JDK源码分析--HashMap

Posted 2020-09-20 willowWind

HashMap为大家常用的java数据结构工具类，下面对HashMap进行源码分析。

类图结构如下：

 

其中AbstractMap实现了

public V get(Object key) ,

public V remove(Object key),

public Set<K> keySet(),

public Collection<V> values()等常用Map操作方法。

 

下面先分析下HashMap中的常量定义：

 /**
     * The default initial capacity - MUST be a power of two.
     */
    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16 初始化容量为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;   //最大容量为230

    /**
     * The load factor used when none specified in constructor.
     */
    static final float DEFAULT_LOAD_FACTOR = 0.75f;  //装载因子

    /**
     * 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; //由链表转换成树的阈值

    /**
     * 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; //由树转换成链表的阈值

    /**
     * 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; //当桶中的bin被树化时最小的hash表容量

 正常情况下HashMap是使用链表数据来存储数据，定义了NODE数据结构：

 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; }
        .............
    }

同时数据存储在Node数组中

    /**
     * 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;  //Node数组

    /**
     * Holds cached entrySet(). Note that AbstractMap fields are used
     * for keySet() and values().
     */
    transient Set<Map.Entry<K,V>> entrySet; //entry集合

下面举例来说明其中的其他使用方法。

通常我们是这样使用HashMap的：

import java.util.HashMap;
import java.util.Map;

public class HashTableMain {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Map<String,String> map = new HashMap<String,String>();
        map.put("key1", "value1");
        map.put("key2", "value2");
        
        if(map.containsKey("key")){
            System.out.println("Find the key");
        }
    }

}

通过Debug代码，我们可以知道，首先HashMap走了默认构造函数,设置了默认装载因子0.75，即map里面元素容量超过设置容量的75%即扩展map容量

当执行put方法添加元素的时候，执行了putVal方法

下面对putVal方法进行详细分析：

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        //在这里初始化，记得1.7之前版本是在构造方法里面初始化
        if ((tab = table) == null || (n = tab.length) == 0)
            //resize方法里面初始化容器大小为16，因子为0.75，阈值为16*0.75
            n = (tab = resize()).length;  
        if ((p = tab[i = (n - 1) & hash]) == null)
　　　　　　　//插入数组tab数据
            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;
　　　　 //如果大于阈值，重新分配数组大小
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }

待续。。。。