理解HashMap，HashTable-基于java1.8

迈不过友情╰ 2022-02-27 14:10 319阅读 0赞

--------------------

title: “理解HashMap-基于java1.8”

url: “[https://wsk1103.github.io/][https_wsk1103.github.io]”

tags:

*  Java

--------------------

**java -version** ：jdk 1.8.0\_191

# HashMap #

## 构造 ##

![image][]

## 类内参数，方法 ##

![image][image 1]

![image][image 2]

![image][image 3]

## 实现 ##

![image][image 4]  
基于**数组 + 链表**实现。

当单条链表的个数达到**8**个时候，链表就会被转换成**红黑树**。

链表时间复杂度**O(n)**，红黑树时间复杂度**O(log n)**

## 静态参数 ##

/**
         * The default initial capacity - MUST be a power of two.
         */
        static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
        初始化时，默认数组的大小
        /**
         * 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;
        数组的最大值，2^30。
        /**
         * The load factor used when none specified in constructor.
         */
        static final float DEFAULT_LOAD_FACTOR = 0.75f;
        默认负载因子，当新增的key在Map中超过 (Map中key的数量 * 负载因子) 的时候，就会扩容。
        扩容是比较耗内存的，所以设计的时候，很多时候都要考虑一下Map的大小，防止频繁扩容或者占用无畏的空间。
        /**
         * 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;
        当单个链表里面的key个数超过8个时候，转换成红黑树。
        /**
         * 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;
        
        //超过这个值则以翻倍形式扩容(resize)
        //threshold = capacity * load factor
        int threshold;
        map的阈值

## 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);
        }

在存入到HashMap的时候，需要先计算一下hash值，然后根据hash值存放到对应的数组和链表中。

static final int hash(Object key) {
            int h;
            return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
        }

函数作用：高16bit不变，低16bit和高16bit做了一个异或

假设 h = hashCode() = 1111 1111 1111 1111 1111 0101 1010 0011

<table> 
 <thead> 
  <tr> 
   <th>步数</th> 
   <th>操作</th> 
   <th>值</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>1</td> 
   <td>h = hashCode()</td> 
   <td>1111 1111 1111 1111 1111 0101 1010 0011</td> 
  </tr> 
  <tr> 
   <td>2</td> 
   <td>h</td> 
   <td>1111 1111 1111 1111 1111 0101 1010 0011</td> 
  </tr> 
  <tr> 
   <td>3</td> 
   <td>h &gt;&gt;&gt; 16</td> 
   <td>0000 0000 0000 0000 1111 1111 1111 1111</td> 
  </tr> 
  <tr> 
   <td>4</td> 
   <td>h ^ (h &gt;&gt;&gt; 16)</td> 
   <td>1111 1111 1111 1111 0000 1010 0101 1100</td> 
  </tr> 
  <tr> 
   <td>5</td> 
   <td>(n - 1) &amp; hash</td> 
   <td>0000 0000 0000 0000 0000 0000 0000 1111(15) <br> 1111 1111 1111 1111 0000 1010 0101 1100</td> 
  </tr> 
  <tr> 
   <td>6</td> 
   <td>结果</td> 
   <td>1100(12)</td> 
  </tr> 
 </tbody> 
</table>

则结果 (n - 1) & hash 值为12。

可以看出，HashMap是允许key为null的，当key为null的时候，hash值为0。

注意：当key存放是一个对象的时候，一般情况下我们需要重写这个对象的hashCode()方法，防止严重碰撞，或者链表/红黑树过长。

/**
         * 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)
                //重置HashMap
                n = (tab = resize()).length;
            if ((p = tab[i = (n - 1) & hash]) == null)
                //当数组tab中对应的位置为空，则直接存放到该tab[]中。
                //i = (n - 1) & hash 取模定位数组位置
                tab[i] = newNode(hash, key, value, null);
            else {
                //已经存在该key的hash值或者碰撞冲突。
                Node<K,V> e; K k;
                if (p.hash == hash &&
                    ((k = p.key) == key || (key != null && key.equals(k))))
                    // 已经存在，保存起来，用于返回oldValue
                    e = p;
                else if (p instanceof TreeNode)
                    //放入树节点中，也就是已经是红黑树的情况。
                    e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
                else {
                    //循环列表
                    for (int binCount = 0; ; ++binCount) {
                        if ((e = p.next) == null) {
                            //放到列表尾部
                            p.next = newNode(hash, key, value, null);
                            //当列表个数超过8个的时候，转换成红黑树。
                            if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                                treeifyBin(tab, hash);
                            break;
                        }
                        //已经存在该key
                        if (e.hash == hash &&
                            ((k = e.key) == key || (key != null && key.equals(k))))
                            break;
                        p = e;
                    }
                }
                //如果已经存在，替换，然后返回被替换前的值。
                if (e != null) { // existing mapping for key
                    V oldValue = e.value;
                    //当参数onlyIfAbsent设置为false的时候，即使相同的key，也不插入。
                    if (!onlyIfAbsent || oldValue == null)
                        e.value = value;
                    afterNodeAccess(e);//这个没有什么用，空方法
                    return oldValue;
                }
            }
            //修改的次数，该值用transient修饰，无法被序列号.
            ++modCount;
            //当key的数量超过阈值时，翻倍并重新将key插入一个新的map中。
            if (++size > threshold)
                resize();
            afterNodeInsertion(evict);//这个没有什么用，空方法
            return null;
        }

## get方法 ##

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

get方法也是用hash值，快速定位到具体的数组位置。

/**
         * 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))))
                    // 找到hash值相等 和 key也相等
                    return first;
                if ((e = first.next) != null) {
                    if (first instanceof TreeNode)
                        //已经转换成红黑树，红黑树搜索
                        return ((TreeNode<K,V>)first).getTreeNode(hash, key);
                    do {
                        //循环遍历链表查找
                        if (e.hash == hash &&
                            ((k = e.key) == key || (key != null && key.equals(k))))
                            return e;
                    } while ((e = e.next) != null);
                }
            }
            //找不到
            return null;
        }

## resize方法 ##

扩容

每次扩容都需要重新分配数组，并把老的值再分配到新的数组，耗能比较高，所以应该尽量避免扩容。例如在初始化的时候，判断该Map的key的数量可以会达到最大的数值，**Map map = new HashMap(keyNum);**

/**
         * Initializes or doubles table size.  If null, allocates in
         * accord with initial capacity target held in field threshold.
         * Otherwise, because we are using power-of-two expansion, the
         * elements from each bin must either stay at same index, or move
         * with a power of two offset in the new table.
         *
         * @return the table
         */
        final Node<K,V>[] resize() {
            Node<K,V>[] oldTab = table;
            int oldCap = (oldTab == null) ? 0 : oldTab.length;
            int oldThr = threshold;
            int newCap, newThr = 0;
            if (oldCap > 0) {
                if (oldCap >= MAXIMUM_CAPACITY) {
                    //当数组tab的个数超过2^32的时候，已经不需要做什么了。
                    threshold = Integer.MAX_VALUE;
                    return oldTab;
                }
                else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                         oldCap >= DEFAULT_INITIAL_CAPACITY)
                        //位运算，翻倍。
                    newThr = oldThr << 1; // double threshold
            }
            else if (oldThr > 0) // initial capacity was placed in threshold
                newCap = oldThr;
            else {               // zero initial threshold signifies using defaults
                newCap = DEFAULT_INITIAL_CAPACITY;
                newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
            }
            if (newThr == 0) {
                float ft = (float)newCap * loadFactor;
                newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                          (int)ft : Integer.MAX_VALUE);
            }
            threshold = newThr;
            @SuppressWarnings({"rawtypes","unchecked"})
                Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
            table = newTab;
            //开始将旧的值存到新的map中
            if (oldTab != null) {
                for (int j = 0; j < oldCap; ++j) {
                    Node<K,V> e;
                    if ((e = oldTab[j]) != null) {
                        oldTab[j] = null;
                        if (e.next == null)
                            newTab[e.hash & (newCap - 1)] = e;
                        else if (e instanceof TreeNode)
                            ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                        else { // preserve order
                            Node<K,V> loHead = null, loTail = null;
                            Node<K,V> hiHead = null, hiTail = null;
                            Node<K,V> next;
                            do {
                                next = e.next;
                                if ((e.hash & oldCap) == 0) {
                                    if (loTail == null)
                                        loHead = e;
                                    else
                                        loTail.next = e;
                                    loTail = e;
                                }
                                else {
                                    if (hiTail == null)
                                        hiHead = e;
                                    else
                                        hiTail.next = e;
                                    hiTail = e;
                                }
                            } while ((e = next) != null);
                            if (loTail != null) {
                                loTail.next = null;
                                newTab[j] = loHead;
                            }
                            if (hiTail != null) {
                                hiTail.next = null;
                                newTab[j + oldCap] = hiHead;
                            }
                        }
                    }
                }
            }
            return newTab;
        }

## 初始化构造器 ##

HashMap提供4中初始化的构造器

#### 1. 参数 int initialCapacity, float loadFactor ####

initialCapacity：初始map的数组大小  
loadFactor：负载因子

/**
         * Constructs an empty <tt>HashMap</tt> with the specified initial
         * capacity and 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);
            this.loadFactor = loadFactor;
            this.threshold = tableSizeFor(initialCapacity);
        }

#### 2. 参数int initialCapacity ####

initialCapacity：初始map的数组大小

/**
         * Constructs an empty <tt>HashMap</tt> with the specified initial
         * capacity and the default load factor (0.75).
         *
         * @param  initialCapacity the initial capacity.
         * @throws IllegalArgumentException if the initial capacity is negative.
         */
        public HashMap(int initialCapacity) {
            this(initialCapacity, DEFAULT_LOAD_FACTOR);
        }

#### 3. 无参 ####

/**
         * Constructs an empty <tt>HashMap</tt> with the default initial capacity
         * (16) and the default load factor (0.75).
         */
        public HashMap() {
            this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
        }

#### 4. 参数 Map<? extends K, ? extends V> m ####

/**
         * Constructs a new <tt>HashMap</tt> with the same mappings as the
         * specified <tt>Map</tt>.  The <tt>HashMap</tt> is created with
         * default load factor (0.75) and an initial capacity sufficient to
         * hold the mappings in the specified <tt>Map</tt>.
         *
         * @param   m the map whose mappings are to be placed in this map
         * @throws  NullPointerException if the specified map is null
         */
        public HashMap(Map<? extends K, ? extends V> m) {
            this.loadFactor = DEFAULT_LOAD_FACTOR;
            putMapEntries(m, false);
        }

## 循环遍历Map ##

map.forEach((key,value)->{
        System.out.println("key=" + key + " value=" + value);
    });

## 其他说明 ##

#### HashMap 和 HashTable 区别 ####

*  HashMap是unsynchronized，使用的时候要注意线程安全。
 *  HashMap允许key为null，而HashTable不允许key为null。
 *  HashTable线程安全的原理是在操作Map的方法上都加入synchronized。

#### HashMap 和 ConcurrentHashMap 区别 ####

*  HashMap是unsynchronized，使用的时候要注意线程安全。
 *  ConcurrentHashMap是synchronized，多线程的情况下，直接使用这个。
 *  ConcurrentHashMap 的原理是在操作每个数组对应的位置加锁，而不是整个Map加锁，所以性能比HashTabe好。

#### Fail-Fast机制 ####

在多个线程同时操作同一个Map的时候，每次修改map，则modCount++，当检测到modCount发生改变的时候，则抛出异常 **throw new ConcurrentModificationException();**

#### 取模 ####

如果数组tab的长度为n，如果要对hash进行取模，一般的做法是

hash % n

但是HashMap的取模是

hash & (n - 1)

因为n在初始化的时候，我们会把n定义为2的次幂，所以n-1 的二进制是01111111…

hash & n-1 实际上就是取保留低位值，结果是在n的范围内，类似取模运算。

假设 n = 16，hash = 1111 1111 1111 1111 1111 0101 1010 0011

当时当在初始化数组的时候，没有设置为2的次幂，那么就和一般的取模运算一样，没有什么性能改进。

#### 使HashMap线程安全 ####

如果不使用ConcurrentHashMap，那么可以使用java.util包。

Map m = Collections.synchronizedMap(new HashMap(...));

同时Set,List也一样。

# HashTable #

实现的接口和继承的类和 HashMap 一致，里面的方法，变量的定义也是基本一致的。  
只是在操作数组和链表的时候，在所有的方法上都添加 **synchronized** 关键字。

例如 **add方法**

/**
         * Maps the specified <code>key</code> to the specified
         * <code>value</code> in this hashtable. Neither the key nor the
         * value can be <code>null</code>. <p>
         *
         * The value can be retrieved by calling the <code>get</code> method
         * with a key that is equal to the original key.
         *
         * @param      key     the hashtable key
         * @param      value   the value
         * @return     the previous value of the specified key in this hashtable,
         *             or <code>null</code> if it did not have one
         * @exception  NullPointerException  if the key or value is
         *               <code>null</code>
         * @see     Object#equals(Object)
         * @see     #get(Object)
         */
        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 = key.hashCode();
            int index = (hash & 0x7FFFFFFF) % tab.length;
            @SuppressWarnings("unchecked")
            Entry<K,V> entry = (Entry<K,V>)tab[index];
            for(; entry != null ; entry = entry.next) {
                if ((entry.hash == hash) && entry.key.equals(key)) {
                    V old = entry.value;
                    entry.value = value;
                    return old;
                }
            }
    
            addEntry(hash, key, value, index);
            return null;
        }

从这里就可以看出， HashTable 是不允许存储的对象为 **null**

并且 HashTable 中的链表是不会转为红黑树。

有趣的是， HashTable 的初始容量是 **11**，而 **扩容操作** 是 **int newCapacity = (oldCapacity << 1) + 1** ，即\*2+1 ，  
计算hash也比较简单

int hash = key.hashCode();
    int index = (hash & 0x7FFFFFFF) % tab.length;

[https_wsk1103.github.io]: https://wsk1103.github.io/
[image]: /images/20220227/5d295ba88c8940d1a47b37fd15d6a745.png
[image 1]: /images/20220227/14e18398cc6c4f4ca6c42a7da47683ab.png
[image 2]: /images/20220227/e72d4a79f1d64679a411b26d8bc2a998.png
[image 3]: /images/20220227/0dd153cb4a7d42deb7d527d65fdf74cd.png
[image 4]: /images/20220227/f50ee7ece997466a86f0284fe08f264b.png