hashMap 1.8.0_91 淺析

我對紅黑樹不是很瞭解，因此解說不是很好。還有remove等方法沒寫，之後再說。linkedHashMap，treeMap，也在說

hashMap由數組、鏈表、紅黑樹組成。

why？

數組，查找快！只要知道下標，Array[index]就查到了，可是向指定下標插入一個值，當該位置有值（我稱之爲原值）時，則要考慮原值的去留問題！

鏈表，插入、刪除快！只要更改prev、next的指向便可，可是，查找慢，得一個個遍歷！

紅黑樹，插入、查找、刪除都快！可是比較複雜。後面有時間，我會慢慢搞透他！

以下圖所示：

圖一 hashMap基礎數據結構

    1. 概述

    hashMap由  transient Node<K,V>[] table （這就是數組） 組成。Node包含鍵值等屬性

/**
     * 基本的桶節點，大多數實體都會用到:存儲的<key,value>對應Node的key，value
     * Basic hash bin node, used for most entries.  (See below for
     * TreeNode subclass, and in LinkedHashMap for its Entry subclass.)
     */
    static class Node<K,V> implements Map.Entry<K,V> {
        final int hash;
        final K key;
        V value;
        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;
        }
        
        ...

     }

    treeNode是紅黑樹的結點，是Node的子類

/**
     * Entry for Tree bins. Extends LinkedHashMap.Entry (which in turn
     * extends Node) so can be used as extension of either regular or
     * linked node.
     */
    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;

        ...
     }

   2. 屬性

/**
     * The table, initialized on first use, and resized as
     * necessary. When allocated, length is always a power of two.
     * (We also tolerate length zero in some operations to allow
     * bootstrapping mechanics that are currently not needed.)
     */
    transient Node<K,V>[] table;

    /**
     * Holds cached entrySet(). Note that AbstractMap fields are used
     * for keySet() and values().
     */
    transient Set<Map.Entry<K,V>> entrySet;

    /**
     * The number of key-value mappings contained in this map.
     */
    transient int size;

    /**
     * The number of times this HashMap has been structurally modified
     * Structural modifications are those that change the number of mappings in
     * the HashMap or otherwise modify its internal structure (e.g.,
     * rehash).  This field is used to make iterators on Collection-views of
     * the HashMap fail-fast.  (See ConcurrentModificationException).
     */
    transient int modCount;

    /**
     * The next size value at which to resize (capacity * load factor).
     *
     * @serial
     */
    // (The javadoc description is true upon serialization.
    // Additionally, if the table array has not been allocated, this
    // field holds the initial array capacity, or zero signifying
    // DEFAULT_INITIAL_CAPACITY.)
    int threshold;

    /**
     * The load factor for the hash table.
     *
     * @serial
     */
    final float loadFactor;

    3. 方法

    3.1 構造方法

     一共有4種構造方法，DEFAULT_LOAD_FACTOR = 0.75f

public HashMap(int initialCapacity, float loadFactor)         //指定table[]初始大小和負載因子

public HashMap(int initialCapacity) {                         //指定table[]初始大小
	this(initialCapacity, DEFAULT_LOAD_FACTOR);
}

public HashMap() {                                            //無參構造函數
	this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}

public HashMap(Map<? extends K, ? extends V> m) {             //根據舊map，生成新map
	this.loadFactor = DEFAULT_LOAD_FACTOR;
	putMapEntries(m, false);
}

重點看第一種

public HashMap(int initialCapacity, float loadFactor) {                  //初始化大小 和 負載因子
    if (initialCapacity < 0)
        throw new IllegalArgumentException("Illegal initial capacity: " +
                                           initialCapacity);
    if (initialCapacity > MAXIMUM_CAPACITY)                             //MAXIMUM_CAPACITY = 1 << 30
        initialCapacity = MAXIMUM_CAPACITY;
    if (loadFactor <= 0 || Float.isNaN(loadFactor))
        throw new IllegalArgumentException("Illegal load factor: " +
                                           loadFactor);
    this.loadFactor = loadFactor;
    this.threshold = tableSizeFor(initialCapacity);
}

調用了tableSizeFor(initialCapacity)，做用是返回>=初始化大小的  最小的  2的n次方。如1->2,15->16,32->32...上圖中將tableSize賦給threshold，在後期會付給table數組的初始化大小——table[]的size始終是2的n次方！這便於快速定位數組的下標(   key.hashCode&(table.size-1)  )

/**
 * Returns a power of two size for the given target capacity.
 */
static final int tableSizeFor(int cap) {
    int n = cap - 1;
    n |= n >>> 1;//無符號右移1位（2進制）
    n |= n >>> 2;
    n |= n >>> 4;
    n |= n >>> 8;
    n |= n >>> 16;
    return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}

 4種構造方法，有三種未初始化table數組，這是一種懶加載機制——（並不須要一開始就建立數組，而是須要用到他的時候建立，好比put）

    3.2 put

/**
 * Associates the specified value with the specified key in this map.
 * If the map previously contained a mapping for the key, the old
 * value is replaced.
 * 插入鍵 - 值，返回該鍵對應的 舊值
 * @param key key with which the specified value is to be associated
 * @param value value to be associated with the specified key
 * @return the previous value associated with <tt>key</tt>, or
 *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
 *         (A <tt>null</tt> return can also indicate that the map
 *         previously associated <tt>null</tt> with <tt>key</tt>.)
 */
public V put(K key, V value) {
    return putVal(hash(key), key, value, false, true);
}

static final int hash(Object key) {
    int h;
    return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);//哈希值與他的無符號高16位 亦或
}

/**
 * Implements Map.put and related methods
 *
 * @param hash hash for key
 * @param key the key
 * @param value the value to put
 * @param onlyIfAbsent if true, don't change existing value
 * @param evict if false, the table is in creation mode.
 * @return previous value, or null if none(若是該key存在對應的value，則修改，並返回舊值。若是不存在，插入，並返回null)
 */
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)//用本地變量tab指向全局table[],
        n = (tab = resize()).length;                   //若是table[]沒有初始化，調用resize初始化，n爲table的長度
    if ((p = tab[i = (n - 1) & hash]) == null)         //(n - 1) & hash,表明該key在table中的下標，記爲 位置A，p指向位置A的，惟一的Node，或鏈表、紅黑樹的頭結點
        tab[i] = newNode(hash, key, value, null);      //若是位置A沒有值，則新建一個Node，next=null
    else {                                             //若是位置A有值，則根據實際狀況：1.修改 2.插入鏈表 3.插入紅黑樹
        Node<K,V> e; K k;
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            e = p;                                    //key與p.key相同，作修改操做（此時，p多是單節點，或鏈表、樹的頭，可是不重要，由判斷條件知道，K要重寫equals和hashCode方法）
        else if (p instanceof TreeNode)               //key與p.key不相同，且p是紅黑樹的節點（頭結點）
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);//修改或插入紅黑樹
        else {                                        //p是鏈表的（頭）結點
            for (int binCount = 0; ; ++binCount) {    //遍歷鏈表
                if ((e = p.next) == null) {           //到了鏈表的尾部-tail
                    p.next = newNode(hash, key, value, null);//建立一個新節點到尾部
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st（鏈表長度大於等於8時，將鏈表轉成紅黑樹）
                        treeifyBin(tab, hash);                //轉成紅黑樹，或擴容
                    break;
                }
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))//key徹底相同，則替換值
                    break;
                p = e;
            }
        }
        if (e != null) { // existing mapping for key
            V oldValue = e.value;
            if (!onlyIfAbsent || oldValue == null)
                e.value = value;
            afterNodeAccess(e);                // Callbacks to allow LinkedHashMap post-actions
            return oldValue;
        }
    }
    ++modCount;                                //走到這一步、說明有新節點加入。modCount，本map的修改次數，便利時用到，防止concurrentModifition
    if (++size > threshold)                    //若是map的大小超過閥值，擴容（1.7版 會rehash）
        resize();
    afterNodeInsertion(evict);                 // Callbacks to allow LinkedHashMap post-actions
    return null;
}

putTreeVal 不太懂

/**
 * Tree version of putVal.
 */
final TreeNode<K,V> putTreeVal(HashMap<K,V> map, Node<K,V>[] tab,
                               int h, K k, V v) {
    Class<?> kc = null;
    boolean searched = false;
    TreeNode<K,V> root = (parent != null) ? root() : this;
    for (TreeNode<K,V> p = root;;) {
        int dir, ph; K pk;
        if ((ph = p.hash) > h)
            dir = -1;
        else if (ph < h)
            dir = 1;
        else if ((pk = p.key) == k || (k != null && k.equals(pk)))
            return p;
        else if ((kc == null &&
                  (kc = comparableClassFor(k)) == null) ||
                 (dir = compareComparables(kc, k, pk)) == 0) {
            if (!searched) {
                TreeNode<K,V> q, ch;
                searched = true;
                if (((ch = p.left) != null &&
                     (q = ch.find(h, k, kc)) != null) ||
                    ((ch = p.right) != null &&
                     (q = ch.find(h, k, kc)) != null))
                    return q;
            }
            dir = tieBreakOrder(k, pk);
        }

        TreeNode<K,V> xp = p;
        if ((p = (dir <= 0) ? p.left : p.right) == null) {
            Node<K,V> xpn = xp.next;
            TreeNode<K,V> x = map.newTreeNode(h, k, v, xpn);
            if (dir <= 0)
                xp.left = x;
            else
                xp.right = x;
            xp.next = x;
            x.parent = x.prev = xp;
            if (xpn != null)
                ((TreeNode<K,V>)xpn).prev = x;
            moveRootToFront(tab, balanceInsertion(root, x));
            return null;
        }
    }
}

不太懂

/**
 * Replaces all linked nodes in bin at index for given hash unless
 * table is too small, in which case resizes instead.
 */
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);
    }
}

小總結：put<key, value>操做，包含着插入、修改兩重語義。當map中存在該key時，執行修改操做，比較簡單。當map中不存在該key，執行插入操做，該操做會致使插入鏈表 || map擴容 || 鏈表轉紅黑樹 || 插入紅黑樹，並修改全局變量modCount，若是遍歷（for循環）map時執行put操做，可能會致使concurrentModifitionException，可使用concurrentHashMap代替，後期會講它

 

    3.3 get方法

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) > 0 &&
            (first = tab[(n - 1) & hash]) != null) {   //hashMap經過(length - 1) & hash快速定位，這是數組的優勢
            if (first.hash == hash && // always check first node
                ((k = first.key) == key || (key != null && key.equals(k))))//若是是鏈表的頭結點/樹的根節點，直接返回
                return first;
            if ((e = first.next) != null) {
                if (first instanceof TreeNode)                             //是樹節點，則經過樹的方式（左子節點-右子節點-左...）返回，
                    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);                            //是鏈表，則遍歷（鏈表最大長度爲8）
            }
        }
        return null;
    }


      /**
       * Calls find for root node.
       */
       final TreeNode<K,V> getTreeNode(int h, Object k) {
           return ((parent != null) ? root() : this).find(h, k, null);    //先獲取樹的root結點，而後調用find方法
       }

        /**
         * Returns root of tree containing this node.
         */
        final TreeNode<K,V> root() {
            for (TreeNode<K,V> r = this, p;;) {
                if ((p = r.parent) == null)
                    return r;
                r = p;
            }
        }

        /**
         * Finds the node starting at root p with the given hash and key.
         * The kc argument caches comparableClassFor(key) upon first use
         * comparing keys.
         */
        final TreeNode<K,V> find(int h, Object k, Class<?> kc) {
            TreeNode<K,V> p = this;
            do {
                int ph, dir; K pk;                                       //ph-當前結點的hash
                TreeNode<K,V> pl = p.left, pr = p.right, q;              //hash = (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16)
                if ((ph = p.hash) > h)     //若是h<當前結點的hash，p = p的左子節點                                     
                    p = pl;
                else if (ph < h)           //若是h>當前結點的hash，p = p的右子節點
                    p = pr;
                else if ((pk = p.key) == k || (k != null && k.equals(pk)))//key相同，直接返回
                    return p;
                else if (pl == null)
                    p = pr;
                else if (pr == null)
                    p = pl;
                else if ((kc != null ||
                          (kc = comparableClassFor(k)) != null) &&
                         (dir = compareComparables(kc, k, pk)) != 0)
                    p = (dir < 0) ? pl : pr;
                else if ((q = pr.find(h, k, kc)) != null)
                    return q;
                else
                    p = pl;
            } while (p != null);
            return null;
        }

上面用到兩個方法，頗有意思，能夠本身意會

/**
 * Returns x's Class if it is of the form "class C implements
 * Comparable<C>", else null.
 */
static Class<?> comparableClassFor(Object x) {
    if (x instanceof Comparable) {
        Class<?> c; Type[] ts, as; Type t; ParameterizedType p;
        if ((c = x.getClass()) == String.class) // bypass checks
            return c;
        if ((ts = c.getGenericInterfaces()) != null) {
            for (int i = 0; i < ts.length; ++i) {
                if (((t = ts[i]) instanceof ParameterizedType) &&
                    ((p = (ParameterizedType)t).getRawType() ==
                     Comparable.class) &&
                    (as = p.getActualTypeArguments()) != null &&
                    as.length == 1 && as[0] == c) // type arg is c
                    return c;
            }
        }
    }
    return null;
}

/**
 * Returns k.compareTo(x) if x matches kc (k's screened comparable
 * class), else 0.
 */
@SuppressWarnings({"rawtypes","unchecked"}) // for cast to Comparable
static int compareComparables(Class<?> kc, Object k, Object x) {
    return (x == null || x.getClass() != kc ? 0 :
            ((Comparable)k).compareTo(x));
}