hashMap理解以及jdk1.7、jdk1.8其中区别


package test;

import java.util.HashMap;

import java.util.Map;

public class testHashMap {

    public static void main(String[] args) {
        Map<Integer, String> map = new HashMap<Integer, String>();
        map.put(1, "小明");
    }
}

网上有很多优秀的讲解hashmap源码的文章，写这个随笔只是想加强下自己的理解，有错误的地方欢迎指正

首先，hashmap的存储结构，图来自网络

注：链表长度大于8切

此处引入的是jdk1.8

从put处（Ctrl+alt+B）进入到源码HashMap.java中的put方法

注意hash(key)

贴一个解释的很好的贴子https://www.cnblogs.com/zxporz/p/11204233.html

putVal源码如下：

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        //jdk1.8 Node<K,V>[] tab是hashMap类中有一个非常重要的字段，即哈希桶数组
        Node<K,V>[] tab; Node<K,V> p; int n, i;
　　　　　//判断table是否为空
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
　　　　　//table不为空，根据(n-1)&hash算出key的落点，并且判断落点处为空，则将数据put进来
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
　　　　　//落点处不为空
        else {
            Node<K,V> e; K k;
　　　　　　　//key值相同则替换
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
　　　　　　　//为红黑树结构，则走红黑树的方法
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            //链表结构，循着链表继续向下寻找
            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)
　　　　　　　//扩容
            resize();
        afterNodeInsertion(evict);
        return null;
    }

说到扩容，就得看一下hashmap中几个重要的参数了，hashmap源码中的构造函数如下：

 /**
     * 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);
    }

    /**
     * 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);
    }

    /**
     * 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
    }

    /**
     * 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);
    }

initialCapacity：初始容量。

capacity：指HashMap中桶的数量。默认值为16。一般第一次扩容时会扩容到64，之后都是以2的幂数增加。

loadFactor：装载因子，默认值为0.75f。

threshold：阀值，满足公式threshold=loadFactor*capacity。当HashMap的size大于threshold时会执行扩容（resize）操作。

关于jdk1.7和jdk1.8的区别：

jdk1.7 table类型为entry jdk1.8table类型为node
jdk1.7 存储结构为数组+链表 jdk1.8存储结构为数组+链表+红黑树

注：并不是链表长度大于8就转红黑树。当数组长度小于64的时候，会先选择扩容，当数组长度扩容到大于64且链表长度大于8的时候才会转红黑树