LinkedHashMap

package java.util;
……
public class LinkedHashMap<K,V>
    extends HashMap<K,V>
    implements Map<K,V>
{
……
transient LinkedHashMap.Entry<K,V> head;
transient LinkedHashMap.Entry<K,V> tail;
}

结点左边的“b”是 before 的意思，是指向前驱的引用类型变量；

结点右边的“a”是 after 的意思，是指向后继的引用类型变量；

next 则是按照 hashcode 取模之后存储元素的hash槽的位置；

测试元素添加和取出的顺序

package io.github.jihch;

import java.util.LinkedHashMap;

public class LinkedHashMapTest {

    public static void main(String[] args) {
        //创建水果集合
        LinkedHashMap<String, String> fruits = new LinkedHashMap<>();
        //添加水果
        fruits.put("apple", "苹果");
        fruits.put("banana", "香蕉");
        fruits.put("pear", "梨子");
        fruits.put("grape", "葡萄");
        fruits.put("lemon", "柠檬");
        fruits.forEach((k, v) -> {
            System.out.printf("key:%s,value:%s\n", k, v);
        });
    }

}

key:apple,value:苹果
key:banana,value:香蕉
key:pear,value:梨子
key:grape,value:葡萄
key:lemon,value:柠檬

可以发现，元素添加的顺序和取出的顺序一致

测试元素访问的顺序

被访问过的元素排在最后，这里的“被访问过”指的是通过

    public V get(Object key) {
        Node<K,V> e;
        if ((e = getNode(hash(key), key)) == null)
            return null;
        if (accessOrder)
            afterNodeAccess(e);
        return e.value;
    }

方法获取过；

举个例子，

添加顺序：<"apple","苹果">，<"banana","香蕉">，<"pear","梨子">，<"grape","葡萄">，<"lemon","柠檬">

如果此时访问：<"apple","苹果">

则取出时顺序：<"banana","香蕉">，<"pear","梨子">，<"grape","葡萄">，<"lemon","柠檬">，<"apple","苹果">

import java.util.LinkedHashMap;

public class LinkedHashMapTest {

    public static void main(String[] args) {
        //创建水果集合
        LinkedHashMap<String, String> fruits = new LinkedHashMap<>(16, 0.75f, true);
        //添加水果
        fruits.put("apple", "苹果");
        fruits.put("banana", "香蕉");
        fruits.put("pear", "梨子");
        fruits.put("grape", "葡萄");
        fruits.put("lemon", "柠檬");

        System.out.println("访问前：");
        fruits.forEach((k, v) -> {
            System.out.printf("key:%s,value:%s\n", k, v);
        });

        fruits.get("apple");

        System.out.println("访问后：");
        fruits.forEach((k, v) -> {
            System.out.printf("key:%s,value:%s\n", k, v);
        });
    }

}

访问前：
key:apple,value:苹果
key:banana,value:香蕉
key:pear,value:梨子
key:grape,value:葡萄
key:lemon,value:柠檬
访问后：
key:banana,value:香蕉
key:pear,value:梨子
key:grape,value:葡萄
key:lemon,value:柠檬
key:apple,value:苹果