HashMap详解

HashMap父类Map ：[url]http://donald-draper.iteye.com/blog/2361603[/url]
Map的简单实现AbstractMap ：[url]http://donald-draper.iteye.com/blog/2361627[/url]
前言：
要将对象存放在一起，如何设计这个容器。目前只有两条路可以走，一种是采用分格技术，每一个对象存放于一个格子中，这样通过对格子的编号就能取到或者遍历对象；另一种技术就是采用串联的方式，将各个对象串联起来，这需要各个对象至少带有下一个对象的索引（或者指针）。显然第一种就是数组的概念，第二种就是链表的概念。所有的容器的实现其实都是基于这两种方式的，不管是数组还是链表，或者二者俱有。HashMap采用的就是数组的方式。
有了存取对象的容器后还需要以下两个条件才能完成Map所需要的条件。
1.能够快速定位元素：Map的需求就是能够根据一个查询条件快速得到需要的结果，
所以这个过程需要的就是尽可能的快。
2.能够自动扩充容量：显然对于容器而然，不需要人工的去控制容器的容量是最好的，
这样对于外部使用者来说越少知道底部细节越好，不仅使用方便，也越安全。

首先条件1，快速定位元素。快速定位元素属于算法和数据结构的范畴，通常情况下哈希（Hash）算法是一种简单可行的算法。所谓哈希算法，是将任意长度的二进制值映射为固定长度的较小二进制值。常见的MD2,MD4,MD5，SHA-1等都属于Hash算法的范畴。具体的算法原理和介绍可以参考相应的算法和数据结构的书籍，但是这里特别提醒一句，由于将一个较大的集合映射到一个较小的集合上，所以必然就存在多个元素映射到同一个元素上的结果，这个叫“碰撞”，后面会用到此知识，暂且不表。条件2，如果满足了条件1，一个元素映射到了某个位置，现在一旦扩充了容量，也就意味着元素映射的位置需要变化。因为对于Hash算法来说，调整了映射的小集合，那么原来映射的路径肯定就不复存在，那么就需要对现有重新计算映射路径，也就是所谓的rehash过程。好了有了上面的理论知识后来看HashMap是如何实现的。

下面我们来看一下HashMap的具体实现：

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

/**
 * Hash table based implementation of the <tt>Map</tt> interface.  This
 * implementation provides all of the optional map operations, and permits
 * <tt>null</tt> values and the <tt>null</tt> key.  (The <tt>HashMap</tt>
 * class is roughly equivalent to <tt>Hashtable</tt>, except that it is
 * unsynchronized and permits nulls.)  This class makes no guarantees as to
 * the order of the map; in particular, it does not guarantee that the order
 * will remain constant over time.
 *
 Hashtable提供了Map接口所有可选的实现，并且允许k和v为null。HashMap基本功能和Hashtable
 相同，允许k和v为null，但有一点，HashMap是非线程安全的。同时不能保证Entry的顺序。

 * <p>This implementation provides constant-time performance for the basic
 * operations (<tt>get</tt> and <tt>put</tt>), assuming the hash function
 * disperses the elements properly among the buckets.  Iteration over
 * collection views requires time proportional to the "capacity" of the
 * <tt>HashMap</tt> instance (the number of buckets) plus its size (the number
 * of key-value mappings).  Thus, it's very important not to set the initial
 * capacity too high (or the load factor too low) if iteration performance is
 * important.
 *
HashMap假设能够将Entry分配到合适的桶中，put和get的时间复杂度为常量。
遍历key或value和Entry的时间复杂度为HashMap的capacity+Entry的数量有关。
如果对遍历Entry有一定的性能要求，那么不能将capacity设置的太高或者load factor
因子太低。


 * <p>An instance of <tt>HashMap</tt> has two parameters that affect its
 * performance: <i>initial capacity</i> and <i>load factor</i>.  The
 * <i>capacity</i> is the number of buckets in the hash table, and the initial
 * capacity is simply the capacity at the time the hash table is created.  The
 * <i>load factor</i> is a measure of how full the hash table is allowed to
 * get before its capacity is automatically increased.  When the number of
 * entries in the hash table exceeds the product of the load factor and the
 * current capacity, the hash table is <i>rehashed</i> (that is, internal data
 * structures are rebuilt) so that the hash table has approximately twice the
 * number of buckets.
 *
 HashMap有两个参数初始化的capacity和load factor可能会影响到它的性能。capacity决定
 者hash table中的桶数量，load factor是一个，衡量是否需要增加capacity的标准。
 当Entry的数量超过容量capacity或load factor，则将会rehashed，内部的数据结构将会重建，
 以保证hash table拥有接近2倍的buckets。
 * <p>As a general rule, the default load factor (.75) offers a good tradeoff
 * between time and space costs.  Higher values decrease the space overhead
 * but increase the lookup cost (reflected in most of the operations of the
 * <tt>HashMap</tt> class, including <tt>get</tt> and <tt>put</tt>).  The
 * expected number of entries in the map and its load factor should be taken
 * into account when setting its initial capacity, so as to minimize the
 * number of rehash operations.  If the initial capacity is greater
 * than the maximum number of entries divided by the load factor, no
 * rehash operations will ever occur.
 *
 作为一个默认条件，load factor为0.75能够在时间和空间性能方面，提供一个折中。
 当空间负载越多，那么将会消耗更多的时间，在get和put操作上。当我们设置初始化容量
capacity，应该考虑肯能会有多少Entry，及负载因子load factor，以减少rehash的可能。
如果实际的Entry数量，达不到capacity*load factor，那么rehash操作将不会发生。
 * <p>If many mappings are to be stored in a <tt>HashMap</tt> instance,
 * creating it with a sufficiently large capacity will allow the mappings to
 * be stored more efficiently than letting it perform automatic rehashing as
 * needed to grow the table.
 *
 如果有许多Entry将会放入到HashMap中，我们应该创建HashMap时，初始化capacity充分
 足够的大，使存储Entry更有效，而不是自动的重hash，增加hash table空间。
 * <p><strong>Note that this implementation is not synchronized.</strong>
 * If multiple threads access a hash map concurrently, and at least one of
 * the threads modifies the map structurally, it <i>must</i> be
 * synchronized externally.  (A structural modification is any operation
 * that adds or deletes one or more mappings; merely changing the value
 * associated with a key that an instance already contains is not a
 * structural modification.)  This is typically accomplished by
 * synchronizing on some object that naturally encapsulates the map.
 *
 HashMap是非线程安全的，如果有多个线程同时访问HashMap，至少有一个可以线程
 安全的修改Map结构。我们可以通过一些object的synchronizing来实现。
 * If no such object exists, the map should be "wrapped" using the
 * {@link Collections#synchronizedMap Collections.synchronizedMap}
 * method.  This is best done at creation time, to prevent accidental
 * unsynchronized access to the map:<pre>
 *   Map m = Collections.synchronizedMap(new HashMap(...));</pre>
 *
 如果没有这样的Object存在，我们应该通过Collections#synchronizedMap将HashMap包装成
 ，线程安全的。这一操作最好在创建的时候做，以防止非安全地访问Map。
 * <p>The iterators returned by all of this class's "collection view methods"
 * are <i>fail-fast</i>: if the map is structurally modified at any time after
 * the iterator is created, in any way except through the iterator's own
 * <tt>remove</tt> method, the iterator will throw a
 * {@link ConcurrentModificationException}.  Thus, in the face of concurrent
 * modification, the iterator fails quickly and cleanly, rather than risking
 * arbitrary, non-deterministic behavior at an undetermined time in the
 * future.
 *
 HashMap的k，v和Entry视图集，都是fail-fast：意思为在iterator创建之后，
 如果Map的结构，通过iterator自己的方法remove将抛出ConcurrentModificationException。
 因为在并发修改方面，iterator将会quickly and cleanly, rather than risking
 arbitrary, non-deterministic behavior at an undetermined time in the
 future.

 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
 * as it is, generally speaking, impossible to make any hard guarantees in the
 * presence of unsynchronized concurrent modification.  Fail-fast iterators
 * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
 * Therefore, it would be wrong to write a program that depended on this
 * exception for its correctness: <i>the fail-fast behavior of iterators
 * should be used only to detect bugs.</i>
 *
iterator的fail-fast是不能保证它自己，换一种说法，在并发访问下，其不能保证
线程安全。 Fail-fast iterators throw ConcurrentModificationException
on a best-effort basis.因此等我们开发程序依赖这个异常时，将会错误，这种行为
只是被用来检测bug。
 * <p>This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @param <K> the type of keys maintained by this map
 * @param <V> the type of mapped values
 *
 * @author  Doug Lea
 * @author  Josh Bloch
 * @author  Arthur van Hoff
 * @author  Neal Gafter
 * @see     Object#hashCode()
 * @see     Collection
 * @see     Map
 * @see     TreeMap
 * @see     Hashtable
 * @since   1.2
 */

public class HashMap<K,V>
    extends AbstractMap<K,V>
    implements Map<K,V>, Cloneable, Serializable
{
     /**
     * The default initial capacity - MUST be a power of two.
     */
     //默认Entry table的初始化容量
    static final int DEFAULT_INITIAL_CAPACITY = 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.
     */
     //最大容量
    static final int MAXIMUM_CAPACITY = 1 << 30;

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

    /**
     * The table, resized as necessary. Length MUST Always be a power of two.
     */
    //存放Entry数组
    transient Entry<K,V>[] table;

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

    /**
     * The next size value at which to resize (capacity * load factor).
     * @serial
     */
     //threshold=capacity * load factor,当table中的实际容量为threshold，则重新rehash
     //扩大容量为初始化容量的两倍
    int threshold;

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

    /**
     * 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).
     */
     //HashMap结构修改的次数，结构性的修改是指，改变Entry的数量
    transient int modCount;

    /**
     * The default threshold of map capacity above which alternative hashing is
     * used for String keys. Alternative hashing reduces the incidence of
     * collisions due to weak hash code calculation for String keys.
     * <p/>
     默认的HashMap，扩容界限
     * This value may be overridden by defining the system property
     * {@code jdk.map.althashing.threshold}. A property value of {@code 1}
     * forces alternative hashing to be used at all times whereas
     * {@code -1} value ensures that alternative hashing is never used.
     */
    static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE;

    /**
     * holds values which can't be initialized until after VM is booted.
     */
    private static class Holder {

            // Unsafe mechanics
        /**
         * Unsafe utilities
         */
        static final sun.misc.Unsafe UNSAFE;

        /**
         * Offset of "final" hashSeed field we must set in readObject() method.
         */
        static final long HASHSEED_OFFSET;

        /**
         * Table capacity above which to switch to use alternative hashing.
         */
        static final int ALTERNATIVE_HASHING_THRESHOLD;

        static {
            String altThreshold = java.security.AccessController.doPrivileged(
                new sun.security.action.GetPropertyAction(
                    "jdk.map.althashing.threshold"));

            int threshold;
            try {
                threshold = (null != altThreshold)
                        ? Integer.parseInt(altThreshold)
                        : ALTERNATIVE_HASHING_THRESHOLD_DEFAULT;

                // disable alternative hashing if -1
                if (threshold == -1) {
                    threshold = Integer.MAX_VALUE;
                }

                if (threshold < 0) {
                    throw new IllegalArgumentException("value must be positive integer.");
                }
            } catch(IllegalArgumentException failed) {
                throw new Error("Illegal value for 'jdk.map.althashing.threshold'", failed);
            }
            ALTERNATIVE_HASHING_THRESHOLD = threshold;

            try {
                UNSAFE = sun.misc.Unsafe.getUnsafe();
                HASHSEED_OFFSET = UNSAFE.objectFieldOffset(
                    HashMap.class.getDeclaredField("hashSeed"));
            } catch (NoSuchFieldException | SecurityException e) {
                throw new Error("Failed to record hashSeed offset", e);
            }
        }
    }
    **
     * If {@code true} then perform alternative hashing of String keys to reduce
     * the incidence of collisions due to weak hash code calculation.
     */
    transient boolean useAltHashing;

    /**
     * A randomizing value associated with this instance that is applied to
     * hash code of keys to make hash collisions harder to find.
     */
     //hash种子
    transient final int hashSeed = sun.misc.Hashing.randomHashSeed(this);
}

来看构造

    /**
     * Constructs an empty <tt>HashMap</tt> with the default initial capacity
     * (16) and the default load factor (0.75).
     */
     默认容量及负载因子
    public HashMap() {
        this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
    }
 //初始化容量，默认负载因子
 public HashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }
/**
     * Constructs an empty <tt>HashMap</tt> with the specified initial
     * capacity and load factor.
     *
     构造一个空HashMap，初始化容量为capacity，负载因子为load factor
     * @param  initialCapacity the initial capacity
     * @param  loadFactor      the load factor
     * @throws IllegalArgumentException if the initial capacity is negative
     *         or the load factor is nonpositive
     */
    public HashMap(int initialCapacity, float loadFactor) {
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal initial capacity: " +
                                               initialCapacity);
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new IllegalArgumentException("Illegal load factor: " +
                                               loadFactor);

        // Find a power of 2 >= initialCapacity
        int capacity = 1;
	//我们给的初始化容量为initialCapacity，而实际初始容量为第一个2^N>initialCapacity
	的容量值，不然我们的initialCapacity为大于16，小于31的数，那么capacity为32
        while (capacity < initialCapacity)
            capacity <<= 1;
        this.loadFactor = loadFactor;
	//计算扩容临界点
        threshold = (int)Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);
        //创建Entry table
	table = new Entry[capacity];
        useAltHashing = sun.misc.VM.isBooted() &&
                (capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
        init();
    }
// internal utilities

    /**
     * Initialization hook for subclasses. This method is called
     * in all constructors and pseudo-constructors (clone, readObject)
     * after HashMap has been initialized but before any entries have
     * been inserted.  (In the absence of this method, readObject would
     * require explicit knowledge of subclasses.)
     */
    void init() {
    //待子类扩展，做一些在初始化之后，Entry被插入之前；
    }

相关方法

/**
     * Returns the number of key-value mappings in this map.
     *当前容量
     * @return the number of key-value mappings in this map
     */
    public int size() {
        return size;
    }

    /**
     * Returns <tt>true</tt> if this map contains no key-value mappings.
     *是否为空
     * @return <tt>true</tt> if this map contains no key-value mappings
     */
    public boolean isEmpty() {
        return size == 0;
    }

在往下看之前，我们先看一下Entry的实现

 static class Entry<K,V> implements Map.Entry<K,V> {
        final K key;
        V value;
        Entry<K,V> next;//当有hash值相同时，放在同一个桶，链接
        int hash;//hash值

        /**
         * Creates new entry.构造一个新的Entry
         */
        Entry(int h, K k, V v, Entry<K,V> n) {
            value = v;
            next = n;//next为n
            key = k;
            hash = h;
        }

        public final K getKey() {
            return key;
        }

        public final V getValue() {
            return value;
        }

        public final V setValue(V newValue) {
            V oldValue = value;
            value = newValue;
            return oldValue;
        }

        public final boolean equals(Object o) {
            if (!(o instanceof Map.Entry))
                return false;
            Map.Entry e = (Map.Entry)o;
            Object k1 = getKey();
            Object k2 = e.getKey();
            if (k1 == k2 || (k1 != null && k1.equals(k2))) {
                Object v1 = getValue();
                Object v2 = e.getValue();
                if (v1 == v2 || (v1 != null && v1.equals(v2)))
                    return true;
            }
            return false;
        }

        public final int hashCode() {
            return (key==null   ? 0 : key.hashCode()) ^
                   (value==null ? 0 : value.hashCode());
        }

        public final String toString() {
            return getKey() + "=" + getValue();
        }

        /**
         * This method is invoked whenever the value in an entry is
         * overwritten by an invocation of put(k,v) for a key k that's already
         * in the HashMap.
         */
        void recordAccess(HashMap<K,V> m) {
        }

        /**
         * This method is invoked whenever the entry is
         * removed from the table.
         */
        void recordRemoval(HashMap<K,V> m) {
        }
    }

Entry与AbstractMap中Entry没有太大的区别，增加了一个hash值，一个Entry next链接

来看put操作

public V put(K key, V value) {
        if (key == null)
	    //如果key为null，执行放一个null Key的操作
            return putForNullKey(value);
	 //获取hash值
        int hash = hash(key);
	//根据hash值和table长度获取，table索引
        int i = indexFor(hash, table.length);
	//遍历table索引i对应的Entry链，如果存在对应的Key，则替换旧值
        for (Entry<K,V> e = table[i]; e != null; e = e.next) {
            Object k;
	    //如果hash值相同，并且key相等，替换key对应的旧值。
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;
            }
        }
        modCount++;
	//如果对应的Key的hash不存在，则添加新Entry
        addEntry(hash, key, value, i);
        return null;
    }

这个操作，我们一步一步的看
分如下几步
1.

if (key == null)
//如果key为null，执行放一个null Key的操作
    return putForNullKey(value);

2.//获取hash值

int hash = hash(key);

3.
//根据hash值和table长度获取，table索引

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

4.
//遍历table索引i对应的Entry链，如果存在对应的Key，则替换旧值

  for (Entry<K,V> e = table[i]; e != null; e = e.next) {
            Object k;
	    //如果hash值相同，并且key相等，替换key对应的旧值。
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;
            }
        }

5.
//如果对应的Key的hash不存在，则添加新Entry

 addEntry(hash, key, value, i);

以上5步第四步就不看了很好理解下面我们分别来看上面几步

1.

if (key == null)
//如果key为null，执行放一个null Key的操作
    return putForNullKey(value);

从下面的代码可以看出，空key对应Entry是放在table的索引0上面

* Offloaded version of put for null keys
     */
    private V putForNullKey(V value) {
    //遍历table索引0对应的Entry链，如果存在对应的null key，则替换旧值
        for (Entry<K,V> e = table[0]; e != null; e = e.next) {
            if (e.key == null) {
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;
            }
        }
        modCount++;
	//否则，添加新Entry，这个与第五步一样，我们第五步统一看
        addEntry(0, null, value, 0);
        return null;
    }

2.//获取hash值

int hash = hash(key);

final int hash(Object k) {
        int h = 0;
        if (useAltHashing) {
            if (k instanceof String) {
                return sun.misc.Hashing.stringHash32((String) k);
            }
            h = hashSeed;
        }

        h ^= k.hashCode();

        // 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);
    }

3.
//根据hash值和table长度获取，table索引

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

 /**
     * Returns index for hash code h.
     */
    static int indexFor(int h, int length) {
        return h & (length-1);
    }

4.

//遍历table索引i对应的Entry链，如果存在对应的Key，则替换旧值

     for (Entry<K,V> e = table[i]; e != null; e = e.next) {
            Object k;
	    //如果hash值相同，并且key相等，替换key对应的旧值。
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;
            }
        }

这步没什么多看的

5.
//如果对应的Key的hash不存在，则添加新Entry

void addEntry(int hash, K key, V value, int bucketIndex) {
        if ((size >= threshold) && (null != table[bucketIndex])) {
	   //若果size大于扩容的临界条件，则扩容
            resize(2 * table.length);
            hash = (null != key) ? hash(key) : 0;
	    //从新获取索引
            bucketIndex = indexFor(hash, table.length);
        }
        //如果size没有达到临界点，则创建createEntry
        createEntry(hash, key, value, bucketIndex);
    }

这个分两步走
A：

if ((size >= threshold) && (null != table[bucketIndex])) {
	   //若果size大于扩容的临界条件，则扩容
            resize(2 * table.length);
            hash = (null != key) ? hash(key) : 0;
	    //重新获取索引
            bucketIndex = indexFor(hash, table.length);
        }

B:
//如果size没有达到临界点，则创建createEntry

 createEntry(hash, key, value, bucketIndex);

我们先看B：
B:
//如果size没有达到临界点，则创建createEntry

  createEntry(hash, key, value, bucketIndex);

void createEntry(int hash, K key, V value, int bucketIndex) {
        //获取索引对应的Entry
        Entry<K,V> e = table[bucketIndex];
        table[bucketIndex] = new Entry<>(hash, key, value, e);
        size++;
    }

从上面来看，首先获取table指定索引bucketIndex，链头的Entry，
添加新的Entry到table指定索引bucketIndex作为链头，next指向之前的链头的Entry


A：

if ((size >= threshold) && (null != table[bucketIndex])) {
	   //若果size大于扩容的临界条件，则扩容
            resize(2 * table.length);
            hash = (null != key) ? hash(key) : 0;
	    //重新获取索引
            bucketIndex = indexFor(hash, table.length);
        }

先看扩容

void resize(int newCapacity) {
        //获取旧的Entry table
        Entry[] oldTable = table;
        int oldCapacity = oldTable.length;
        if (oldCapacity == MAXIMUM_CAPACITY) {
	    //扩容失败，已经达到最大容量
            threshold = Integer.MAX_VALUE;
            return;
        }
        //创建新Entry table
        Entry[] newTable = new Entry[newCapacity];
        boolean oldAltHashing = useAltHashing;
        useAltHashing |= sun.misc.VM.isBooted() &&
                (newCapacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
        boolean rehash = oldAltHashing ^ useAltHashing;
	//重新hash，转移旧的table，到扩容后的table
        transfer(newTable, rehash);
        table = newTable;
	//重新设定扩容临界点
        threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
    }

 /**
     * Transfers all entries from current table to newTable.
     */
    void transfer(Entry[] newTable, boolean rehash) {
        int newCapacity = newTable.length;
	//遍历旧的table，获取table中Entry
        for (Entry<K,V> e : table) {
            while(null != e) {
                Entry<K,V> next = e.next;
                if (rehash) {
                    e.hash = null == e.key ? 0 : hash(e.key);
                }
		//获取Entry的新table索引index
                int i = indexFor(e.hash, newCapacity);
                e.next = newTable[i];
                newTable[i] = e;
                e = next;
            }
        }
    }

小节一下put操作，

如果key为null则放在Entry table的index为0的上面，如果存在null key则替换旧值；
如果key不为null，则获取key的hash值，然后再获取Entry在table中的index，如果对应
的index上有Key对应的Entry，则替换旧值返回，否则添加新的Entry到table的index对应
的链表中。当添加Entry时的size达到临界条件，则扩容table为原来的两倍，重新hash旧table中Entry对应的key，并定位Entry应该存放的桶索引index，放到新table中。如果在扩容是旧容量已经达到最大容量，则扩容失败；添加新的Entry到table的index对应的桶链表时，将Entry放在链表头，next指向原始的链表头。

再看get操作
//获取key对应的值

public V get(Object key) {
        if (key == null)
	   //返回当key为null的值
            return getForNullKey();
	 //获取Key对应的Entry
        Entry<K,V> entry = getEntry(key);
	//返回Entry的值
        return null == entry ? null : entry.getValue();
    }

先看为key为null，再看不为null
1.

 if (key == null)
	   //返回当key为null的值
            return getForNullKey();

2
//获取Key对应的Entry

    Entry<K,V> entry = getEntry(key);
	//返回Entry的值
        return null == entry ? null : entry.getValue();

1.

if (key == null)
	   //返回当key为null的值
            return getForNullKey();

//获取table索引为0的链表中key为null的值

  private V getForNullKey() {
        for (Entry<K,V> e = table[0]; e != null; e = e.next) {
            if (e.key == null)
                return e.value;
        }
        return null;
    }

2
//获取Key对应的Entry

 Entry<K,V> entry = getEntry(key);
	//返回Entry的值
        return null == entry ? null : entry.getValue();

final Entry<K,V> getEntry(Object key) {
        int hash = (key == null) ? 0 : hash(key);
        for (Entry<K,V> e = table[indexFor(hash, table.length)];
             e != null;
             e = e.next) {
            Object k;
            if (e.hash == hash &&
                ((k = e.key) == key || (key != null && key.equals(k))))
                return e;
        }
        return null;
    }

从代码可以看出，先获取key的hash值，在获取key对应的table index，遍历table的
index对应的Entry链表，找出与key相同的Entry，返回。

从上面可以看出:
get操作首先判断key是否为null，如果为null，则获取table索引为0的链表中key为null的值
否则先获取key的hash值，在获取key对应的table index，遍历table的
index对应的Entry链表，找出与key相同的Entry，返回Entry对应的值。

再看是否包含key

public boolean containsKey(Object key) {
        return getEntry(key) != null;
    }

是否包含value
//遍历table中的所有Entry链表，找value相等的，则返回ture，
从时间复杂度上，来讲，最好不要调用者这个方法

public boolean containsValue(Object value) {
        if (value == null)
            return containsNullValue();

        Entry[] tab = table;
        for (int i = 0; i < tab.length ; i++)
            for (Entry e = tab[i] ; e != null ; e = e.next)
                if (value.equals(e.value))
                    return true;
        return false;
    }

是否包含null值

 /**
     * Special-case code for containsValue with null argument
     */
    private boolean containsNullValue() {
        Entry[] tab = table;
        for (int i = 0; i < tab.length ; i++)
            for (Entry e = tab[i] ; e != null ; e = e.next)
                if (e.value == null)
                    return true;
        return false;
    }

//克隆

public Object clone() {
        HashMap<K,V> result = null;
        try {
            result = (HashMap<K,V>)super.clone();
        } catch (CloneNotSupportedException e) {
            // assert false;
        }
        result.table = new Entry[table.length];
        result.entrySet = null;
        result.modCount = 0;
        result.size = 0;
        result.init();
        result.putAllForCreate(this);

        return result;
    }

移除key

/**
     * Removes the mapping for the specified key from this map if present.
     *
     * @param  key key whose mapping is to be removed from the map
     * @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 remove(Object key) {
        Entry<K,V> e = removeEntryForKey(key);
        return (e == null ? null : e.value);
    }

    /**
     * Removes and returns the entry associated with the specified key
     * in the HashMap.  Returns null if the HashMap contains no mapping
     * for this key.
     */
    final Entry<K,V> removeEntryForKey(Object key) {
        int hash = (key == null) ? 0 : hash(key);
        int i = indexFor(hash, table.length);
        Entry<K,V> prev = table[i];
        Entry<K,V> e = prev;

        while (e != null) {
            Entry<K,V> next = e.next;
            Object k;
            if (e.hash == hash &&
                ((k = e.key) == key || (key != null && key.equals(k)))) {
                modCount++;
                size--;
                if (prev == e)
                    table[i] = next;
                else
                    prev.next = next;
                e.recordRemoval(this);
                return e;
            }
            prev = e;
            e = next;
        }

从代码可以看出，移除操作首先获取key对应的hash值，在定位table的index，遍历index对应
的Entry链表，找到key和hash值相等的，移除。


//遍历所有Entry，放到Map中

public void putAll(Map<? extends K, ? extends V> m) {
        int numKeysToBeAdded = m.size();
        if (numKeysToBeAdded == 0)
            return;

        /*
         * Expand the map if the map if the number of mappings to be added
         * is greater than or equal to threshold.  This is conservative; the
         * obvious condition is (m.size() + size) >= threshold, but this
         * condition could result in a map with twice the appropriate capacity,
         * if the keys to be added overlap with the keys already in this map.
         * By using the conservative calculation, we subject ourself
         * to at most one extra resize.
         */
        if (numKeysToBeAdded > threshold) {
            int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1);
            if (targetCapacity > MAXIMUM_CAPACITY)
                targetCapacity = MAXIMUM_CAPACITY;
            int newCapacity = table.length;
            while (newCapacity < targetCapacity)
                newCapacity <<= 1;
            if (newCapacity > table.length)
                resize(newCapacity);
        }

        for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
            put(e.getKey(), e.getValue());
    }

下面来看一下Map的k，v和Entry视图

// Views

    private transient Set<Map.Entry<K,V>> entrySet = null;

public Set<Map.Entry<K,V>> entrySet() {
        //委托给entrySet0
        return entrySet0();
    }

    private Set<Map.Entry<K,V>> entrySet0() {
        Set<Map.Entry<K,V>> es = entrySet;
        return es != null ? es : (entrySet = new EntrySet());
    }

EntrySet的所有操作以HashMap相关联

 private final class EntrySet extends AbstractSet<Map.Entry<K,V>> {
        public Iterator<Map.Entry<K,V>> iterator() {
            return newEntryIterator();
        }
        public boolean contains(Object o) {
            if (!(o instanceof Map.Entry))
                return false;
            Map.Entry<K,V> e = (Map.Entry<K,V>) o;
            Entry<K,V> candidate = getEntry(e.getKey());
            return candidate != null && candidate.equals(e);
        }
        public boolean remove(Object o) {
            return removeMapping(o) != null;
        }
        public int size() {
            return size;
        }
        public void clear() {
            HashMap.this.clear();
        }
    }

创建EntryIterator

Iterator<Map.Entry<K,V>> newEntryIterator()   {
        return new EntryIterator();
    }

 private final class EntryIterator extends HashIterator<Map.Entry<K,V>> {
        public Map.Entry<K,V> next() {
            return nextEntry();
        }
    }

private abstract class HashIterator<E> implements Iterator<E> {
        Entry<K,V> next;        // next entry to return
        int expectedModCount;   // For fast-fail
        int index;              // current slot
        Entry<K,V> current;     // current entry

        HashIterator() {
	//构造
            expectedModCount = modCount;
            if (size > 0) { // advance to first entry
                Entry[] t = table;
                while (index < t.length && (next = t[index++]) == null)
                    ;
            }
        }

        public final boolean hasNext() {
            return next != null;
        }

        final Entry<K,V> nextEntry() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            Entry<K,V> e = next;
            if (e == null)
                throw new NoSuchElementException();

            if ((next = e.next) == null) {
                Entry[] t = table;
		//遍历table
                while (index < t.length && (next = t[index++]) == null)
                    ;
            }
            current = e;
            return e;
        }

        public void remove() {
            if (current == null)
                throw new IllegalStateException();
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            Object k = current.key;
            current = null;
            HashMap.this.removeEntryForKey(k);
            expectedModCount = modCount;
        }
    }

private final class ValueIterator extends HashIterator<V> {
        public V next() {
            return nextEntry().value;
        }
    }

   private final class KeyIterator extends HashIterator<K> {
        public K next() {
            return nextEntry().getKey();
        }
    }

Value视图

 public Collection<V> values() {
        Collection<V> vs = values;
        return (vs != null ? vs : (values = new Values()));
    }

 private final class Values extends AbstractCollection<V> {
        public Iterator<V> iterator() {
            return newValueIterator();
        }
        public int size() {
            return size;
        }
        public boolean contains(Object o) {
            return containsValue(o);
        }
        public void clear() {
            HashMap.this.clear();
        }
    }

Key视图

  public Set<K> keySet() {
        Set<K> ks = keySet;
        return (ks != null ? ks : (keySet = new KeySet()));
    }

private final class KeySet extends AbstractSet<K> {
        public Iterator<K> iterator() {
            return newKeyIterator();
        }
        public int size() {
            return size;
        }
        public boolean contains(Object o) {
            return containsKey(o);
        }
        public boolean remove(Object o) {
            return HashMap.this.removeEntryForKey(o) != null;
        }
        public void clear() {
            HashMap.this.clear();
        }
    }

从上面可以看出Key视图和Value视图的实现是在Entry的基础上，所以我们遍历HashMap的时候
最好用EntrySet。


// These methods are used when serializing HashSets

 int   capacity()     { return table.length; }
    float loadFactor()   { return loadFactor;   }

清空Map

/**
     * Removes all of the mappings from this map.
     * The map will be empty after this call returns.
     */
    public void clear() {
        modCount++;
        Entry[] tab = table;
        for (int i = 0; i < tab.length; i++)
            tab[i] = null;
        size = 0;
    }

序列化

private void writeObject(java.io.ObjectOutputStream s)
        throws IOException
    {
        Iterator<Map.Entry<K,V>> i =
            (size > 0) ? entrySet0().iterator() : null;

        // Write out the threshold, loadfactor, and any hidden stuff
        s.defaultWriteObject();

        // Write out number of buckets
        s.writeInt(table.length);

        // Write out size (number of Mappings)
        s.writeInt(size);

        // Write out keys and values (alternating)
        if (size > 0) {
            for(Map.Entry<K,V> e : entrySet0()) {
                s.writeObject(e.getKey());
                s.writeObject(e.getValue());
            }
        }
    }

 private static final long serialVersionUID = 362498820763181265L;

从上面可以看出序列化时，先调用默认的序列化，在写table长度，size，
遍历Entry，写key和value。

反序列:

 private void readObject(java.io.ObjectInputStream s)
         throws IOException, ClassNotFoundException
    {
        // Read in the threshold (ignored), loadfactor, and any hidden stuff
        s.defaultReadObject();
        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new InvalidObjectException("Illegal load factor: " +
                                               loadFactor);

        // set hashSeed (can only happen after VM boot)
        Holder.UNSAFE.putIntVolatile(this, Holder.HASHSEED_OFFSET,
                sun.misc.Hashing.randomHashSeed(this));

        // Read in number of buckets and allocate the bucket array;
        s.readInt(); // ignored

        // Read number of mappings
        int mappings = s.readInt();
        if (mappings < 0)
            throw new InvalidObjectException("Illegal mappings count: " +
                                               mappings);

        int initialCapacity = (int) Math.min(
                // capacity chosen by number of mappings
                // and desired load (if >= 0.25)
                mappings * Math.min(1 / loadFactor, 4.0f),
                // we have limits...
                HashMap.MAXIMUM_CAPACITY);
        int capacity = 1;
        // find smallest power of two which holds all mappings
        while (capacity < initialCapacity) {
            capacity <<= 1;
        }

        table = new Entry[capacity];
        threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);
        useAltHashing = sun.misc.VM.isBooted() &&
                (capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);

        init();  // Give subclass a chance to do its thing.

        // Read the keys and values, and put the mappings in the HashMap
        for (int i=0; i<mappings; i++) {
            K key = (K) s.readObject();
            V value = (V) s.readObject();
            putForCreate(key, value);
        }
    }

     private void putForCreate(K key, V value) {
        int hash = null == key ? 0 : hash(key);
        int i = indexFor(hash, table.length);

        /**
         * Look for preexisting entry for key.  This will never happen for
         * clone or deserialize.  It will only happen for construction if the
         * input Map is a sorted map whose ordering is inconsistent w/ equals.
         */
        for (Entry<K,V> e = table[i]; e != null; e = e.next) {
            Object k;
            if (e.hash == hash &&
                ((k = e.key) == key || (key != null && key.equals(k)))) {
                e.value = value;
                return;
            }
        }

        createEntry(hash, key, value, i);
    }

总结：
[color=green]HashMap中有一个Entry数组table用于存储Entry，size描述的是Map中元素的大小，threshold描述的是扩容的临界条件，loadFactor是扩容因子(loadFactor>0)，计算临界条件threshold（capacity*loadFactor）的。那么容量就是table.length（capacity），也就是数组的大小。换句话说，如果存取的Entry的达到了整个容量threshold，那么就需要扩充容量了。在HashMap中每次扩容就是将扩大数组的一倍，使数组大小为原来的两倍。在初始化Map时，给的容量最好是2的N次方，在构造中我们，看到了怎么回事。扩充过程会导致元素数据的所有元素进行重新hash计算，这个过程也叫rehash。显然这是一个非常耗时的过程，否则扩容都会导致所有元素重新计算hash。因此尽可能的选择合适的初始化大小是有效提高HashMap效率的关键。太大了会导致过多的浪费空间，太小了就可能会导致繁重的rehash过程。在这个过程中loadFactor也可以考虑。举个例子来说，如果要存储1000个元素，采用默认扩容因子0.75，那么1024显然是不够的，因为1000>0.75*1024了，所以选择2048是必须的，显然浪费了1048个空间。如果确定最多只有1000个元素，那么扩容因子为1，那么1024是不错的选择。另外需要强调的一点是扩容因此越大，从统计学角度讲意味着链表的长度就也大，也就是在查找元素的时候就需要更多次的循环。所以凡事必然是一个平衡的过程。这里可能有人要问题，一旦我将Map的容量扩大后（也就是数组的大小），这个容量还能减小么？比如说刚开始Map中可能有10000个元素，运行一旦时间以后Map的大小永远不会超过10个，那么Map的容量能减小到10个或者16个么？答案就是不能，这个capacity一旦扩大后就不能减小了，只能通过构造一个新的Map来控制capacity了。

put操作:
如果key为null则放在Entry table的index为0的上面，如果存在null key则替换旧值；
如果key不为null，则获取key的hash值，然后再获取Entry在table中的index，如果对应
的index上有Key对应的Entry，则替换旧值返回，否则添加新的Entry到table的index对应
的链表中。当添加Entry时的size达到临界条件，则扩容table为原来的两倍，重新hash旧table中Entry对应的key，并定位Entry应该存放的桶索引index，放到新table中。如果在扩容是旧容量已经达到最大容量，则扩容失败；添加新的Entry到table的index对应的桶链表时，将Entry放在链表头，next指向原始的链表头。
get操作：
get操作首先判断key是否为null，如果为null，则获取table索引为0的链表中key为null的值
否则先获取key的hash值，在获取key对应的table index，遍历table的index对应的Entry链表，找出与key相同的Entry，返回Entry对应的值。

遍历：
Key视图和Value视图的实现是在Entry的基础上，所以我们遍历HashMap的时候最好用EntrySet。[/color]
附Hashtable源码以便比较：

*
 * Java Collections Framework</a>.  Unlike the new collection
 * implementations, {@code Hashtable} is synchronized.  If a
 * thread-safe implementation is not needed, it is recommended to use
 * {@link HashMap} in place of {@code Hashtable}.  If a thread-safe
 * highly-concurrent implementation is desired, then it is recommended
 * to use {@link java.util.concurrent.ConcurrentHashMap} in place of
 * {@code Hashtable}.
 *
 * @author  Arthur van Hoff
 * @author  Josh Bloch
 * @author  Neal Gafter
 * @see     Object#equals(java.lang.Object)
 * @see     Object#hashCode()
 * @see     Hashtable#rehash()
 * @see     Collection
 * @see     Map
 * @see     HashMap
 * @see     TreeMap
 * @since JDK1.0
 */
Hashtable是线程安全的，只是在方法上加了synchronized，保证多线程访问的安全。
如果需要多线程访问我们可以用ConcurrentHashMap
public class Hashtable<K,V>
    extends Dictionary<K,V>
    implements Map<K,V>, Cloneable, java.io.Serializable {

    /**
     * The hash table data.
     */
    private transient Entry<K,V>[] table;

    /**
     * The total number of entries in the hash table.
     */
    private transient int count;

    /**
     * The table is rehashed when its size exceeds this threshold.  (The
     * value of this field is (int)(capacity * loadFactor).)
     *
     * @serial
     */
    private int threshold;

    /**
     * The load factor for the hashtable.
     *
     * @serial
     */
    private float loadFactor;

    /**
     * The number of times this Hashtable has been structurally modified
     * Structural modifications are those that change the number of entries in
     * the Hashtable or otherwise modify its internal structure (e.g.,
     * rehash).  This field is used to make iterators on Collection-views of
     * the Hashtable fail-fast.  (See ConcurrentModificationException).
     */
    private transient int modCount = 0;

    /** use serialVersionUID from JDK 1.0.2 for interoperability */
    private static final long serialVersionUID = 1421746759512286392L；
    public synchronized int size() {
        return count;
    }

    /**
     * Tests if this hashtable maps no keys to values.
     *
     * @return  <code>true</code> if this hashtable maps no keys to values;
     *          <code>false</code> otherwise.
     */
    public synchronized boolean isEmpty() {
        return count == 0;
    }
    public synchronized V get(Object key) {
        Entry tab[] = table;
        int hash = hash(key);
        int index = (hash & 0x7FFFFFFF) % tab.length;
        for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
            if ((e.hash == hash) && e.key.equals(key)) {
                return e.value;
            }
        }
        return null;
    }
    public synchronized V put(K key, V value) {
        // Make sure the value is not null
        if (value == null) {
            throw new NullPointerException();
        }

        // Makes sure the key is not already in the hashtable.
        Entry tab[] = table;
        int hash = hash(key);
        int index = (hash & 0x7FFFFFFF) % tab.length;
        for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
            if ((e.hash == hash) && e.key.equals(key)) {
                V old = e.value;
                e.value = value;
                return old;
            }
        }

        modCount++;
        if (count >= threshold) {
            // Rehash the table if the threshold is exceeded
            rehash();

            tab = table;
            hash = hash(key);
            index = (hash & 0x7FFFFFFF) % tab.length;
        }

        // Creates the new entry.
        Entry<K,V> e = tab[index];
        tab[index] = new Entry<>(hash, key, value, e);
        count++;
        return null;
    }
    ...
    线程安全，只是在方法上加了synchronized，保证多线程访问的安全。
}