数据结构及算法基础--哈希图（HashMap）

http://www.cnblogs.com/DSNFZ/articles/7634042.html

https://www.cnblogs.com/DSNFZ/articles/7675347.html

 

HashMap能够说是java中最多见的几种集合了。

在了解HashMap前咱们要先了解Object的两个方法：Equals和hashCode（）

首先咱们来看一下object内的源码是怎样实现的：

hashcode（）：

/**
     * Returns a hash code value for the object. This method is
     * supported for the benefit of hash tables such as those provided by
     * {@link java.util.HashMap}.
     * <p>
     * The general contract of {@code hashCode} is:
     * <ul>
     * <li>Whenever it is invoked on the same object more than once during
     *     an execution of a Java application, the {@code hashCode} method
     *     must consistently return the same integer, provided no information
     *     used in {@code equals} comparisons on the object is modified.
     *     This integer need not remain consistent from one execution of an
     *     application to another execution of the same application.
     * <li>If two objects are equal according to the {@code equals(Object)}
     *     method, then calling the {@code hashCode} method on each of
     *     the two objects must produce the same integer result.
     * <li>It is <em>not</em> required that if two objects are unequal
     *     according to the {@link java.lang.Object#equals(java.lang.Object)}
     *     method, then calling the {@code hashCode} method on each of the
     *     two objects must produce distinct integer results.  However, the
     *     programmer should be aware that producing distinct integer results
     *     for unequal objects may improve the performance of hash tables.
     * </ul>
     * <p>
     * As much as is reasonably practical, the hashCode method defined by
     * class {@code Object} does return distinct integers for distinct
     * objects. (This is typically implemented by converting the internal
     * address of the object into an integer, but this implementation
     * technique is not required by the
     * Java&trade; programming language.)
     *
     * @return  a hash code value for this object.
     * @see     java.lang.Object#equals(java.lang.Object)
     * @see     java.lang.System#identityHashCode
     */
    public native int hashCode();

可是这个方法没有实现！注意上面这句话：

but this implementation technique is not required by the Java™ programming language.
咱们不须要知道具体怎样实现的hashCode的运行过程，咱们须要知道的是它返回这个对象的特定的类型为整数的hashcode

equals（）：

/**
     * Indicates whether some other object is "equal to" this one.
     * <p>
     * The {@code equals} method implements an equivalence relation
     * on non-null object references:
     * <ul>
     * <li>It is <i>reflexive</i>: for any non-null reference value
     *     {@code x}, {@code x.equals(x)} should return
     *     {@code true}.
     * <li>It is <i>symmetric</i>: for any non-null reference values
     *     {@code x} and {@code y}, {@code x.equals(y)}
     *     should return {@code true} if and only if
     *     {@code y.equals(x)} returns {@code true}.
     * <li>It is <i>transitive</i>: for any non-null reference values
     *     {@code x}, {@code y}, and {@code z}, if
     *     {@code x.equals(y)} returns {@code true} and
     *     {@code y.equals(z)} returns {@code true}, then
     *     {@code x.equals(z)} should return {@code true}.
     * <li>It is <i>consistent</i>: for any non-null reference values
     *     {@code x} and {@code y}, multiple invocations of
     *     {@code x.equals(y)} consistently return {@code true}
     *     or consistently return {@code false}, provided no
     *     information used in {@code equals} comparisons on the
     *     objects is modified.
     * <li>For any non-null reference value {@code x},
     *     {@code x.equals(null)} should return {@code false}.
     * </ul>
     * <p>
     * The {@code equals} method for class {@code Object} implements
     * the most discriminating possible equivalence relation on objects;
     * that is, for any non-null reference values {@code x} and
     * {@code y}, this method returns {@code true} if and only
     * if {@code x} and {@code y} refer to the same object
     * ({@code x == y} has the value {@code true}).
     * <p>
     * Note that it is generally necessary to override the {@code hashCode}
     * method whenever this method is overridden, so as to maintain the
     * general contract for the {@code hashCode} method, which states
     * that equal objects must have equal hash codes.
     *
     * @param   obj   the reference object with which to compare.
     * @return  {@code true} if this object is the same as the obj
     *          argument; {@code false} otherwise.
     * @see     #hashCode()
     * @see     java.util.HashMap
     */
    public boolean equals(Object obj) {
        return (this == obj);
    }

 

这里我将jdk源码中全部相关信息都给出来了，但愿在某些地方理解的时候，会提供必定的帮助。

 

固然咱们能够重写这两个函数，可是在java1.8中定义的函数最好不要进行重写，否则对hashmap的性能产生很大的影响；

 

1）HashMap概述

HashMap是基于哈希表的map接口的非同步实现，此实现提供全部可选的映射操做，并容许使用null值和null键。此类不保证映射的顺序，特别是它不保证该顺序恒久不变。

 

2）HashMap数据结构

在java语言编程中，最基本的数据结构就两种：数组和引用，其余全部的数据结构均可以经过这两个基本的数据结构来实现，在jkd 1.7之前，hashmap就是一个链表散列的结构，可是在jdk1.8发布后，hashmap的链表长度大于必定值事后，变编程红黑树，关于红黑树的概念，在上篇文章中进行了讲解：

其HashMap的具体结构以下图所示：

 

java中采用的即是链地址法，即是每一个数组元素上都是一个链表。当数据被hash后，获得数组下标，将数据放在对应数组下标的链表上

其中每一个元素都用node节点表示：

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;
        }
}

node是hashmap的一个内部类，用来储存数据和保持链表结构的。它的本质就是一个映射（键值对）。

固然，会产生两个key值产生同一个位置，（最主要的即是由于index的产生原理，固然也有多是产生了同样的hash值）这种状况叫哈希碰撞。固然hash算法计算结果越分散均匀，发生hash碰撞的机率就越小，map的存储效率就越高。

 

hashmap中又一个很重要的字段就是Node[] table。如上图所示，这就是hashmap的基本结构，构成链表的数组。

若是哈希桶数组很大，即便较差的Hash算法也会比较分散，若是哈希桶数组数组很小，即便好的Hash算法也会出现较多碰撞，因此就须要在空间成本和时间成本之间权衡，其实就是在根据实际状况肯定哈希桶数组的大小，并在此基础上设计好的hash算法减小Hash碰撞。那么经过什么方式来控制map使得Hash碰撞的几率又小，哈希桶数组（Node[] table）占用空间又少呢？答案就是好的Hash算法和扩容机制。

 

在此以前，咱们先来了解一下hashmap一些很是很是重要的参数。源代码中以下：

/**
     * The default initial capacity - MUST be a power of two.
     */
    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

    /**
     * The maximum capacity, used if a higher value is implicitly specified
     * by either of the constructors with arguments.
     * MUST be a power of two <= 1<<30.
     */
    static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * The load factor used when none specified in constructor.
     */
    static final float DEFAULT_LOAD_FACTOR = 0.75f;

    /**
     * The bin count threshold for using a tree rather than list for a
     * bin.  Bins are converted to trees when adding an element to a
     * bin with at least this many nodes. The value must be greater
     * than 2 and should be at least 8 to mesh with assumptions in
     * tree removal about conversion back to plain bins upon
     * shrinkage.
     */
    static final int TREEIFY_THRESHOLD = 8;

    /**
     * The bin count threshold for untreeifying a (split) bin during a
     * resize operation. Should be less than TREEIFY_THRESHOLD, and at
     * most 6 to mesh with shrinkage detection under removal.
     */
    static final int UNTREEIFY_THRESHOLD = 6;

    /**
     * The smallest table capacity for which bins may be treeified.
     * (Otherwise the table is resized if too many nodes in a bin.)
     * Should be at least 4 * TREEIFY_THRESHOLD to avoid conflicts
     * between resizing and treeification thresholds.
     */
    static final int MIN_TREEIFY_CAPACITY = 64;
    
    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;
    /**
     * The number of key-value mappings contained in this map.
     */

上面这些参数的是很是很是重要的，其重要性至关于hashmap的数据结构的重要性。在本篇中，咱们运用到并重点讲解的为一下几个参数：

static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;
static final float DEFAULT_LOAD_FACTOR = 0.75f;
transient int size;
transient int modCount;
int threshold;
final float loadFactor;

首先能够单刀，Node[] table的默认长度是16，loadFactor的默认大小为0.75，threshold是hashmap所能容纳的最大数据量的Node个数，默认为0.75，threshold=DEFAULT_INITIAL_CAPACITY*loadFactor;当添加元素数量超过这个数量事后，就要进行扩容，扩容后hashmap的容量是以前的两倍。对于0.75，建议你们不要轻易修改。除非在时间和空间比较特殊的状况下，若是内存空间不少而又对时间效率要求很高，能够下降负载因子Load factor的值；相反，若是内存空间紧张而对时间效率要求不高，能够增长负载因子loadFactor的值，这个值能够大于1。

 

size就是在这个hashmpa中实际存在的node数量。modCount即是hashmap结构修改的次数。在以前对iterator（迭代器）进行讲解的时候我已经进行了说明，须要注意的是在hashmap中modcount指的是结构更改的次数，例如添加新的node，可是若是是替换原有node的value，modcount是不变的，由于它不属于结构变化。

 

有兴趣能够了解下：在HashMap中，哈希桶数组table的长度length大小必须为2的n次方(必定是合数)，这是一种很是规的设计，常规的设计是把桶的大小设计为素数。相对来讲素数致使冲突的几率要小于合数，具体证实能够参考http://blog.csdn.net/liuqiyao_01/article/details/14475159，Hashtable初始化桶大小为11，就是桶大小设计为素数的应用（Hashtable扩容后不能保证仍是素数）。HashMap采用这种很是规设计，主要是为了在取模和扩容时作优化，同时为了减小冲突，HashMap定位哈希桶索引位置时，也加入了高位参与运算的过程。

 

3）确认hashmap索引位置

代码：

/**
     * Computes key.hashCode() and spreads (XORs) higher bits of hash
     * to lower.  Because the table uses power-of-two masking, sets of
     * hashes that vary only in bits above the current mask will
     * always collide. (Among known examples are sets of Float keys
     * holding consecutive whole numbers in small tables.)  So we
     * apply a transform that spreads the impact of higher bits
     * downward. There is a tradeoff between speed, utility, and
     * quality of bit-spreading. Because many common sets of hashes
     * are already reasonably distributed (so don't benefit from
     * spreading), and because we use trees to handle large sets of
     * collisions in bins, we just XOR some shifted bits in the
     * cheapest possible way to reduce systematic lossage, as well as
     * to incorporate impact of the highest bits that would otherwise
     * never be used in index calculations because of table bounds.
     */
    static final int hash(Object key) {
        int h;
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
    }

 

这里的Hash算法本质上就是三步：取key的hashCode值、高位运算、取模运算。

对于任意给定的对象，只要它的hashCode()返回值相同，那么程序调用方法一所计算获得的Hash码值老是相同的。咱们首先想到的就是把hash值对数组长度取模运算，这样一来，元素的分布相对来讲是比较均匀的。可是，模运算的消耗仍是比较大的，在HashMap中是这样作的：咱们经过h & (table.length -1)来计算该对象应该保存在table数组的哪一个索引处。

这个方法很是巧妙，它经过h & (table.length -1)来获得该对象的保存位，而HashMap底层数组的长度老是2的n次方，这是HashMap在速度上的优化。当length老是2的n次方时，h& (length-1)运算等价于对length取模，也就是h%length，可是&比%具备更高的效率。

在JDK1.8的实现中，优化了高位运算的算法，经过hashCode()的高16位异或低16位实现的：(h = k.hashCode()) ^ (h >>> 16)，主要是从速度、功效、质量来考虑的，这么作能够在数组table的length比较小的时候，也能保证考虑到高低Bit都参与到Hash的计算中，同时不会有太大的开销。

咱们举个栗子：

大概的获得索引的流程就是上面所示。

 

4）hashmap的put实现方法：

put函数大体的思路为：

1. 对key的hashCode()作hash，而后再计算index;
2. 若是没碰撞直接放到bucket里；
3. 若是碰撞了，以链表的形式存在buckets后；
4. 若是碰撞致使链表过长(大于等于TREEIFY_THRESHOLD)，就把链表转换成红黑树；
5. 若是节点已经存在就替换old value(保证key的惟一性)
6. 若是bucket满了(超过load factor*current capacity)，就要resize。

代码以下

public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }

    /**
     * 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
     */
    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;
        if ((p = tab[i = (n - 1) & hash]) == null)
            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))))
                e = p;
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        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)
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }

 

5）hashmap的get方法：

思路以下：

1. bucket里的第一个节点，直接命中；
2. 若是有冲突，则经过key.equals(k)去查找对应的entry 
   若为树，则在树中经过key.equals(k)查找，O(logn)； 
   若为链表，则在链表中经过key.equals(k)查找，O(n)。

代码以下：

public V get(Object key) {
        Node<K,V> e;
        return (e = getNode(hash(key), key)) == null ? null : e.value;
    }

    /**
     * Implements Map.get and related methods
     *
     * @param hash hash for key
     * @param key the key
     * @return the node, or null if none
     */
    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) {
            if (first.hash == hash && // always check first node
                ((k = first.key) == key || (key != null && key.equals(k))))
                return first;
            if ((e = first.next) != null) {
                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;
    }

 

 

注意：上述put的思路从putval的方法中是正确的，可是若是将putval方法打碎了分析，这个思路是不彻底的，这就涉及到了hashmap的扩容机制，我会在下一篇hashmap的讲解中来具体讲解，putval在不一样状况下是怎么运行的，以及扩容机制中最重要的函数，resize（）；

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