HashMap| 深度剥析Java SE 源码合集Ⅱ | 你会吗?

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目录

一. 🦁 HashMap介绍

1.1 特点

HashMap 是 Map 接口的接口实现类,它采用哈希算法实现,是 Map 接口最常用的实现类。 由于底层采用了哈希表存储数据,所以要求键不能重复,如果发生重复,新的值会替换旧的值。 HashMap 在查找、删除、修改方面都有非常高的效率

1.2 底层实现

HashMap 底层实现采用了哈希表,既集合了数组(占用空间连续。 寻址容易,查询速度快)的优点,又集合了链表(增加和删除效率非常高)的优点。其实哈希表的本质就是”数组+链表“。

二. 🦁 结构以及对应方法分析

HashMap中,当维互链表节点个数的过程中,链表节点数大于8时,则会转化成红黑树来存储,从而提高查询效率。

2.1 结构组成

2.1.1 成员变量

DEFAULT_INITIAL_CAPACITY = 1 << 4: 默认的初始容量为16,而且注解有说明这个默认初始化容量必须是2的倍数
MAXIMUM_CAPACITY = 1 << 30:最大初始化容量为2^30
DEFAULT_LOAD_FACTOR = 0.75f:负载因子,用来决定数组什么时候开始扩容,即当数组长度达到75%时会进行扩容
TREEIFY_THRESHOLD = 8:阈值,当前数组长度>64,会将节点个数大于8的链表做红黑树转换
UNTREEIFY_THRESHOLD = 6:同理,当红黑树节点数小于6时,将这个红黑树转换成链表
MIN_TREEIFY_CAPACITY = 64:设置当数组长度超过多少时,才会对链表节点个数大于8的做红黑树转换
transient Node<K,V>[] table:就是前面说的神秘的数组。(为啥是Node<K,V>l类型?)

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

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

    /**
     * The load factor used when none specified in constructor.
     */
    static final float DEFAULT_LOAD_FACTOR = 0.75f;
    /**
     * The bin count threshold for using a tree rather than list for a
     * bin.  Bins are converted to trees when adding an element to a
     * bin with at least this many nodes. The value must be greater
     * than 2 and should be at least 8 to mesh with assumptions in
     * tree removal about conversion back to plain bins upon
     * shrinkage.
     */
    static final int TREEIFY_THRESHOLD = 8;

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

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

 /**
     * The table, initialized on first use, and resized as
     * necessary. When allocated, length is always a power of two.
     * (We also tolerate length zero in some operations to allow
     * bootstrapping mechanics that are currently not needed.)
     */
    transient Node<K,V>[] table;

2.1.2 存储元素的节点类型

既然说了哈希表是由数组+链表组成,而且到后面还会转为红黑树,那么他肯定会有对应的节点类。其源码类型如下:

2.1.2.1 链表Node类

   /**
     * Basic hash bin node, used for most entries.  (See below for
     * TreeNode subclass, and in LinkedHashMap for its Entry subclass.)
     */
    static class Node<K,V> implements Map.Entry<K,V> 
        final int hash;
        final K key;
        V value;
        Node<K,V> next;

        Node(int hash, K key, V value, Node<K,V> next) 
            this.hash = hash;
            this.key = key;
            this.value = value;
            this.next = next;
        

        public final K getKey()         return key; 
        public final V getValue()       return value; 
        public final String toString()  return key + "=" + value; 

        public final int hashCode() 
            return Objects.hashCode(key) ^ Objects.hashCode(value);
        

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

        public final boolean equals(Object o) 
            if (o == this)
                return true;
            if (o instanceof Map.Entry) 
                Map.Entry<?,?> e = (Map.Entry<?,?>)o;
                if (Objects.equals(key, e.getKey()) &&
                    Objects.equals(value, e.getValue()))
                    return true;
            
            return false;
        
    

由源码可知,链表的Node节点类型实现了Map接口的内部接口类Entry<K,V>,这个接口定义的就是能操作HashMap的一个key——>value存储结构的一些行为(例如 获取键值对的key)。
成员遍历
hash:记录存储key的hash值,不可改变(final修饰)
key:记录key,不可改变(final修饰),所以hashmap的key是唯一的,不能重复。
value:记录value,可改变。
next:当前节点记录下一个节点的地址(由此可知,该链表是单向链表)

2.1.2.2 树节点类

/**
 * Entry for Tree bins. Extends LinkedHashMap.Entry (which in turn
 * extends Node) so can be used as extension of either regular or
 * linked node.
 */
static final class TreeNode<K,V> extends LinkedHashMap.Entry<K,V> 
  TreeNode<K,V> parent; // red-black tree links
  TreeNode<K,V> left;
  TreeNode<K,V> right;
  TreeNode<K,V> prev;  // needed to unlink next upon deletion
  boolean red;
  TreeNode(int hash, K key, V val, Node<K,V> next) 
    super(hash, key, val, next);
   


  /**
   * Returns root of tree containing this node.
   */
  final TreeNode<K,V> root() 
    for (TreeNode<K,V> r = this, p;;) 
      if ((p = r.parent) == null)
        return r;
      r = p;
     
   

成员变量:
parent:记录父节点
left: 左子树
right:右子树
prev:前节点
red:记录红黑树的状态(true是红树,反之。)

2.1.2.3 继承关系


HashMap的数组既有链表,又有红黑树,为什么这个神秘的数组是Node类型?我觉得到这里就可以讲的通了:

链表节点类Node实现了Entry接口,而LinkedHashMap的内部类Entry又继承了Node类,而TreeNode又继承了Entry,所以红黑树的节点类是和链表的Node是有继承关系的,可以统一当成一个类型来看待,所以Node<K,V>类型的数组既可以存放链表,又可以存放红黑树。

2.2 方法实现

2.2.1 HashMap的数组初始化

在 JDK11 的 HashMap 中对于数组的初始化采用的是延迟初始化方式。通过 resize 方法
实现初始化处理。resize 方法既实现数组初始化,也实现数组扩容处理

tips:啥叫延迟初始化?
向数组添加第一个元素的时候,才开始对数组做初始化处理。

  /**
     * 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) 
                threshold = Integer.MAX_VALUE;
                return oldTab;
            
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
                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;
        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;
    

首先,回到刚刚的HashMap的成员变量时,成员变量table只是作了一个声明,如图:

所以table为null,所以在执行int oldCap = (oldTab == null) ? 0 : oldTab.length时,oldCap=0,而此时 threshold也为0,所以在执行第一个if的时候,两个变量都为0,所以直接执行else里面的语句。
newCap = DEFAULT_INITIAL_CAPACITY;
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY); 即将将初始化数组长度的成员变量(16)赋值给newCap,而下一句则是将下一次扩容的长度给newThr(此时为12),然后跳过if语句,给成员变量threshold重新赋值。再执行
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap],将newTab赋值给成员变量table,然后返回newTab,这样一次初始化完成。

2.2.2 计算hash值

在map的存储中,我们是根据key的hash值来存放元素的。所以需要对key的hash值进行一系列的运算:

1.获取key的hashCode。
2.根据hashCode计算出hash值。(但是由于要求要转换成table数组的长度-1的范围内,所以还需要一系列的运算。)
3.转化算法:hash = hashcode&(n-1)得到数组中的存放位置。

/**
 * 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);

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


这里是计算key的hash值的方法。先将key的hashcode值赋给h,然后与h的高16位进行异或运算(也就是h的低16位和高16位进行异或运算)。
下面来演示一下运算过程:
假定:key = 123456,使用计算器计算得到其二进制为:

然后进行异或运算(相同为0,相异为1):

计算得到10进制为:

到这一步返回123457,下面回到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;

putVal方法拿到key的hashCode后,和15进行&运算(相同为1,相异为0):

最终得到存入数组位置为1。

2.2.3 添加元素put(K key,V value)方法

调用了putVal()方法(源码在上面)

  1. putVal()主要是计算hash值从而获取元素在数组中的位置(前面已经分析过了)、如果该位置数组没有元素,则将新节点放入;
  2. 我们都知道,hashMap对于key相同的值,是将其value值覆盖,key不变,以下则是实现方法:

  3. 如果p节点与TreeNode节点是同类(红黑树),则将其挂到红黑树上:
  4. 前面都不执行的话,最后就是挂载到数组所在位置的链表了末尾了:

我们再来看看链表——>红黑树的方法( treeifyBin(Node<K,V>[] tab, int hash))

 /**
     * Replaces all linked nodes in bin at index for given hash unless
     * table is too small, in which case resizes instead.
     */
    final void treeifyBin(Node<K,V>[] tab, int hash) 
        int n, index; Node<K,V> e;
        if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
            resize();
        else if ((e = tab[index = (n - 1) & hash]) != null) 
            TreeNode<K,V> hd = null, tl = nullJava开发面试常见问题合集

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