java hashmap 速度慢_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：