HashMap

HashMap源碼分析

package java.util;
import java.io.*;

public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable {
    // 默認容量16
    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;
    // 最大容量，/mæksɪməm/ 最大值的，是Integer最大值2147483647的一半1073741824
    static final int MAXIMUM_CAPACITY = 1 << 30;
    // 負載因子，/ˈfæktə/ 因素、因子
    static final float DEFAULT_LOAD_FACTOR = 0.75f;
    
　　// simple–simplify 簡化 ，beautiful-beautify 美化，ugly-uglify 醜化
　　// treeifys樹化 threshold /ˈθreʃhəʊld/ 門檻，閾值，臨界值
　　static final int TREEIFY_THRESHOLD = 8;
　　static final int UNTREEIFY_THRESHOLD = 6;
　　static final int MIN_TREEIFY_CAPACITY = 64;

    // 存入元素個數
    transient int size;

    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;
        ...
    }
    
    /* /ˈθreʃhəʊld/ 閾(yù)值,容量大小*0.75
     * new一個HashMap完成時，
     * HashMap構造方法傳入的容量大小先由threshold先臨時緩存起來，
     * HashMap內部的數組仍是null
     *
     * 第一次put元素時
     * 會把這個閾值真正賦值給數組大小
     */
    int threshold;
    // 負載因子，若是不指定，會把默認的DEFAULT_LOAD_FACTOR——0.75賦值給它
    final float loadFactor;
    transient int modCount;
    static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE;
    // 無參構造方法，調用本類的兩個參數的構造方法
    public HashMap() {
        // DEFAULT_INITIAL_CAPACITY,默認的初始化容量，16
        // DEFAULT_LOAD_FACTOR,默認的負載因子0.75
        this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
    }
    // 一個參數的構造方法，傳入參數初始化容量
    public HashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }
    // 也能夠指定負載因子
    public HashMap(int initialCapacity, float loadFactor) {
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal initial capacity: " + initialCapacity);
        // HashMap的最大容量是Interger最大值的一半                                        
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new IllegalArgumentException("Illegal load factor: " + loadFactor);
        this.loadFactor = loadFactor;
        // 肯定數組容量
        this.threshold = tableSizeFor(initialCapacity);
    }
    /* HashMap初始化時數組容量由tableSizeFor()方法肯定
     * tableSizeFor()方法返回的結果是2的冪
     * 傳入1，返回1；傳入2，返回4；傳入7，返回8；傳入13，返回16
     * 二、四、八、1六、32減1每一個位置都是1，如16-1=15,1111
     * 計算數組下標，i = (n - 1) & hash —— &都爲1結果爲1
     * i是下標，n是數組容量，hash是hash值
     * 這樣在進行按位與的時候，11...11，能夠使每一位都真正有效
     */
    static final int tableSizeFor(int cap) {
        int n = cap - 1;
        n |= n >>> 1;
        n |= n >>> 2;
        n |= n >>> 4;
        n |= n >>> 8;
        n |= n >>> 16;
        return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
    }
    public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }
    static final int hash(Object key) {
        int h;
        // 異或，不一樣爲1，相同爲0
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
    }
    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)
            n = (tab = resize()).length;
        /* 
         * 計算下標
         * i = (n - 1) & hash, &都爲1結果爲1
         * 若是這個下標的數組元素是null，以前沒有插入過，直接插入
         */
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        // 衝突了    
        else {
            Node<K,V> e; K k;
            // 相同的 key
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            else if (p instanceof TreeNode)
                // 插入樹
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        // 插入鏈表
                        p.next = newNode(hash, key, value, null);
                        // 若是鏈表元素大於等於8個，把數據放入一個紅黑樹
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            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)
                    // 若是是key相同，會把新的value覆蓋舊的value
                    e.value = value;
                afterNodeAccess(e);
                // 並返回舊的value
                return oldValue;
            }
        }
        ++modCount;
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }
    final Node<K,V>[] resize() {
        // 第一次put元素時，table爲null
        Node<K,V>[] oldTab = table;
        // 第一次put元素時，舊數組容量oldCab爲0
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        int oldThr = threshold;
        int newCap, newThr = 0;
        if (oldCap > 0) {
            // 擴容
            if (oldCap >= MAXIMUM_CAPACITY) {
                // 原數組長度大於最大容量(1073741824) 則將threshold設爲Integer.MAX_VALUE=2147483647
                // 接近MAXIMUM_CAPACITY的兩倍
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            /* 數組容量擴展至原來的兩倍
             * 閾值也擴展至原來的兩倍
             * 16  12 —— 0.75 
             * 32  24 —— 0.75
             * 64  48 —— 0.75
             * 128 96 —— 0.75
             */
            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
            /* 第一次put元素時，
             * 把構造HashMap時由threshold緩存起來的數組大小真正賦值給新數組容量
             */
            newCap = oldThr;
        else {               // zero initial threshold signifies using defaults
            newCap = DEFAULT_INITIAL_CAPACITY;
            newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
        }
        if (newThr == 0) {
            /* 第一次put元素時，
             * 計算新數組閾值
             */
            float ft = (float)newCap * loadFactor;
            newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                      (int)ft : Integer.MAX_VALUE);
        }
        /* 第一次put元素時，
         * 爲閾值賦值
         */
        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;
                    if (e.next == null)
                        newTab[e.hash & (newCap - 1)] = e;
                    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;
                            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);
                        if (loTail != null) {
                            loTail.next = null;
                            newTab[j] = loHead;
                        }
                        if (hiTail != null) {
                            hiTail.next = null;
                            newTab[j + oldCap] = hiHead;
                        }
                    }
                }
            }
        }
        return newTab;
    }
    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) {
            // 比較hash，且比較key，若是相同，就返回
            if (first.hash == hash && // always check first node
                ((k = first.key) == key || (key != null && key.equals(k))))
                return first;
            if ((e = first.next) != null) {
                // 判斷是否是紅黑樹
                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;
    }
    final void treeifyBin(Node<K,V>[] tab, int hash) {
        int n, index; Node<K,V> e;
        if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
            resize();
        else if ((e = tab[index = (n - 1) & hash]) != null) {
            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)
                hd.treeify(tab);
        }
    }
    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
        // 紅黑樹
        boolean red;
        TreeNode(int hash, K key, V val, Node<K,V> next) {
            super(hash, key, val, next);
        }
    }
}