0x00.新老HashMap區別

本文使用jdk7（1.7.0_79）與 jdk8（1.8.0_45）進行對比，主要學習數據結構區別

數據結構

jdk7內部數據結構為數組+鏈表，通過key的hash值計算數據所在數組下標，多個key的hash相同或hash計算的數組下標相同，但是key值不同時，往鏈表尾追加Entry。

transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;

static class Entry<K,V> implements Map.Entry<K,V> {
    final K key;
    V value;
    Entry<K,V> next;
    int hash;
}

jdk8內部數據結構為數組+(鏈表 或 紅黑樹)，通過key的hash值計算數據所在數組下標，多個key的hash相同或hash計算的數組下標相同，但是key值不同時，檢查節點是否為樹節點，是樹節點則往樹節點添加，如果是普通節點則往鏈表尾追加Entry，當鏈表長度大于8時，則將鏈表轉為紅黑樹。

transient Node<K,V>[] table;

static class Node<K,V> implements Map.Entry<K,V> {
    final int hash;
    final K key;
    V value;
    Node<K,V> next;
}

static final class TreeNode<K,V> extends LinkedHashMap.Entry<K,V> {
    TreeNode<K,V> parent;  // red-black tree links
    TreeNode<K,V> left;
    TreeNode<K,V> right;
    TreeNode<K,V> prev;    // needed to unlink next upon deletion
    boolean red;
}

//LinkedHashMap.Entry
static class Entry<K,V> extends HashMap.Node<K,V> {
    Entry<K,V> before, after;
    Entry(int hash, K key, V value, Node<K,V> next) {
        super(hash, key, value, next);
    }
}

0x01. HashMap.put源碼閱讀

源碼學習，邊看源碼邊加注釋，邊debug，邊理解。

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;
    if ((tab = table) == null || (n = tab.length) == 0)
        //當HashMap的內部table為空時，觸發resize()
        n = (tab = resize()).length;
    if ((p = tab[i = (n - 1) & hash]) == null)
        //當通過key的hash值計算數據所在數組下標值為null時直接生成新節點放入桶
        tab[i] = newNode(hash, key, value, null);
    else {
        Node<K,V> e; K k;
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            //key完全相同
            e = p;
        else if (p instanceof TreeNode)
            //桶內對象已經時紅黑樹
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        else {
            //桶內已經存在普通節點則遍歷鏈表，往鏈表尾部添加節點
            for (int binCount = 0; ; ++binCount) {
                if ((e = p.next) == null) { //第1個循環是獲取第2個節點了（0->root.next）
                    //binCount為0時插入的第二個節點
                    p.next = newNode(hash, key, value, null);
                    //如果鏈表內的數據已經超過8個則將鏈表轉成紅黑樹
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                        //binCount等于7時插入的第9個節點
                        treeifyBin(tab, hash);
                    break;
                }
                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;
            if (!onlyIfAbsent || oldValue == null)
                e.value = value;
            afterNodeAccess(e);
            return oldValue;
        }
    }
    ++modCount;
    if (++size > threshold)
        resize();
    afterNodeInsertion(evict);
    return null;
}

final void treeifyBin(Node<K,V>[] tab, int hash) {
    int n, index; Node<K,V> e;
    //檢查tab的長度是否大于等于64，小于則擴容
    if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
        resize();
    else if ((e = tab[index = (n - 1) & hash]) != null) {
        //將Node對象轉為TreeNode
        TreeNode<K,V> hd = null, tl = null;
        do {
            TreeNode<K,V> p = replacementTreeNode(e, null);
            if (tl == null)
                hd = p;
            else {
                p.prev = tl;
                tl.next = p;
            }
            tl = p;
        } while ((e = e.next) != null);
        if ((tab[index] = hd) != null)
            //將TreeNode鏈表轉成紅黑樹
            hd.treeify(tab);
    }
}

final void treeify(Node<K,V>[] tab) {
    TreeNode<K,V> root = null;
    for (TreeNode<K,V> x = this, next; x != null; x = next) {
        next = (TreeNode<K,V>)x.next;
        x.left = x.right = null;
        if (root == null) {
            //root為空時直接第一個元素存入root
            x.parent = null;
            x.red = false;
            root = x;
        }
        else {
            K k = x.key;
            int h = x.hash;
            Class<?> kc = null;
            //往二叉樹中插入當前節點
            for (TreeNode<K,V> p = root;;) {
                int dir, ph;
                K pk = p.key;

                if ((ph = p.hash) > h)//優先直接比較hash值
                    dir = -1;
                else if (ph < h)//優先直接比較hash值
                    dir = 1;
                else if ((kc == null &&
                          (kc = comparableClassFor(k)) == null) || //hash值相同則判斷key是否實現Comparable接口
                         (dir = compareComparables(kc, k, pk)) == 0) //實現了Comparable接口則直接比較
                    //沒有實現Comparable接口則用System.identityHashCode比較
                    dir = tieBreakOrder(k, pk);

                TreeNode<K,V> xp = p;
                //檢查樹節點是否為空，不為空繼續比較，直到找到空的節點放入當前節點。
                if ((p = (dir <= 0) ? p.left : p.right) == null) {
                    x.parent = xp;
                    if (dir <= 0)
                        xp.left = x;
                    else
                        xp.right = x;
                    //進行二叉樹平衡操作
                    root = balanceInsertion(root, x);
                    break;
                }
            }
        }
    }
    //將root接口放入table的第一個元素
    moveRootToFront(tab, root);
}

static <K,V> TreeNode<K,V> balanceInsertion(TreeNode<K,V> root,
                                            TreeNode<K,V> x) {
    x.red = true;
    for (TreeNode<K,V> xp, xpp, xppl, xppr;;) {
        //判斷當前節點的父節點為空 
        if ((xp = x.parent) == null) {
            //則當前節點為root 直接將顏色設置為黑色 
            x.red = false;
            return x;
        }
        else if (!xp.red || (xpp = xp.parent) == null)
            //當前節點父節點為黑色節點 或者當前節點的父節點時root節點
            return root;
        if (xp == (xppl = xpp.left)) {//當前節點的父節點是左子節點（父節點為紅色節點）
            if ((xppr = xpp.right) != null && xppr.red) {
                //當前節點的父節點以及父節點的兄弟節點都是紅色 則顏色反轉
                xppr.red = false;
                xp.red = false;
                xpp.red = true;
                x = xpp;
            }
            else {
                //當前節點為右節點 則先進行左旋
                if (x == xp.right) {
                    root = rotateLeft(root, x = xp);
                    xpp = (xp = x.parent) == null ? null : xp.parent;
                }
                //父節點不為空 則變色 右旋
                if (xp != null) {
                    xp.red = false;
                    if (xpp != null) {
                        xpp.red = true;
                        root = rotateRight(root, xpp);
                    }
                }
            }
        }
        else {//當前節點的父節點是右子節點
            if (xppl != null && xppl.red) { 
                //當前節點的父節點以及父節點的兄弟節點都是紅色 則顏色反轉
                xppl.red = false;
                xp.red = false;
                xpp.red = true;
                x = xpp;
            }
            else {
                //節點在左側則右旋
                if (x == xp.left) {
                    root = rotateRight(root, x = xp);
                    xpp = (xp = x.parent) == null ? null : xp.parent;
                }
                //父節點不為空 則變色 左旋
                if (xp != null) {
                    xp.red = false;
                    if (xpp != null) {
                        xpp.red = true;
                        root = rotateLeft(root, xpp);
                    }
                }
            }
        }
    }
}

0x02. 測試代碼

觸發鏈表轉紅黑樹的測試代碼 可直接用來debug

import java.util.HashMap;

/**
 * Created by qiyan on 2017/4/9.
 */
public class HashMapTest {
    public static void main(String[] args) throws Exception {
        //map內的數組容量大于等于64 且 鏈表數量大于8才會進行紅黑樹轉換
        HashMap map = new HashMap(64);
        for (int i = 0; i <= 8; i++) {
            map.put(new HashCodeOneObj(i), i);
        }
        System.out.println(map);
    }

    private static class HashCodeOneObj implements Comparable<HashCodeOneObj> {
        private int val;

        public HashCodeOneObj(int val) {
            this.val = val;
        }

        public int getVal() {
            return val;
        }

        @Override
        public int hashCode() {
            return 1; //讓所有數據都存入一個桶
        }

        @Override
        public int compareTo(HashCodeOneObj o) {
            if (null == o) {
                return -1;
            }
            return Integer.compare(this.getVal(), o.getVal());
        }
    }
}

鏈表轉紅黑樹過程示意圖：

紅黑樹v2(insert-0-8).png

HashMap中的紅黑樹