java8 HashMap 性能质疑

我们都知道java8对HashMap的实现进行了优化(链表超过一定长度则采用红黑树处理)，但以下简单的代码证明:java8中HashMap存在严重的性能问题。

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

/**
 * java8 在hashCode不均匀的情形下 性能极差(相比6与7)
 * java8对于链表长度太长（默认超过8）时，链表就转换为红黑树，利用红黑树快速增删改查的特点提高HashMap的性能
 * 为什么采用红黑树反而起来相反的效果?
 * 

 * Created by donghq on 17/7/28.
 */
public class TestJava8HashMap {

    public static final int TIMES = 40000;  //4万


    public static void main(String[] args) {
        Map users = new HashMap(TIMES);
        User u;
        long b = System.currentTimeMillis();
        for (int i = 0; i < TIMES; i++) {
            u = new User(String.valueOf(i), i);
            users.put(u, String.valueOf(i));
        }
        long e = System.currentTimeMillis();
        System.out.println("put耗时: " + (e - b) + " ms");

        b = System.currentTimeMillis();
        for (int i = 0; i < TIMES; i++) {
            u = new User(String.valueOf(i), i);
            users.get(u);
        }
        e = System.currentTimeMillis();
        System.out.println("get耗时: " + (e - b) + " ms");
    }
}

class User {

    private String name;
    private int age;

    public User(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;

        User user = (User) o;

        if (age != user.age) return false;
        return name != null ? name.equals(user.name) : user.name == null;
    }

    @Override
    public int hashCode() {
        //int result = name != null ? name.hashCode() : 0;
        //result = 31 * result + age;
        //使hashCode 极不均匀
        return 1;
    }

    @Override
    public String toString() {
        return "User{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

以下是测试结果：

 

转载于:https://my.oschina.net/u/3601137/blog/1492001