HashMap也是我们使用非常多的Collection，它是基于哈希表的 Map 接口的实现，以key-value的形式存在。 

HashMap是基于哈希表的Map接口的非同步实现。此实现提供所有可选的映射操作，并允许使用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。

本文使用的是JDK1.7的HashMap源码，因为JDK1.8的HashMap改变还很多，以后再去单一分析。

HashMap内部结构

查看HashMap源代码，可以发现其继承自AbstractMap, 并实现了Map, Cloneable, Serializable接口~

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

HashMap包含一个数组对象table[]，

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

数组中包含的元素为Entry对象，Entry是HashMap中定义的静态内部类，内容如下：

 static class Entry<K,V> implements Map.Entry<K,V> {         final K key;         V value;         Entry<K,V> next;         int hash;          /**          * Creates new entry.          */         Entry(int h, K k, V v, Entry<K,V> n) {             value = v;             next = 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 Objects.hashCode(getKey()) ^ Objects.hashCode(getValue());         }          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) {         }     } 

从Entry的类内容可以看出，其主要包含四个部分，

 static class Entry<K,V> implements Map.Entry<K,V> {         final K key;         V value;         Entry<K,V> next;         int hash;

key      --   键

Value   --  值

next     --  下一个Entry对象

hash    --   hash值

从上述的描述可以看出，HashMap的内部结构基本上是通过 “数组” + “链表”来实现~ 如下图所示：

HashMap默认的大小为16

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

接下来，使用一个简单示例来感受一下HashMap的数组+链表的结构存储。

如下是一个以名字为键，年龄为值的HashMap示例~

import java.util.HashMap; import java.util.Map;  public class MapExample {  	public static void main(String[] args) { 		 		Map<String, Integer> nameAgeMap = new HashMap<>(); 		 		nameAgeMap.put("Eric", 20); 		nameAgeMap.put("John", 21); 		nameAgeMap.put("LiLei", 19); 		nameAgeMap.put("Wang", 28); 		nameAgeMap.put("Zhang", 24); 		 		System.out.println(nameAgeMap); 	} } 

在输出结果之前，Debug一下nameAgeMap中的内容，如下图所示：

从上述的示例中可以看出，不同的Key根据hash映射到数组（table[]）的不同部分上去。

HashMap通过如下方式选择元素存放在哪一个table[]元素中去：

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

其中，

• hash(Key)的内容为

    /**      * Retrieve object hash code and applies a supplemental hash function to the      * result hash, which defends against poor quality hash functions.  This is      * critical because HashMap uses power-of-two length hash tables, that      * otherwise encounter collisions for hashCodes that do not differ      * in lower bits. Note: Null keys always map to hash 0, thus index 0.      */     final int hash(Object k) {         int h = hashSeed;         if (0 != h && k instanceof String) {             return sun.misc.Hashing.stringHash32((String) k);         }          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);     }

•  indexFor(hash, table.length)的内容为

    /**      * Returns index for hash code h.      */     static int indexFor(int h, int length) {         // assert Integer.bitCount(length) == 1 : "length must be a non-zero power of 2";         return h & (length-1);     }

如果不同元素计算出来的hash值是一样的，HashMap使用链表的方式来解决。

就像上述示例中，

table[6]存放了key为“LiLei”的Entry，并且这个Entry的next对象为key为“Eric”的Entry。

对HashMap的数据结构有了大致了解之后，就可以来看看，HashMap的主要方法实现是怎么样的~

方法put的实现

源代码

    /**      * 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.      *      * @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) {         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) {             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);         return null;     }

put方法步骤

put方法主要包含的部分如下图所示：

从源码可以看出，HashMap是支持键为null值的

使用代码来验证一下：

import java.util.HashMap; import java.util.Map;  public class Test {      public static void main(String[] args) {          Map<String, Integer> nameAgeMap = new HashMap<>();          nameAgeMap.put("One", 1);         nameAgeMap.put("Two", 2);         nameAgeMap.put("Three", 3);         nameAgeMap.put(null, 4);          for (Map.Entry<String, Integer> entry : nameAgeMap.entrySet()) {             System.out.println(entry.getKey() + ' ' + entry.getValue());         }      }  } 

输出：

null 4 Three 3 One 1 Two 2

addEntry方法

    /**      * Adds a new entry with the specified key, value and hash code to      * the specified bucket.  It is the responsibility of this      * method to resize the table if appropriate.      *      * Subclass overrides this to alter the behavior of put method.      */     void addEntry(int hash, K key, V value, int bucketIndex) {         if ((size >= threshold) && (null != table[bucketIndex])) {             resize(2 * table.length);             hash = (null != key) ? hash(key) : 0;             bucketIndex = indexFor(hash, table.length);         }          createEntry(hash, key, value, bucketIndex);     } 

• addEntry包含两个部分：

1. 如果元素越来越多，达到阈值（threshold），则需要将table大小乘2，并重新hash
2. 如果未到达阈值，则添加元素到相应的table[bucketIndex]中

createEntry方法

     /**      * Like addEntry except that this version is used when creating entries      * as part of Map construction or "pseudo-construction" (cloning,      * deserialization).  This version needn't worry about resizing the table.      *      * Subclass overrides this to alter the behavior of HashMap(Map),      * clone, and readObject.      */     void createEntry(int hash, K key, V value, int bucketIndex) {         Entry<K,V> e = table[bucketIndex];         table[bucketIndex] = new Entry<>(hash, key, value, e);         size++;     }

从上述可以看出，table[bucketIndex]中的Entry永远是存放最新的元素的~

方法get的实现

源代码

    /**      * Returns the value to which the specified key is mapped,      * or {@code null} if this map contains no mapping for the key.      *      * <p>More formally, if this map contains a mapping from a key      * {@code k} to a value {@code v} such that {@code (key==null ? k==null :      * key.equals(k))}, then this method returns {@code v}; otherwise      * it returns {@code null}.  (There can be at most one such mapping.)      *      * <p>A return value of {@code null} does not <i>necessarily</i>      * indicate that the map contains no mapping for the key; it's also      * possible that the map explicitly maps the key to {@code null}.      * The {@link #containsKey containsKey} operation may be used to      * distinguish these two cases.      *      * @see #put(Object, Object)      */     public V get(Object key) {         if (key == null)             return getForNullKey();         Entry<K,V> entry = getEntry(key);          return null == entry ? null : entry.getValue();     }

从上述源码可以看出，HashMap的在实现上考虑了key为null值或者不为null值。

三个步骤==>

1. 如果key为null值，则调用getForNullKey方法
2. 如果key不为null值，则调用 getEntry(key)来获取Entry
3. 如果获取的Entry为null，则返回null；不为null则返回entry.getValue()

getForNullKey方法

    /**      * Offloaded version of get() to look up null keys.  Null keys map      * to index 0.  This null case is split out into separate methods      * for the sake of performance in the two most commonly used      * operations (get and put), but incorporated with conditionals in      * others.      */     private V getForNullKey() {         if (size == 0) {             return null;         }         for (Entry<K,V> e = table[0]; e != null; e = e.next) {             if (e.key == null)                 return e.value;         }         return null;     }

如果Map中没有元素，则返回null，否则从table[0]中查找~

getEntry(Object key)方法

    /**      * Returns the entry associated with the specified key in the      * HashMap.  Returns null if the HashMap contains no mapping      * for the key.      */     final Entry<K,V> getEntry(Object key) {         if (size == 0) {             return null;         }          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;     }

1. 如果size为0，也就是HashMap中没有元素，则返回null
2. 如果HashMap中有元素，则先计算出key值对应的数组元素table[bucketIndex], 然后通过遍历链表获取元素

方法clear的实现

源代码

    /**      * Removes all of the mappings from this map.      * The map will be empty after this call returns.      */     public void clear() {         modCount++;         Arrays.fill(table, null);         size = 0;     }

HashMap中clear方法的实现包含了如下几个部分

1. 修改次数加1
2. 数组table的元素设置成null，调用Arrays.fill方法实现
3. HashMap中元素的个数size设置成0

Arrays.fill方法如下：

    /**      * Assigns the specified Object reference to each element of the specified      * array of Objects.      *      * @param a the array to be filled      * @param val the value to be stored in all elements of the array      * @throws ArrayStoreException if the specified value is not of a      *         runtime type that can be stored in the specified array      */     public static void fill(Object[] a, Object val) {         for (int i = 0, len = a.length; i < len; i++)             a[i] = val;     }

方法containsKey和containsValue的实现

containsKey的实现

    /**      * Returns <tt>true</tt> if this map contains a mapping for the      * specified key.      *      * @param   key   The key whose presence in this map is to be tested      * @return <tt>true</tt> if this map contains a mapping for the specified      * key.      */     public boolean containsKey(Object key) {         return getEntry(key) != null;     }      /**      * Returns the entry associated with the specified key in the      * HashMap.  Returns null if the HashMap contains no mapping      * for the key.      */     final Entry<K,V> getEntry(Object key) {         if (size == 0) {             return null;         }          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;     }

getEntry(key)方法说明如下：

1. 如果size为0，也就是HashMap中没有元素，则返回null
2. 如果HashMap中有元素，则先计算出key值对应的数组元素table[bucketIndex], 然后通过遍历链表获取元素

containsValue的实现

    /**      * Returns <tt>true</tt> if this map maps one or more keys to the      * specified value.      *      * @param value value whose presence in this map is to be tested      * @return <tt>true</tt> if this map maps one or more keys to the      *         specified value      */     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;     }      /**      * 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;     }

无论value的值是否为null值，都是采用双重循环来实现~

方法resize的实现

源代码

   /**      * Rehashes the contents of this map into a new array with a      * larger capacity.  This method is called automatically when the      * number of keys in this map reaches its threshold.      *      * If current capacity is MAXIMUM_CAPACITY, this method does not      * resize the map, but sets threshold to Integer.MAX_VALUE.      * This has the effect of preventing future calls.      *      * @param newCapacity the new capacity, MUST be a power of two;      *        must be greater than current capacity unless current      *        capacity is MAXIMUM_CAPACITY (in which case value      *        is irrelevant).      */     void resize(int newCapacity) {         Entry[] oldTable = table;         int oldCapacity = oldTable.length;         if (oldCapacity == MAXIMUM_CAPACITY) {             threshold = Integer.MAX_VALUE;             return;         }          Entry[] newTable = new Entry[newCapacity];         transfer(newTable, initHashSeedAsNeeded(newCapacity));         table = newTable;         threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);     }

就想上面提到过的，如果HashMap中添加元素超过了阈值，则需要扩容，并重新分配元素到新的数组( table[] )中去~

如transfer方法如下所示，其作用就是将所有的元素从当前table数组中，转移到新的table数组中

transfer方法

    /**      * Transfers all entries from current table to newTable.      */     void transfer(Entry[] newTable, boolean rehash) {         int newCapacity = newTable.length;         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);                 }                 int i = indexFor(e.hash, newCapacity);                 e.next = newTable[i];                 newTable[i] = e;                 e = next;             }         }     }

resize方法在添加单个元素的时候，需要考虑，同样在添加多个元素的时候更要考虑~

putAll方法

  /**      * Copies all of the mappings from the specified map to this map.      * These mappings will replace any mappings that this map had for      * any of the keys currently in the specified map.      *      * @param m mappings to be stored in this map      * @throws NullPointerException if the specified map is null      */     public void putAll(Map<? extends K, ? extends V> m) {         int numKeysToBeAdded = m.size();         if (numKeysToBeAdded == 0)             return;          if (table == EMPTY_TABLE) {             inflateTable((int) Math.max(numKeysToBeAdded * loadFactor, threshold));         }          /*          * 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());     }

putAll方法就是确定好足够的大小之后，循环调用put方法来实现的~

方法remove的实现

源代码

   /**      * 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);     }

remove方法其实就是调用了removeEntryForKey方法~

removeEntryForKey如下：

方法removeEntryForKey

   /**      * 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) {         if (size == 0) {             return null;         }         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;         }          return e;     } 

removeEntryForKey其实就是链表的删除的实现，其包含如下几个部分：

1. 如果没有元素，则返回null值
2. 然后根据key计算hash值，并确定到哪一个table[bucketIndex]中删除
3. 循环遍历元素，删除符合条件的Entry节点即可，比较好理解

方法indexFor的实现

源代码

  /**      * Returns index for hash code h.      */     static int indexFor(int h, int length) {         // assert Integer.bitCount(length) == 1 : "length must be a non-zero power of 2";         return h & (length-1);     }

 indexFor方法在HashMap中用于定位数组位置table[bucketIndex]~

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

在HashMap中，table的长度定义推荐使用2的N次方~ 

h & (length - 1) 等价与 h % length，但他们是等价（效果）不等效（效率）的，位运算结果与取模一致，但效率更高；

这样做的好处还有：

当数组长度为2的n次幂的时候，不同的key算得得index相同的几率较小，那么数据在数组上分布就比较均匀，也就是说碰撞的几率小，相对的，查询的时候就不用遍历某个位置上的链表，这样查询效率也就较高了。 参考【http://www.iteye.com/topic/539465】

modCount是干嘛的

源代码

    /**      * 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;

modCount记录的HashMap结构变化的次数，在迭代器初始化过程中会将这个值赋给迭代器的 expectedModCount，在迭代过程中，判断 modCount 跟 expectedModCount 是否相等，如果不相等就表示已经有其他线程修改了 Map，那么将抛出ConcurrentModificationException，这就是所谓fail-fast策略。

可以查看HashMap中的抽象类HashIterator：

• 构造函数中expectedModCount 赋值

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

• remove改变抛ConcurrentModificationException异常

         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;                 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;         }

Fail-Fast简单示例

import java.util.HashMap; import java.util.Map;  public class Test {      public static void main(String[] args) {          Map<String, Integer> nameAgeMap = new HashMap<>();          nameAgeMap.put("One", 1);         nameAgeMap.put("Two", 2);         nameAgeMap.put("Three", 3);         nameAgeMap.put(null, 4);         nameAgeMap.put("five", null);          for (Map.Entry<String, Integer> entry : nameAgeMap.entrySet()) {             if(null == entry.getKey()) {                 nameAgeMap.remove(null);             }             System.out.println(entry.getKey() + ' ' + entry.getValue());         }      }  }

运行结果：

null 4 Exception in thread "main" java.util.ConcurrentModificationException 	at java.util.HashMap$HashIterator.nextEntry(Unknown Source) 	at java.util.HashMap$EntryIterator.next(Unknown Source) 	at java.util.HashMap$EntryIterator.next(Unknown Source) 	at Test.main(Test.java:16)

这里就不对HashMap中的方法一一列举了~

JDK 1.8 HashMap调整

HashMap在JDK1.8中，相比JDK 1.7做了较多的改动。

HashMap的数据结构（数组+链表+红黑树），桶中的结构可能是链表，也可能是红黑树，红黑树的引入是为了提高效率

如put方法~

    /**      * 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.      *      * @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);     }

putVal方法

 /**      * 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;     } 

其中， treeifyBin(tab, hash)部分就是红黑树的相关处理部分~ 

参考

http://coding-geek.com/how-does-a-hashmap-work-in-java/

http://javaconceptoftheday.com/how-hashmap-works-internally-in-java/