如何實現ArrayList排序

package mianshi;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

/**
 * @description
 * 
 * @author shilvfei
 * 
 * @date 2018年4月28日
 */
public class ArrayListTest {

	public static void main(String[] args) {
		User u1 = new User(1,"張三",23);
		User u2 = new User(3,"李四",24);
		User u3 = new User(2,"王五",29);
		User u4 = new User(5,"趙流",21);
		User u5 = new User(4,"黑八",22);
		
		List<User> list = new ArrayList<>();

		list.add(u1);
		list.add(u2);
		list.add(u3);
		list.add(u4);
		list.add(u5);
		System.out.println("排序前:");
		for (User user : list) {
			System.out.println(user);
		}
		
		//Collections.sort(list, new SortByAge()); //按照年齡排序
		
		Collections.sort(list,new SortByUserId());//按UserId排序
		
		System.out.println("排序後:");
		for (User user : list) {
			System.out.println(user);
		}
	}
	
}
//年齡排序
class SortByAge implements Comparator<User>{

	@Override
	public int compare(User u1, User u2) {
		if(u1.getAge()>u2.getAge()){
			return 1;
		}
		return -1;
	}

}

//UserId排序
class SortByUserId implements Comparator<User>{

	@Override
	public int compare(User u1, User u2) {
		if(u1.getId()>u2.getId()){
			return 1;
		}
		return -1;
	}

}

//User實體
class User {
	private int id ;
	private String userName;
	private int age;
	
	public User(int id, String userName, int age) {
		this.id = id;
		this.userName = userName;
		this.age = age;
	}
	public int getId() {
		return id;
	}
	public void setId(int id) {
		this.id = id;
	}
	public String getUserName() {
		return userName;
	}
	public void setUserName(String userName) {
		this.userName = userName;
	}
	public int getAge() {
		return age;
	}
	public void setAge(int age) {
		this.age = age;
	}
	@Override
	public String toString() {
		return "User [id=" + id + ", userName=" + userName + ", age=" + age + "]";
	}
}