmap又叫散列鏈表:基于一個(gè)數(shù)組以及多個(gè)鏈表的實(shí)現(xiàn),hash值沖突的時(shí)候,就將對(duì)應(yīng)節(jié)點(diǎn)以鏈表的形式存儲(chǔ)。
JDK8中,當(dāng)同一個(gè)hash值(Table上元素)的鏈表節(jié)點(diǎn)數(shù)不小于8時(shí),將不再以單鏈表的形式存儲(chǔ)了,會(huì)被調(diào)整成一顆紅黑樹(shù)。這就是JDK7與JDK8中HashMap實(shí)現(xiàn)的最大區(qū)別。
源代碼如下:
1 /** 2 * Implements Map.put and related methods 3 * 實(shí)現(xiàn)Map的put和相關(guān)操作 4 * @param hash hash for key 計(jì)算出來(lái)的key的hash值 5 * @param key the key 鍵 6 * @param value the value to put 值 7 * @param onlyIfAbsent if true, don't change existing value 為true則不覆蓋已存在值 8 * @param evict if false, the table is in creation mode. 9 * @return previous value, or null if none10 */11 final V putVal(int hash, K key, V value, boolean onlyIfAbsent,12 boolean evict) {13 //tab數(shù)組節(jié)點(diǎn) p當(dāng)前需要插入的節(jié)點(diǎn) 14 Node<K,V>[] tab; Node<K,V> p; int n, i;15 //如果當(dāng)前map中無(wú)數(shù)據(jù),執(zhí)行resize方法。并且返回n16 if ((tab = table) == null || (n = tab.length) == 0)17 n = (tab = resize()).length;18 //如果要插入的鍵值對(duì)在tab上剛好沒(méi)有元素,那么把他封裝成Node對(duì)象,放在這個(gè)位置上結(jié)束。19 if ((p = tab[i = (n - 1) & hash]) == null)20 tab[i] = newNode(hash, key, value, null);21 //有元素,在table的i位置發(fā)生碰撞22 else {23 Node<K,V> e; K k;24 //1.hash值一樣,key一樣,替換25 if (p.hash == hash &&26 ((k = p.key) == key || (key != null && key.equals(k))))27 e = p;28 //2.如果當(dāng)前節(jié)點(diǎn)是TreeNode類型的數(shù)據(jù),插入紅黑樹(shù)29 else if (p instanceof TreeNode)30 e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);31 //3.不是TreeNode,遍歷這條鏈子上的節(jié)點(diǎn),跟jdk7一樣32 else {33 for (int binCount = 0; ; ++binCount) {34 if ((e = p.next) == null) {35 p.next = newNode(hash, key, value, null);36 //如果大于等于7,即準(zhǔn)備插入第8個(gè),將鏈表轉(zhuǎn)換成紅黑樹(shù)(有條件,具體看下圖)。37 if (binCount >= TREEIFY_THRESHOLD - 1) 38 treeifyBin(tab, hash);39 break;40 }41 if (e.hash == hash &&42 ((k = e.key) == key || (key != null && key.equals(k))))43 break;44 p = e;45 }46 }47 if (e != null) { // existing mapping for key48 V oldValue = e.value;49 if (!onlyIfAbsent || oldValue == null)50 e.value = value;51 afterNodeAccess(e);52 return oldValue;53 }54 }55 ++modCount;56 //判斷閾值,決定是否擴(kuò)容57 if (++size > threshold)58 resize();59 afterNodeInsertion(evict);60 return null;61 }
treeifyBin方法源碼如下:
1 final void treeifyBin(Node<K,V>[] tab, int hash) { 2 int n, index; Node<K,V> e; 3 //當(dāng)tab為null或tab.length<64需要進(jìn)行擴(kuò)容 4 if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY) 5 resize(); 6 else if ((e = tab[index = (n - 1) & hash]) != null) { 7 TreeNode<K,V> hd = null, tl = null; 8 do { 9 TreeNode<K,V> p = replacementTreeNode(e, null);10 if (tl == null)11 hd = p;12 else {13 p.prev = tl;14 tl.next = p;15 }16 tl = p;17 } while ((e = e.next) != null);18 if ((tab[index] = hd) != null)19 //存儲(chǔ)在紅黑樹(shù)20 hd.treeify(tab);21 }22 }
現(xiàn)在我們來(lái)看一下為什么需要擴(kuò)容:
1 /** 2 * The bin count threshold for using a tree rather than list for a 3 * bin. Bins are converted to trees when adding an element to a 4 * bin with at least this many nodes. The value must be greater 5 * than 2 and should be at least 8 to mesh with assumptions in 6 * tree removal about conversion back to plain bins upon 7 * shrinkage. 8 */ 9 static final int TREEIFY_THRESHOLD = 8;10 11 /**12 * The smallest table capacity for which bins may be treeified.13 * (Otherwise the table is resized if too many nodes in a bin.)14 * Should be at least 4 * TREEIFY_THRESHOLD to avoid conflicts15 * between resizing and treeification thresholds.16 */17 static final int MIN_TREEIFY_CAPACITY = 64;
MIN_TREEIFY_CAPACITY=64 > 4 * TREEIFY_THRESHOLD=4*8=32
如上圖:轉(zhuǎn)化紅黑樹(shù)的table容量最小容量64(JDK8定義的是64),至少是4*TREEIFY_THRESHOLD(單鏈表節(jié)點(diǎn)個(gè)數(shù)閥值,用以判斷是否需要樹(shù)形化)=32。用以避免 在擴(kuò)容和樹(shù)形結(jié)構(gòu)化的閥值 可能產(chǎn)生的沖突。所以如果小于64必須要擴(kuò)容。
可見(jiàn)外部查找和JDK7差別不大,只是原本是entry[]查詢,JDK8變成Node[]查詢.都是 hash(key)找到table中元素位置,再遍歷找到鏈表(或者是紅黑樹(shù))元素的key。根據(jù)元素類型判斷到底是查詢哪個(gè)類型
1 public V get(Object key) { 2 Node<K,V> e; 3 return (e = getNode(hash(key), key)) == null ? null : e.value; 4 } 5 6 /** 7 * Implements Map.get and related methods 8 * 9 * @param hash hash for key10 * @param key the key11 * @return the node, or null if none12 */13 final Node<K,V> getNode(int hash, Object key) {14 Node<K,V>[] tab; Node<K,V> first, e; int n; K k;15 if ((tab = table) != null && (n = tab.length) > 0 &&16 (first = tab[(n - 1) & hash]) != null) {17 if (first.hash == hash && // always check first node18 ((k = first.key) == key || (key != null && key.equals(k))))19 return first;20 if ((e = first.next) != null) {21 if (first instanceof TreeNode)22 return ((TreeNode<K,V>)first).getTreeNode(hash, key);23 do {24 if (e.hash == hash &&25 ((k = e.key) == key || (key != null && key.equals(k))))26 return e;27 } while ((e = e.next) != null);28 }29 }30 return null;31 }
第二部put的時(shí)候,有可能需要resize。 因?yàn)閚ewCap是oldCap的兩倍所以原節(jié)點(diǎn)的索引值要么和原來(lái)一樣,要么就是原(索引+oldCap)和JDK 1.7中實(shí)現(xiàn)不同這里不存在rehash
1 /** 2 * Initializes or doubles table size. If null, allocates in 3 * accord with initial capacity target held in field threshold. 4 * Otherwise, because we are using power-of-two expansion, the 5 * elements from each bin must either stay at same index, or move 6 * with a power of two offset in the new table. 7 * 8 * @return the table 9 */10 final Node<K,V>[] resize() {11 Node<K,V>[] oldTab = table;12 int oldCap = (oldTab == null) ? 0 : oldTab.length;13 int oldThr = threshold;14 int newCap, newThr = 0;15 if (oldCap > 0) {16 if (oldCap >= MAXIMUM_CAPACITY) {17 threshold = Integer.MAX_VALUE;18 return oldTab;19 }20 else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&21 oldCap >= DEFAULT_INITIAL_CAPACITY)22 newThr = oldThr << 1; // double threshold23 }24 else if (oldThr > 0) // initial capacity was placed in threshold25 newCap = oldThr;26 else { // zero initial threshold signifies using defaults27 newCap = DEFAULT_INITIAL_CAPACITY;28 newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);29 }30 if (newThr == 0) {31 float ft = (float)newCap * loadFactor;32 newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?33 (int)ft : Integer.MAX_VALUE);34 }35 threshold = newThr;36 @SuppressWarnings({"rawtypes","unchecked"})37 Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];38 table = newTab;39 if (oldTab != null) {40 for (int j = 0; j < oldCap; ++j) {41 Node<K,V> e;42 if ((e = oldTab[j]) != null) {43 oldTab[j] = null;44 if (e.next == null)45 newTab[e.hash & (newCap - 1)] = e;46 else if (e instanceof TreeNode)47 ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);48 else { // preserve order49 Node<K,V> loHead = null, loTail = null;50 Node<K,V> hiHead = null, hiTail = null;51 Node<K,V> next;52 do {53 next = e.next;54 if ((e.hash & oldCap) == 0) {55 if (loTail == null)56 loHead = e;57 else58 loTail.next = e;59 loTail = e;60 }61 else {62 if (hiTail == null)63 hiHead = e;64 else65 hiTail.next = e;66 hiTail = e;67 }68 } while ((e = next) != null);69 if (loTail != null) {70 loTail.next = null;71 newTab[j] = loHead;72 }73 if (hiTail != null) {74 hiTail.next = null;75 //把節(jié)點(diǎn)移動(dòng)新的位置j+oldCap,這種情況不適用與鏈表的節(jié)點(diǎn)數(shù)大于8的情況76 //鏈表節(jié)點(diǎn)大于8的情況會(huì)轉(zhuǎn)換為紅黑樹(shù)存儲(chǔ)77 newTab[j + oldCap] = hiHead;78 }79 }80 }81 }82 }83 return newTab;84 }
好了終于到treeify了,大部分內(nèi)容都在注解中
1 final void treeify(Node<K,V>[] tab) { 2 TreeNode<K,V> root = null; 3 for (TreeNode<K,V> x = this, next; x != null; x = next) { 4 next = (TreeNode<K,V>)x.next; 5 x.left = x.right = null; 6 if (root == null) { 7 x.parent = null; 8 x.red = false; 9 root = x;10 }11 else {12 K k = x.key;13 int h = x.hash;14 Class<?> kc = null;15 //遍歷root,把節(jié)點(diǎn)x插入到紅黑樹(shù)中,執(zhí)行先插入,然后進(jìn)行紅黑樹(shù)修正16 for (TreeNode<K,V> p = root;;) {17 int dir, ph;18 K pk = p.key;19 if ((ph = p.hash) > h)20 dir = -1;21 else if (ph < h)22 dir = 1;23 else if ((kc == null &&24 (kc = comparableClassFor(k)) == null) ||25 (dir = compareComparables(kc, k, pk)) == 0)26 dir = tieBreakOrder(k, pk);//比較k和pk的值,用于判斷是遍歷左子樹(shù)還是右子樹(shù)27 TreeNode<K,V> xp = p;28 if ((p = (dir <= 0) ? p.left : p.right) == null) {29 x.parent = xp;30 if (dir <= 0)31 xp.left = x;32 else33 xp.right = x;34 //修正紅黑樹(shù)35 root = balanceInsertion(root, x);36 //退出循環(huán)37 break;38 }39 }40 }41 }42 moveRootToFront(tab, root);43 }
上面主要做的是紅黑樹(shù)的insert,我們知道紅黑樹(shù)insert后是需要修復(fù)的,為了保持紅黑樹(shù)的平衡,我們來(lái)看下紅黑樹(shù)平衡的幾條性質(zhì):
1.節(jié)點(diǎn)是紅色或黑色。
2.根是黑色。
3.所有葉子都是黑色(葉子是NIL節(jié)點(diǎn))。
4.每個(gè)紅色節(jié)點(diǎn)必須有兩個(gè)黑色的子節(jié)點(diǎn)。(從每個(gè)葉子到根的所有路徑上不能有兩個(gè)連續(xù)的紅色節(jié)點(diǎn)。)
5.從任一節(jié)點(diǎn)到其每個(gè)葉子的所有簡(jiǎn)單路徑都包含相同數(shù)目的黑色節(jié)點(diǎn)。
當(dāng)insert一個(gè)節(jié)點(diǎn)之后為了達(dá)到平衡,我們可能需要對(duì)節(jié)點(diǎn)進(jìn)行旋轉(zhuǎn)和顏色翻轉(zhuǎn)(上面的balanceInsertion方法)。
1 static <K,V> TreeNode<K,V> balanceInsertion(TreeNode<K,V> root, 2 TreeNode<K,V> x) { 3 //插入的節(jié)點(diǎn)必須是紅色的,除非是根節(jié)點(diǎn) 4 x.red = true; 5 //遍歷到x節(jié)點(diǎn)為黑色,整個(gè)過(guò)程是一個(gè)上濾的過(guò)程 6 //xp=x.parent;xpp=xp.parent;xppl=xpp.left;xppr=xpp.right; 7 for (TreeNode<K,V> xp, xpp, xppl, xppr;;) { 8 if ((xp = x.parent) == null) { 9 x.red = false;10 return x;11 }12 //如果xp的黑色就直接完成,最簡(jiǎn)單的情況13 else if (!xp.red || (xpp = xp.parent) == null)14 return root;15 //如果x的父節(jié)點(diǎn)是x父節(jié)點(diǎn)的左節(jié)點(diǎn)16 if (xp == (xppl = xpp.left)) {17 //x的父親節(jié)點(diǎn)的兄弟是紅色的(需要顏色翻轉(zhuǎn))18 if ((xppr = xpp.right) != null && xppr.red) {19 //x父親節(jié)點(diǎn)的兄弟節(jié)點(diǎn)置成黑色20 xppr.red = false;21 //父幾點(diǎn)和其兄弟節(jié)點(diǎn)一樣是黑色22 xp.red = false;23 //祖父節(jié)點(diǎn)置成紅色24 xpp.red = true;25 //然后上濾(就是不斷的重復(fù)上面的操作)26 x = xpp;27 }28 else {29 //如果x是xp的右節(jié)點(diǎn)整個(gè)要進(jìn)行兩次旋轉(zhuǎn),先左旋轉(zhuǎn)再右旋轉(zhuǎn)30 if (x == xp.right) {31 root = rotateLeft(root, x = xp);32 xpp = (xp = x.parent) == null ? null : xp.parent;33 }34 if (xp != null) {35 xp.red = false;36 if (xpp != null) {37 xpp.red = true;38 root = rotateRight(root, xpp);39 }40 }41 }42 }43 //以左節(jié)點(diǎn)鏡像對(duì)稱就不做具體分析了 44 else {45 if (xppl != null && xppl.red) {46 xppl.red = false;47 xp.red = false;48 xpp.red = true;49 x = xpp;50 }51 else {52 if (x == xp.left) {53 root = rotateRight(root, x = xp);54 xpp = (xp = x.parent) == null ? null : xp.parent;55 }56 if (xp != null) {57 xp.red = false;58 if (xpp != null) {59 xpp.red = true;60 root = rotateLeft(root, xpp);61 }62 }63 }64 }65 }66 }
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參考:http://www.cnblogs.com/huaizuo/p/5371099.html
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