HashMap 学习笔记

先摆上JDK1.8中HashMap的类注释；我翻译了一下

/**
 * 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.
 * 哈希表实现了Map接口.哈希表允许空的键和值.HashMap相当于Hashtable,只是它是线程不同步的且允许空值.
 * 这个实现类不保证映射的顺序,尤其是随着时间变化不保证映射的顺序不发生变化.
 *
 * <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.
 * 当哈希表中的元素都是正常分布的,get,put操作时间复杂度都是O(1).迭代一个哈希表所需要的时间与哈希表的容量成正比,
 * 因此如果迭代性能不是很重要,不要将初始容量设置太高（或负载因子太低）.
 *
 * <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.
 * 一个HashMap两个参数会影响它的性能,初始容量和负载因子.容量是哈希表中的数据量,初始容量只是创建哈希表时的指定的哈希表容量.
 * 负载因子loadFactor= HashMap中的数据量/HashMap中的总容量(initial capacity),map中的数据量达到 总容量*负载因子时,
 * HashMap的总容量就会自动扩张一倍.
 * 注：HashMap的三个构造函数能帮助理解,见本人博客 "HashMap构造函数"
 * 
 *
 * <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.
 * 作为一般规则,默认负载因子是0.75,这在时间成本和空间成本之间提供了一个比较好的平衡.
 * 较高的值会降低空间开销,但增加查找成本（大部分体现在对HashMap的操作上,比如get,put）.
 * 在设置初始容量时,应当考虑map中的数据量和负载因子,以便最小化重新对map结构进行扩容的操作.
 * 注：这和ArrayList的ensureCapaciry(int size)有点类似.
 * 
 *
 * <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.  Note that using
 * many keys with the same {@code hashCode()} is a sure way to slow
 * down performance of any hash table. To ameliorate impact, when keys
 * are {@link Comparable}, this class may use comparison order among
 * keys to help break ties.
 * 如果许多映射要存储在HashMap实例中,那么将实例的初始容量设置比较大要比按需要自动扩容效率要高.
 * 请注意,使用相同的键（hashCode）是降低任何哈希表的一个可靠方法,为了改善这种性能问题,使用
 * 连续的键.
 * 注：hashMap允许相同的key,只是值会被覆盖.
 *
 * <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.
 * 注意,此实现线程不同步.如果多个线程同时访问hashMap,其中有一个线程修改了hashMap的结构.那么必须在
 * 外部进行线程同步处理.这通常是对hashMap的操作上进行同步处理封装
 * 
 * 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>
 * 因为线程同步问题,可以调用Collections.synchronizedMap来(Map<K,V> m)来返回一个线程安全的hashMap.    
 *
 * <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同样是快速失败机制.参考我的博客 "Iterator fail-fast"
 * 
 * <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>
 * 快速失败机制,是一种错误检测机制.它只能被用来检测错误,JDK并不保证fail-fast一定会发生.
 *
 * <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

总结

1.数据结构

初始化一个HashMap<String,String>，存储了一些测试数据，

 1     Map map = new HashMap();
 2         map.put("What", "chenyz");
 3         map.put("You", "chenyz");
 4         map.put("Don't", "chenyz");
 5         map.put("Know", "chenyz");
 6         map.put("About", "chenyz");
 7         map.put("Geo", "chenyz");
 8         map.put("APIs", "chenyz");
 9         map.put("Can't", "chenyz");
10         map.put("Hurt", "chenyz");
11         map.put("you", "chenyz");
12         map.put("google", "chenyz");
13         map.put("map", "chenyz");
14         map.put("hello", "chenyz");
15 //        每个<k,v>的hash(key)如下：
16 //        What-->hash值：8
17 //        You-->hash值：3
18 //        Don't-->hash值：7
19 //        Know-->hash值：13
20 //        About-->hash值：11
21 //        Geo-->hash值：12
22 //        APIs-->hash值：1
23 //        Can't-->hash值：7
24 //        Hurt-->hash值：1
25 //        you-->hash值：10
26 //        google-->hash值：3
27 //        map-->hash值：8
28 //        hello-->hash值：0

此时HashMap的内部如图1：

在Java中，最基础的数据类型是数组和链表，HashMap是两者结合体。所以在数据结构中被称作“链表散列”。每创建一个HashMap,就初始化一个数组。

关于这个数组源码是这样定义的：

 1 /**
 2  * The table, initialized on first use, and resized as
 3  * necessary. When allocated, length is always a power of two.
 4  * (We also tolerate length zero in some operations to allow
 5  * bootstrapping mechanics that are currently not needed.)
 6  * 该表首先使用初始化,并根据需要调整大小.分配时长度总是2的幂.
 7  * (这个数组的长度有些时候也允许为0)
 8  */
 9 transient Node<K,V>[] table;
10 
11 static class Node<K,V> implements Map.Entry<K,V> {
12     final int hash;
13     final K key;
14     V value;
15     Node<K,V> next;
16 
17     Node(int hash, K key, V value, Node<K,V> next) {
18         this.hash = hash;
19         this.key = key;
20         this.value = value;
21         this.next = next;
22     }
23 }

所以，数组中存储的是一个Map.Entry<K,V>,它由hash,key,value,next组成。next是指向下一个元素的引用，这就形成了链表。

2.存储元素

源码如下：

 1     /**
 2      * Associates the specified value with the specified key in this map.
 3      * If the map previously contained a mapping for the key, the old
 4      * value is replaced.
 5      *
 6      * @param key key with which the specified value is to be associated
 7      * @param value value to be associated with the specified key
 8      * @return the previous value associated with <tt>key</tt>, or
 9      *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
10      *         (A <tt>null</tt> return can also indicate that the map
11      *         previously associated <tt>null</tt> with <tt>key</tt>.)
12      */
13     public V put(K key, V value) {
14         return putVal(hash(key), key, value, false, true);
15     }
16 
17     /**
18      * Implements Map.put and related methods
19      *
20      * @param hash hash for key
21      * @param key the key
22      * @param value the value to put
23      * @param onlyIfAbsent if true, don't change existing value
24      * @param evict if false, the table is in creation mode.
25      * @return previous value, or null if none
26      */
27     final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
28                    boolean evict) {
29         Node<K,V>[] tab; Node<K,V> p; int n, i;
30         if ((tab = table) == null || (n = tab.length) == 0)
31             n = (tab = resize()).length;
32         if ((p = tab[i = (n - 1) & hash]) == null)
33             tab[i] = newNode(hash, key, value, null);
34         else {
35             Node<K,V> e; K k;
36             if (p.hash == hash &&
37                 ((k = p.key) == key || (key != null && key.equals(k))))
38                 e = p;
39             else if (p instanceof TreeNode)
40                 e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
41             else {
42                 for (int binCount = 0; ; ++binCount) {
43                     if ((e = p.next) == null) {
44                         p.next = newNode(hash, key, value, null);
45                         if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
46                             treeifyBin(tab, hash);
47                         break;
48                     }
49                     if (e.hash == hash &&
50                         ((k = e.key) == key || (key != null && key.equals(k))))
51                         break;
52                     p = e;
53                 }
54             }
55             if (e != null) { // existing mapping for key
56                 V oldValue = e.value;
57                 if (!onlyIfAbsent || oldValue == null)
58                     e.value = value;
59                 afterNodeAccess(e);
60                 return oldValue;
61             }
62         }
63         ++modCount;
64         if (++size > threshold)
65             resize();
66         afterNodeInsertion(evict);
67         return null;
68     }

不难理解，首先hash(key)得到key的哈希值,这些值就是“数据结构”中数组的元素。根据hash值确定当前元素的存储格子，如果当前格子已经存储了元素，那么就以链表形式

存到这个格子中，新加入的放在链表的头部，所以最早加入的就被排在了链尾部。如图1中hash值为1的位置，就以链表的形式存储了两个值。

每一次添加元素都会修改modCount全局变量，这是是用来检测HashMap在迭代的过程中是否发生了结构变化

同时这里还会设计到扩容resize().详见“HashMap负载因子”

3.索引元素

源码如下：

 1 public V get(Object key) {
 2         Node<K,V> e;
 3         return (e = getNode(hash(key), key)) == null ? null : e.value;
 4     }
 5 
 6     /**
 7      * Implements Map.get and related methods
 8      *
 9      * @param hash hash for key
10      * @param key the key
11      * @return the node, or null if none
12      */
13     final Node<K,V> getNode(int hash, Object key) {
14         Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
15         if ((tab = table) != null && (n = tab.length) > 0 &&
16             (first = tab[(n - 1) & hash]) != null) {
17             if (first.hash == hash && // always check first node
18                 ((k = first.key) == key || (key != null && key.equals(k))))
19                 return first;
20             if ((e = first.next) != null) {
21                 if (first instanceof TreeNode)
22                     return ((TreeNode<K,V>)first).getTreeNode(hash, key);
23                 do {
24                     if (e.hash == hash &&
25                         ((k = e.key) == key || (key != null && key.equals(k))))
26                         return e;
27                 } while ((e = e.next) != null);
28             }
29         }
30         return null;
31     }

首先计算出key的hash值，找到数组中对应的下标，然后通过key的equals方法匹配出对应位置的链表中的元素，这就完成了元素get的操作。这也是为什么要成对复写hashCode

和equals方法的原因。由此可知HashMap的get操作，效率会有瓶颈，如果一个位置存放了大量的元素，由于是链表存储的，就需要挨个比对，才能找到匹配的值。

所以让HashMap中的元素均匀的分布，每个位置上的链表只存储一个元素那么HashMap的效率就会很高。

4.性能因素

创建一个HashMap时，有initialCapacity和loadFactor这两个初始化参数，初始容量和负载因子。

这两个参数的设置会决定一个HashMap结构是否合理。详见“HashMap 负载因子”

5.hash算法

这是整个HashMap最核心的方法了，因为get(),put()操作都会用到这个方法，用于定位。确定新元素的存放位置。这关系着元素分布是否合理。

 1 /**
 2      * Computes key.hashCode() and spreads (XORs) higher bits of hash
 3      * to lower.  Because the table uses power-of-two masking, sets of
 4      * hashes that vary only in bits above the current mask will
 5      * always collide. (Among known examples are sets of Float keys
 6      * holding consecutive whole numbers in small tables.)  So we
 7      * apply a transform that spreads the impact of higher bits
 8      * downward. There is a tradeoff between speed, utility, and
 9      * quality of bit-spreading. Because many common sets of hashes
10      * are already reasonably distributed (so don't benefit from
11      * spreading), and because we use trees to handle large sets of
12      * collisions in bins, we just XOR some shifted bits in the
13      * cheapest possible way to reduce systematic lossage, as well as
14      * to incorporate impact of the highest bits that would otherwise
15      * never be used in index calculations because of table bounds.
16      */
17     static final int hash(Object key) {
18         int h;
19         return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
20     }