HashMap源碼怎么分析

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static final int hash(Object key) {

  int h;

  // key.hashCode()：返回散列值也就是hashcode

  // ^ ：按位異或

  // >>>:無符號右移，忽略符號位，空位都以0補齊

  return (key == null) ?  : (h = key.hashCode()) ^ (h >>> 16);

}

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static int hash(int h) {

    // This function ensures that hashCodes that differ only by

    // constant multiples at each bit position have a bounded

    // number of collisions (approximately 8 at default load factor).

    h ^= (h >>> 20) ^ (h >>> 12);

    return h ^ (h >>> 7) ^ (h >>> 4);

}

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public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable {

    // 序列號

    private static final long serialVersionUID = 362498820763181265L;   

    // 默認的初始容量是16

    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;  

    // 最大容量

    static final int MAXIMUM_CAPACITY = 1 << 30;

    // 默認的填充因子

    static final float DEFAULT_LOAD_FACTOR = .75f;

    // 當桶(bucket)上的結點數大于這個值時會轉成紅黑樹

    static final int TREEIFY_THRESHOLD = 8;

    // 當桶(bucket)上的結點數小于這個值時樹轉鏈表

    static final int UNTREEIFY_THRESHOLD = 6;

    // 桶中結構轉化為紅黑樹對應的table的最小大小

    static final int MIN_TREEIFY_CAPACITY = 64;

    // 存儲元素的數組，總是2的冪次倍

    transient Node<k,v>[] table;

    // 存放具體元素的集

    transient Set<map.entry<k,v>> entrySet;

    // 存放元素的個數，注意這個不等于數組的長度。

    transient int size;

    // 每次擴容和更改map結構的計數器

    transient int modCount;  

    // 臨界值 當實際大小(容量*填充因子)超過臨界值時，會進行擴容

    int threshold;

    // 填充因子

    final float loadFactor;

}

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// 繼承自 Map.Entry<K,V>

static class Node<K,V> implements Map.Entry<K,V> {

       final int hash;// 哈希值，存放元素到hashmap中時用來與其他元素hash值比較

       final K key;//鍵

       V value;//值

       // 指向下一個節點

       Node<K,V> next;

       Node(int hash, K key, V value, Node<K,V> next) {

            this.hash = hash;

            this.key = key;

            this.value = value;

            this.next = next;

        }

        public final K getKey()        { return key; }

        public final V getValue()      { return value; }

        public final String toString() { return key + "=" + value; }

        // 重寫hashCode()方法

        public final int hashCode() {

            return Objects.hashCode(key) ^ Objects.hashCode(value);

        }

        public final V setValue(V newValue) {

            V oldValue = value;

            value = newValue;

            return oldValue;

        }

        // 重寫 equals() 方法

        public final boolean equals(Object o) {

            if (o == this)

                return true;

            if (o instanceof Map.Entry) {

                Map.Entry<?,?> e = (Map.Entry<?,?>)o;

                if (Objects.equals(key, e.getKey()) &&

                    Objects.equals(value, e.getValue()))

                    return true;

            }

            return false;

        }

}

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static final class TreeNode<K,V> extends LinkedHashMap.Entry<K,V> {

        TreeNode<K,V> parent;  // 父

        TreeNode<K,V> left;    // 左

        TreeNode<K,V> right;   // 右

        TreeNode<K,V> prev;    // needed to unlink next upon deletion

        boolean red;           // 判斷顏色

        TreeNode(int hash, K key, V val, Node<K,V> next) {

            super(hash, key, val, next);

        }

        // 返回根節點

        final TreeNode<K,V> root() {

            for (TreeNode<K,V> r = this, p;;) {

                if ((p = r.parent) == null)

                    return r;

                r = p;

       }

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// 默認構造函數。

public More ...HashMap() {

    this.loadFactor = DEFAULT_LOAD_FACTOR; // all   other fields defaulted

 }

 // 包含另一個“Map”的構造函數

 public More ...HashMap(Map<? extends K, ? extends V> m) {

     this.loadFactor = DEFAULT_LOAD_FACTOR;

     putMapEntries(m, false);//下面會分析到這個方法

 }

 // 指定“容量大小”的構造函數

 public More ...HashMap(int initialCapacity) {

     this(initialCapacity, DEFAULT_LOAD_FACTOR);

 }

 // 指定“容量大小”和“加載因子”的構造函數

 public More ...HashMap(int initialCapacity, float loadFactor) {

     if (initialCapacity < )

         throw new IllegalArgumentException("Illegal initial capacity: " + initialCapacity);

     if (initialCapacity > MAXIMUM_CAPACITY)

         initialCapacity = MAXIMUM_CAPACITY;

     if (loadFactor <=  || Float.isNaN(loadFactor))

         throw new IllegalArgumentException("Illegal load factor: " + loadFactor);

     this.loadFactor = loadFactor;

     this.threshold = tableSizeFor(initialCapacity);

 }

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final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {

    int s = m.size();

    if (s > ) {

        // 判斷table是否已經初始化

        if (table == null) { // pre-size

            // 未初始化，s為m的實際元素個數

            float ft = ((float)s / loadFactor) + 1.0F;

            int t = ((ft < (float)MAXIMUM_CAPACITY) ?

                    (int)ft : MAXIMUM_CAPACITY);

            // 計算得到的t大于閾值，則初始化閾值

            if (t > threshold)

                threshold = tableSizeFor(t);

        }

        // 已初始化，并且m元素個數大于閾值，進行擴容處理

        else if (s > threshold)

            resize();

        // 將m中的所有元素添加至HashMap中

        for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) {

            K key = e.getKey();

            V value = e.getValue();

            putVal(hash(key), key, value, false, evict);

        }

    }

}

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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;

    // table未初始化或者長度為0，進行擴容

    if ((tab = table) == null || (n = tab.length) == )

        n = (tab = resize()).length;

    // (n - 1) & hash 確定元素存放在哪個桶中，桶為空，新生成結點放入桶中(此時，這個結點是放在數組中)

    if ((p = tab[i = (n - 1) & hash]) == null)

        tab[i] = newNode(hash, key, value, null);

    // 桶中已經存在元素

    else {

        Node<K,V> e; K k;

        // 比較桶中第一個元素(數組中的結點)的hash值相等，key相等

        if (p.hash == hash &&

            ((k = p.key) == key || (key != null && key.equals(k))))

                // 將第一個元素賦值給e，用e來記錄

                e = p;

        // hash值不相等，即key不相等；為紅黑樹結點

        else if (p instanceof TreeNode)

            // 放入樹中

            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);

        // 為鏈表結點

        else {

            // 在鏈表最末插入結點

            for (int binCount = ; ; ++binCount) {

                // 到達鏈表的尾部

                if ((e = p.next) == null) {

                    // 在尾部插入新結點

                    p.next = newNode(hash, key, value, null);

                    // 結點數量達到閾值，轉化為紅黑樹

                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st

                        treeifyBin(tab, hash);

                    // 跳出循環

                    break;

                }

                // 判斷鏈表中結點的key值與插入的元素的key值是否相等

                if (e.hash == hash &&

                    ((k = e.key) == key || (key != null && key.equals(k))))

                    // 相等，跳出循環

                    break;

                // 用于遍歷桶中的鏈表，與前面的e = p.next組合，可以遍歷鏈表

                p = e;

            }

        }

        // 表示在桶中找到key值、hash值與插入元素相等的結點

        if (e != null) {

            // 記錄e的value

            V oldValue = e.value;

            // onlyIfAbsent為false或者舊值為null

            if (!onlyIfAbsent || oldValue == null)

                //用新值替換舊值

                e.value = value;

            // 訪問后回調

            afterNodeAccess(e);

            // 返回舊值

            return oldValue;

        }

    }

    // 結構性修改

    ++modCount;

    // 實際大小大于閾值則擴容

    if (++size > threshold)

        resize();

    // 插入后回調

    afterNodeInsertion(evict);

    return null;

}

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public V put(K key, V value)

    if (table == EMPTY_TABLE) {

    inflateTable(threshold);

} 

    if (key == null)

        return putForNullKey(value);

    int hash = hash(key);

    int i = indexFor(hash, table.length);

    for (Entry<K,V> e = table[i]; e != null; e = e.next) { // 先遍歷

        Object k;

        if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {

            V oldValue = e.value;

            e.value = value;

            e.recordAccess(this);

            return oldValue;

        }

    }

    modCount++;

    addEntry(hash, key, value, i);  // 再插入

    return null;

}

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public V get(Object key) {

    Node<K,V> e;

    return (e = getNode(hash(key), key)) == null ? null : e.value;

}

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) >  &&

        (first = tab[(n - 1) & hash]) != null) {

        // 數組元素相等

        if (first.hash == hash && // always check first node

            ((k = first.key) == key || (key != null && key.equals(k))))

            return first;

        // 桶中不止一個節點

        if ((e = first.next) != null) {

            // 在樹中get

            if (first instanceof TreeNode)

                return ((TreeNode<K,V>)first).getTreeNode(hash, key);

            // 在鏈表中get

            do {

                if (e.hash == hash &&

                    ((k = e.key) == key || (key != null && key.equals(k))))

                    return e;

            } while ((e = e.next) != null);

        }

    }

    return null;

}

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final Node<K,V>[] resize() {

    Node<K,V>[] oldTab = table;

    int oldCap = (oldTab == null) ?  : oldTab.length;

    int oldThr = threshold;

    int newCap, newThr = ;

    if (oldCap > ) {

        // 超過最大值就不再擴充了，就只好隨你碰撞去吧

        if (oldCap >= MAXIMUM_CAPACITY) {

            threshold = Integer.MAX_VALUE;

            return oldTab;

        }

        // 沒超過最大值，就擴充為原來的2倍

        else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY && oldCap >= DEFAULT_INITIAL_CAPACITY)

            newThr = oldThr << 1; // double threshold

    }

    else if (oldThr > ) // initial capacity was placed in threshold

        newCap = oldThr;

    else {

        signifies using defaults

        newCap = DEFAULT_INITIAL_CAPACITY;

        newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);

    }

    // 計算新的resize上限

    if (newThr == ) {

        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) {

        // 把每個bucket都移動到新的buckets中

        for (int j = ; 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 {

                    Node<K,V> loHead = null, loTail = null;

                    Node<K,V> hiHead = null, hiTail = null;

                    Node<K,V> next;

                    do {

                        next = e.next;

                        // 原索引

                        if ((e.hash & oldCap) == ) {

                            if (loTail == null)

                                loHead = e;

                            else

                                loTail.next = e;

                            loTail = e;

                        }

                        // 原索引+oldCap

                        else {

                            if (hiTail == null)

                                hiHead = e;

                            else

                                hiTail.next = e;

                            hiTail = e;

                        }

                    } while ((e = next) != null);

                    // 原索引放到bucket里

                    if (loTail != null) {

                        loTail.next = null;

                        newTab[j] = loHead;

                    }

                    // 原索引+oldCap放到bucket里

                    if (hiTail != null) {

                        hiTail.next = null;

                        newTab[j + oldCap] = hiHead;

                    }

                }

            }

        }

    }

    return newTab;

}

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package map;

import java.util.Collection;

import java.util.HashMap;

import java.util.Set;

public class HashMapDemo {

    public static void main(String[] args) {

        HashMap<String, String> map = new HashMap<String, String>();

        // 鍵不能重復，值可以重復

        map.put("san", "張三");

        map.put("si", "李四");

        map.put("wu", "王五");

        map.put("wang", "老王");

        map.put("wang", "老王2");// 老王被覆蓋

        map.put("lao", "老王");

        System.out.println("-------直接輸出hashmap:-------");

        System.out.println(map);

        /**

         * 遍歷HashMap

         */

        // 1.獲取Map中的所有鍵

        System.out.println("-------foreach獲取Map中所有的鍵:------");

        Set<String> keys = map.keySet();

        for (String key : keys) {

            System.out.print(key+"  ");

        }

        System.out.println();//換行

        // 2.獲取Map中所有值

        System.out.println("-------foreach獲取Map中所有的值:------");

        Collection<String> values = map.values();

        for (String value : values) {

            System.out.print(value+"  ");

        }

        System.out.println();//換行

        // 3.得到key的值的同時得到key所對應的值

        System.out.println("-------得到key的值的同時得到key所對應的值:-------");

        Set<String> keys2 = map.keySet();

        for (String key : keys2) {

            System.out.print(key + "：" + map.get(key)+"   ");

        }

        /**

         * 另外一種不常用的遍歷方式

         */

        // 當我調用put(key,value)方法的時候，首先會把key和value封裝到

        // Entry這個靜態內部類對象中，把Entry對象再添加到數組中，所以我們想獲取

        // map中的所有鍵值對，我們只要獲取數組中的所有Entry對象，接下來

        // 調用Entry對象中的getKey()和getValue()方法就能獲取鍵值對了

        Set<java.util.Map.Entry<String, String>> entrys = map.entrySet();

        for (java.util.Map.Entry<String, String> entry : entrys) {

            System.out.println(entry.getKey() + "--" + entry.getValue());

        }

        /**

         * HashMap其他常用方法

         */

        System.out.println("after map.size()："+map.size());

        System.out.println("after map.isEmpty()："+map.isEmpty());

        System.out.println(map.remove("san"));

        System.out.println("after map.remove()："+map);

        System.out.println("after map.get(si)："+map.get("si"));

        System.out.println("after map.containsKey(si)："+map.containsKey("si"));

        System.out.println("after containsValue(李四)："+map.containsValue("李四"));

        System.out.println(map.replace("si", "李四2"));

        System.out.println("after map.replace(si, 李四2):"+map);

    }

}