Java中的Collections.sort()

Java中的Collections.sort

0.码仙励志
1.普通数字排序
2.对象单字段排序
3.对象多字段排序

0.码仙励志

时间是治疗心灵创伤的大师，但绝不是解决问题的高手。

1.普通数字排序

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;public class ListSortTest {public static void main(String[] args) {List<Integer> list = new ArrayList<Integer>();list.add(3);list.add(2);list.add(2);list.add(4);list.add(1);System.out.println(list);// 从小到大排序Collections.sort(list);System.out.println(list);// 倒序Collections.reverse(list);System.out.println(list);}
}

运行结果：

[3, 2, 2, 4, 1]
[1, 2, 2, 3, 4]
[4, 3, 2, 2, 1]

2.对象单字段排序

先创建一个对象Student，我们用age来排序

public class Student {private String name;private int age;private int height;public Student(String name, int age, int height) {super();this.name = name;this.age = age;this.height = height;}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;}public int getHeight() {return height;}public void setHeight(int height) {this.height = height;}@Overridepublic String toString() {return "Student [name=" + name + ", age=" + age + ", height=" + height + "]";}}

开始排序：
这里说一下重写的 public int compareTo(Student o){} 这个方法，它返回三种 int 类型的值：负整数，零，正整数。

负整数:当前对象的值 < 比较对象的值，位置排在前。
零:当前对象的值 = 比较对象的值，位置不变。
正整数:当前对象的值 > 比较对象的值，位置排在后。

让Student实现 Comparable 接口,然后重写里面的
compareTo()方法。

public class Student implements Comparable<Student> {private String name;private int age;private int height;public Student(String name, int age, int height) {super();this.name = name;this.age = age;this.height = height;}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;}public int getHeight() {return height;}public void setHeight(int height) {this.height = height;}@Overridepublic String toString() {return "Student [name=" + name + ", age=" + age + ", height=" + height + "]";}@Overridepublic int compareTo(Student o) {// 从大到小// return o.age - this.age;// 从小到大return this.age - o.age;}}

测试一下：

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;public class ListSortTest {public static void main(String[] args) {List<Student> list = new ArrayList<Student>();list.add(new Student("喜羊羊", 25, 175));list.add(new Student("美羊羊", 18, 165));list.add(new Student("懒羊羊", 23, 170));list.add(new Student("沸羊羊", 23, 175));list.add(new Student("慢羊羊", 80, 169));System.out.println("默认：");for (Student student : list) {System.out.println(student);}System.out.println("排序：");Collections.sort(list);for (Student student : list) {System.out.println(student);}System.out.println("倒序：");Collections.reverse(list);for (Student student : list) {System.out.println(student);}}
}

运行结果：

默认：
Student [name=喜羊羊, age=25, height=175]
Student [name=美羊羊, age=18, height=165]
Student [name=懒羊羊, age=23, height=170]
Student [name=沸羊羊, age=23, height=175]
Student [name=慢羊羊, age=80, height=169]
排序：
Student [name=美羊羊, age=18, height=165]
Student [name=懒羊羊, age=23, height=170]
Student [name=沸羊羊, age=23, height=175]
Student [name=喜羊羊, age=25, height=175]
Student [name=慢羊羊, age=80, height=169]
倒序：
Student [name=慢羊羊, age=80, height=169]
Student [name=喜羊羊, age=25, height=175]
Student [name=沸羊羊, age=23, height=175]
Student [name=懒羊羊, age=23, height=170]
Student [name=美羊羊, age=18, height=165]

3.对象多字段排序

接下来根据age和height排序
把Student的compareTo重写一下

public class Student implements Comparable<Student> {private String name;private int age;private int height;public Student(String name, int age, int height) {super();this.name = name;this.age = age;this.height = height;}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;}public int getHeight() {return height;}public void setHeight(int height) {this.height = height;}@Overridepublic String toString() {return "Student [name=" + name + ", age=" + age + ", height=" + height + "]";}@Overridepublic int compareTo(Student o) {// 从大到小if (o.age == this.age) {return o.height - this.height;}return o.age - this.age;}}

ListSortTest不用变
运行结果：

默认：
Student [name=喜羊羊, age=25, height=175]
Student [name=美羊羊, age=18, height=165]
Student [name=懒羊羊, age=23, height=170]
Student [name=沸羊羊, age=23, height=175]
Student [name=慢羊羊, age=80, height=169]
排序：
Student [name=慢羊羊, age=80, height=169]
Student [name=喜羊羊, age=25, height=175]
Student [name=沸羊羊, age=23, height=175]
Student [name=懒羊羊, age=23, height=170]
Student [name=美羊羊, age=18, height=165]
倒序：
Student [name=美羊羊, age=18, height=165]
Student [name=懒羊羊, age=23, height=170]
Student [name=沸羊羊, age=23, height=175]
Student [name=喜羊羊, age=25, height=175]
Student [name=慢羊羊, age=80, height=169]

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