Java List排序『Collections.sort()比较对List排序』

在使用List集合时，通常情况下，希望从集合中得到的对象是按照一定顺序排列的，但是List集合的默认排序方式为按照对象的插入顺序来排序的。但是可以通过 java.util.Collections类的静态方法sort(List<T> list)、sort(List<T> list, Comparator<? super T> c)或reverse(List<?> list)对集合中的对象进行客户化排序。那么接下来就看看sort排序的两种具体实现。

方法一：实现java.lang.Comparable接口

在创建实体类UserA时就实现java.lang.Comparable接口，然后在该接口方法中自定义排序原则，如下：

package com.pes_soft.ppms.sort;

import java.io.Serializable;

@SuppressWarnings("serial")
public class UserA implements Serializable, Comparable<UserA> {
	private String userId;
	private String name;
	private int age;
	
	public UserA() {}
	
	public UserA(String userId, String name, int age) {
		this.userId = userId;
		this.name = name;
		this.age = age;
	}
	
	/**
	 * 排序比较方法
	 */
	@Override
	public int compareTo(UserA user) {
		// 首先比较年龄(age)，如果年龄相同，则比较名字(userId)
		int flag = String.valueOf(this.age).compareTo(String.valueOf(user.getAge()));
		if (flag == 0) {
			return this.userId.compareTo(user.getUserId());
		} else {
			return flag;
		}
	}

	public String getUserId() {
		return userId;
	}

	public void setUserId(String userId) {
		this.userId = userId;
	}

	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;
	}
}

方法二：实现java.util.Comparator接口

1、创建实体类UserB，不实现任何接口：

package com.pes_soft.ppms.sort;

public class UserB {
	private String userId;
	private String name;
	private int age;
	
	public UserB() {}
	
	public UserB(String userId, String name, int age) {
		this.userId = userId;
		this.name = name;
		this.age = age;
	}
	
	public String getUserId() {
		return userId;
	}

	public void setUserId(String userId) {
		this.userId = userId;
	}

	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;
	}
}

2、创建比较器类，实现java.util.Comparator接口，然后在该接口方法中自定义排序原则，如下：

package com.pes_soft.ppms.sort;

import java.util.Comparator;

public class UserBComparator implements Comparator<UserB> {

	/**
	 * 排序比较方法
	 */
	@Override
	public int compare(UserB user1, UserB user2) {
		//首先比较年龄(age)，如果年龄相同，则比较名字(userId) 
        int flag = String.valueOf(user1.getAge()).compareTo(String.valueOf(user2.getAge())); 
        if (flag == 0) { 
            return user1.getUserId().compareTo(user2.getUserId()); 
        }else { 
            return flag; 
        }
	}
}

测试类：

package com.pes_soft.ppms.sort;

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

public class SortTest {
	
    public static void main(String[] args) { 
        List<UserA> userlistA = new ArrayList<UserA>(); 
        List<UserB> userlistB = new ArrayList<UserB>(); 
        
        userlistA.add(new UserA("ff","诸葛亮",5)); 
        userlistA.add(new UserA("dd","张三",4)); 
        userlistA.add(new UserA("aa","刘德华",1)); 
        userlistA.add(new UserA("ee","刘表",5)); 
        userlistA.add(new UserA("bb","曹操",2));   
        userlistA.add(new UserA("cc","深田恭子",3)); 
        userlistA.add(new UserA("gg","SAP",6)); 
        
        userlistB.add(new UserB("ff","诸葛亮",5)); 
        userlistB.add(new UserB("dd","张三",4)); 
        userlistB.add(new UserB("aa","刘德华",1)); 
        userlistB.add(new UserB("ee","刘表",5)); 
        userlistB.add(new UserB("bb","曹操",2));   
        userlistB.add(new UserB("cc","深田恭子",3)); 
        userlistB.add(new UserB("gg","SAP",6)); 
         
        Collections.sort(userlistA);
        
        UserBComparator userBComparator = new UserBComparator();
        Collections.sort(userlistB, userBComparator);
         
        for (UserA user : userlistA) { 
            System.out.println(user.getUserId() + " | " + user.getAge() + " | " + user.getName()); 
        }
        
        System.out.println("=======分隔线=======");
        
        for (UserB user : userlistB) { 
            System.out.println(user.getUserId() + " | " + user.getAge() + " | " + user.getName()); 
        }
	}
}

测试结果，两种排序方式的排序结果是一样的：

aa | 1 | 刘德华
bb | 2 | 曹操
cc | 3 | 深田恭子
dd | 4 | 张三
ee | 5 | 刘表
ff | 5 | 诸葛亮
gg | 6 | SAP
=======分隔线=======
aa | 1 | 刘德华
bb | 2 | 曹操
cc | 3 | 深田恭子
dd | 4 | 张三
ee | 5 | 刘表
ff | 5 | 诸葛亮
gg | 6 | SAP