[Java 8 HashMap 详解系列] 文章目录

1.HashMap 的存储数据结构

2.HashMap 中 Key 的 index 是怎样计算的?

3.HashMap 的 put() 方法执行原理

4.HashMap 的 get() 方法执行原理

5.HashMap 的 remove() 方法执行原理

6.HashMap 的扩容 resize() 原理

7.HashMap 中的红黑树原理

6.HashMap 的扩容 resize() 原理

我们先来上一段测试代码,直观感受一下 HashMap 的真实的扩容过程:

package i

import java.util.*

/**
 * @author: Jack
 * 2020-03-21 21:55
 */
fun main(args: Array<String>) {
    val map = HashMap<String, Int>(2)
    val clz = map::class.java
    val capacity = clz.getDeclaredMethod("capacity")
    capacity.isAccessible = true
    val size = clz.getDeclaredMethod("size")
    size.isAccessible = true

    println("capacity=${capacity.invoke(map)}")
    println("size=${size.invoke(map)}")
    println(map)

    map["a"] = 1
    println("capacity=${capacity.invoke(map)}")
    println("size=${size.invoke(map)}")
    println(map)

    map["b"] = 2
    println("capacity=${capacity.invoke(map)}")
    println("size=${size.invoke(map)}")
    println(map)

    map["c"] = 3
    println("capacity=${capacity.invoke(map)}")
    println("size=${size.invoke(map)}")
    println(map)

    map["ab"] = 12
    println("capacity=${capacity.invoke(map)}")
    println("size=${size.invoke(map)}")
    println(map)

    map["bc"] = 23
    println("capacity=${capacity.invoke(map)}")
    println("size=${size.invoke(map)}")
    println(map)

    map["abc"] = 123
    println("capacity=${capacity.invoke(map)}")
    println("size=${size.invoke(map)}")
    println(map)

    map["abcd"] = 1234
    println("capacity=${capacity.invoke(map)}")
    println("size=${size.invoke(map)}")
    println(map)

}

输出:

capacity=2
size=0
{}
capacity=2
size=1
{a=1}
capacity=4
size=2
{a=1, b=2}
capacity=4
size=3
{a=1, b=2, c=3}
capacity=8
size=4
{a=1, ab=12, b=2, c=3}
capacity=8
size=5
{a=1, ab=12, bc=23, b=2, c=3}
capacity=8
size=6
{a=1, ab=12, bc=23, b=2, c=3, abc=123}
capacity=16
size=7
{a=1, ab=12, bc=23, b=2, c=3, abc=123, abcd=1234}

源码分析

    /**
     * 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;
            }
// 新容量 = oldCap << 1
            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;
// 新的阈值 = 0.75 * newCap
            newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                      (int)ft : Integer.MAX_VALUE);
        }
        threshold = newThr;

// 新创建一个 Node<K,V>[] 数组, 容量是原来的 2 倍
        @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;
    }

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