Java Map查詢時間探究

總結一下Map的三個實現類HashMap、LinkedHashMap、TreeMap的消耗時間，例子通過用String爲鍵，索引到Integer值和倒過來建立Map，不難發現如下規律：

Types	HashMap	LinkedHashMap	TreeMap
Map<Integer,String>	112ms，數字遞增排序	2ms，數字遞增排序	1ms，數字遞增排序
Map<String,Integer>	2ms，插入順序	<1ms，插入順序	<1ms，字母表遞增排序

TODO: 源碼解析

不多說，直接上代碼：

import java.util.*;

public class MapDeatil {


    public static void testMap1(){
        Map<Integer, String> hashMap=new HashMap<>();
        hashMap.put(1,"Smith");
        hashMap.put(6,"Ant");
        hashMap.put(3,"AHS");
        hashMap.put(8,"JKA");
        hashMap.put(5,"BVmm");
        long preTime=System.currentTimeMillis();
        hashMap.forEach((num,name)->System.out.println(num+","+name));
        long postTime=System.currentTimeMillis();
        System.out.println("hashset時間："+(postTime-preTime));

        Map<Integer, String> linkedHashMap=new LinkedHashMap<>(hashMap);
        preTime=System.currentTimeMillis();
        linkedHashMap.forEach((num,name)->System.out.println(num+","+name));
        postTime=System.currentTimeMillis();
        System.out.println("hashset時間："+(postTime-preTime));

        Map<Integer,String> treeMap=new TreeMap<Integer, String>(hashMap);
        preTime=System.currentTimeMillis();
        treeMap.forEach((num,name)->System.out.println(num+","+name));
        postTime=System.currentTimeMillis();
        System.out.println("hashset時間："+(postTime-preTime));
    }

    public static void testMap2(){
        Map<String,Integer> hashMap=new HashMap<>();
        hashMap.put("Smith",30);
        hashMap.put("Ant",20);
        hashMap.put("AHS",21);
        hashMap.put("JKA",25);
        hashMap.put("BVmm",33);
        long preTime=System.currentTimeMillis();
        hashMap.forEach((num,name)->System.out.println(num+","+name));
        long postTime=System.currentTimeMillis();
        System.out.println("hashset時間："+(postTime-preTime));

        Map<String, Integer> linkedHashMap=new LinkedHashMap<>(hashMap);
        preTime=System.currentTimeMillis();
        linkedHashMap.forEach((num,name)->System.out.println(num+","+name));
        postTime=System.currentTimeMillis();
        System.out.println("linkedhashset時間："+(postTime-preTime));

        Map<String, Integer> treeMap=new TreeMap<>(hashMap);
        preTime=System.currentTimeMillis();
        treeMap.forEach((num,name)->System.out.println(num+","+name));
        postTime=System.currentTimeMillis();
        System.out.println("treeset時間："+(postTime-preTime));
    }

    public static void main(String[] args) {
        testMap1();
        testMap2();//如果以String爲key查詢Integer，比反過來更快
    }
}

輸出如下：

1,Smith
3,AHS
5,BVmm
6,Ant
8,JKA
hashset時間：112
1,Smith
3,AHS
5,BVmm
6,Ant
8,JKA
hashset時間：2
1,Smith
3,AHS
5,BVmm
6,Ant
8,JKA
hashset時間：1
JKA,25
Ant,20
Smith,30
BVmm,33
AHS,21
hashset時間：2
JKA,25
Ant,20
Smith,30
BVmm,33
AHS,21
linkedhashset時間：0
AHS,21
Ant,20
BVmm,33
JKA,25
Smith,30
treeset時間：0