List排序方法

主要有三種方法（按推薦度排序）：

1. JDK8的stream
2. Comparator#compare()
3. Comparable#compareTo()

法1：list的sort()

package com.example.a;import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;class User{private Integer score;private Integer age;public User(Integer score, Integer age){super();this.score = score;this.age = age;}public Integer getScore() {return score;}public void setScore(Integer score) {this.score = score;}public Integer getAge() {return age;}public void setAge(Integer age) {this.age = age;}}public class Demo {public static void main(String[] args) {List<User> users = new ArrayList<>();users.add(new User(95, 26));users.add(new User(84, 23));users.add(new User(96, 25));users.add(new User(95, 24));// 單字段排序users.sort(Comparator.comparing(User::getAge));for(User user : users){System.out.println(user.getScore() + "," + user.getAge());}System.out.println("---------------------------------");// 多字段排序（法1）users.sort((o1, o2) -> {// 這里必須要在中間加分隔符。否則，若都是數字，會變成數字相加，再轉為字符串String tmp1 = o1.getScore() + ":" + o1.getAge();String tmp2 = o2.getScore() + ":" + o2.getAge();return tmp1.compareTo(tmp2);});for(User user : users){System.out.println(user.getScore() + ":" + user.getAge());}System.out.println("---------------------------------");// 多字段排序（法2）users.sort((o1, o2) -> {int i = o2.getScore() - o1.getScore();if (i == 0) {return o1.getAge() - o2.getAge();}return i;});for(User user : users){System.out.println(user.getScore() + "," + user.getAge());}}}//測試輸出
84,23
95,24
96,25
95,26
---------------------------------
84:23
95:24
95:26
96:25
---------------------------------
96,25
95,24
95,26
84,23

法2：JDK8的stream

方法	說明
sorted()	自然排序（從小到大），流中元素需實現Comparable接口。 例：list.stream().sorted()
sorted(Comparator com)	定制排序。常用以下幾種： list.stream().sorted(Comparator.reverseOrder()) //倒序排序（從大到小） list.stream().sorted(Comparator.comparing(Student::getAge)) //順序排序（從小到大） list.stream().sorted(Comparator.comparing(Student::getAge).reversed()) // 倒序排序（從大到小）

返回排序后的流

//4、sorted：排序，根據名字倒序
userList.stream().sorted(Comparator.comparing(User::getName).reversed()).collect(Collectors.toList()).forEach(System.out::println);

原始類型排序

List<String> list = Arrays.asList("aa", "ff", "dd");
//String 類自身已實現Comparable接口
list.stream().sorted().forEach(System.out::println);//結果：
aa
dd
ff

對象單字段排序

User u1 = new User("dd", 40);
User u2 = new User("bb", 20);
User u3 = new User("aa", 20);
User u4 = new User("aa", 30);
List<User> userList = Arrays.asList(u1, u2, u3, u4);//按年齡升序
userList.stream().sorted(Comparator.comparing(User::getAge)).forEach(System.out::println);//結果
User(name=bb, age=20)
User(name=aa, age=20)
User(name=aa, age=30)
User(name=dd, age=40)

對象多字段、全部升序排序?

//先按年齡升序，年齡相同則按姓名升序
User u1 = new User("dd", 40);
User u2 = new User("bb", 20);
User u3 = new User("aa", 20);
User u4 = new User("aa", 30);
List<User> userList = Arrays.asList(u1, u2, u3, u4);// 寫法1（推薦）
userList.stream().sorted(Comparator.comparing(User::getAge).thenComparing(User::getName)// 可以寫多個.thenComparing
).forEach(System.out::println);System.out.println("------------------------------------");// 寫法2
userList.stream().sorted((o1, o2) -> {String tmp1 = o1.getAge() + o1.getName();String tmp2 = o2.getAge() + o2.getName();return tmp1.compareTo(tmp2);}
).forEach(System.out::println);System.out.println("------------------------------------");// 寫法3
userList.stream().sorted((o1, o2) -> {if (!o1.getAge().equals(o2.getAge())) {return o1.getAge().compareTo(o2.getAge());} else {return o1.getName().compareTo(o2.getName());}}
).forEach(System.out::println);//結果
User(name=aa, age=20)
User(name=bb, age=20)
User(name=aa, age=30)
User(name=dd, age=40)
------------------------------------
User(name=aa, age=20)
User(name=bb, age=20)
User(name=aa, age=30)
User(name=dd, age=40)
------------------------------------
User(name=aa, age=20)
User(name=bb, age=20)
User(name=aa, age=30)
User(name=dd, age=40)

對象多字段、升序+降序

//先按年齡升序，年齡相同則按姓名降序
User u1 = new User("dd", 40);
User u2 = new User("bb", 20);
User u3 = new User("aa", 20);
User u4 = new User("aa", 30);
List<User> userList = Arrays.asList(u1, u2, u3, u4);userList.stream().sorted((o1, o2) -> {if (!o1.getAge().equals(o2.getAge())) {return o1.getAge().compareTo(o2.getAge());} else {return o2.getName().compareTo(o1.getName());}}
).forEach(System.out::println);//結果
User(name=bb, age=20)
User(name=aa, age=20)
User(name=aa, age=30)
User(name=dd, age=40)

?

法3：Comparator#compare()

需求：用戶有成績和年齡。按成績排序，若成績相同，則按年齡排序。

package org.example.a;import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;class User{private int score;private int age;public User(int score, int age){super();this.score = score;this.age = age;}public int getScore() {return score;}public void setScore(int score) {this.score = score;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}}public class Demo {public static void main(String[] args) {List<User> users = new ArrayList<User>();users.add(new User(95, 26));users.add(new User(84, 23));users.add(new User(96, 25));users.add(new User(95, 24));Collections.sort(users, new Comparator<User>() {@Overridepublic int compare(User o1, User o2) {int i = o2.getScore() - o1.getScore();if(i == 0){return o1.getAge() - o2.getAge();}return i;}});for(User user : users){System.out.println(user.getScore() + "," + user.getAge());}}}//執行結果
96,25
95,24
95,26
84,23

法4：Comparable#compareTo()

默認按增序排序：

需求：用戶有成績和年齡。按成績降序排序，若成績相同，則按年齡正序排序。

package org.example.a;import java.util.ArrayList;
import java.util.Collections;
import java.util.List;class User implements Comparable<User>{private int score;private int age;public User(int score, int age){super();this.score = score;this.age = age;}public int getScore() {return score;}public void setScore(int score) {this.score = score;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}@Overridepublic int compareTo(User o) {int i = o.getScore() - this.getScore();if(i == 0){return this.getAge() - o.getAge();}return i;}
}public class Demo {public static void main(String[] args) {List<User> users = new ArrayList<User>();users.add(new User(95, 26));users.add(new User(84, 23));users.add(new User(96, 25));users.add(new User(95, 24));Collections.sort(users);for(User user : users){System.out.println(user.getScore() + "," + user.getAge());}}}//執行結果
96,25
95,24
95,26
84,23