參考：https://www.cnblogs.com/igoodful/p/9517784.html

Collections有兩種比較規則方式，第一種是使用自身的比較規則：

該類必須實現Comparable接口並重寫comparTo方法。

this可以想象為1，傳入對象o想象為2，返回1-2即按升序排序。返回2-1即按降序排序。

1、首先編寫一個實現Comparable接口的實體類

 1 package com.abc;
 2 //Comparable接口後面一定要加上需要比較的數據類型
 3 public class Person implements Comparable<Person>
 
{
 4 
 5     private String name;
 6     private int age;
 7     private int salary;
 8 
 9     public Person() {
10     }
11 
12     public Person(String name, int age, int salary) {
13         this.name = name;
14         this.age = age;
15         this.salary = salary;
16     }
17 
18
 
     public String getName() {
19         return name;
20     }
21 
22     public void setName(String name) {
23         this.name = name;
24     }
25 
26     public int getAge() {
27         return age;
28     }
29 
30     public void setAge(int age) {
31         this.age = age;
32     }
 
33 
34     public int getSalary() {
35         return salary;
36     }
37 
38     public void setSalary(int salary) {
39         this.salary = salary;
40     }
41 
42     @Override
43     public String toString() {
44         return "Person{" +
45                 "name=‘" + name + ‘\‘‘ +
46                 ", age=" + age +
47                 ", salary=" + salary +
48                 ‘}‘;
49     }
50 
51     //自身定義年齡升序
52     @Override
53     public int compareTo(Person o) {
54         return this.age-o.age;
55     }
56 }

2、編寫測試代碼

package com.abc;

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

public class PersonTest {

    public static void main(String[] args){
        List<Person> people = new ArrayList<>();
        people.add(new Person("AAA",20,100));
        people.add(new Person("BBB",18,109));
        people.add(new Person("CCC",30,58));

        System.out.println(people);
        Collections.sort(people);
        System.out.println(people);
    }
}

3、運行結果

[Person{name=‘AAA‘, age=20, salary=100}, Person{name=‘BBB‘, age=18, salary=109}, Person{name=‘CCC‘, age=30, salary=58}]
//完成了年齡的升序排列
[Person{name=‘BBB‘, age=18, salary=109}, Person{name=‘AAA‘, age=20, salary=100}, Person{name=‘CCC‘, age=30, salary=58}]

第二個參數為比較器，可以使用它來定義針對集合排序時的比較元素大小的規則。

使用這種方式時，sort方法不要求集合元素必須實現Comparable接口了，因為不會使用元素自身的比較規則。

1、編寫一個普通的實體類，不需要實現任何接口

package com.abcd;

public class Person{

    private String name;
    private int age;
    private int salary;

    public Person() {
    }

    public Person(String name, int age, int salary) {
        this.name = name;
        this.age = age;
        this.salary = salary;
    }

    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 getSalary() {
        return salary;
    }

    public void setSalary(int salary) {
        this.salary = salary;
    }

    @Override
    public String toString() {
        return "Person{" +
                "name=‘" + name + ‘\‘‘ +
                ", age=" + age +
                ", salary=" + salary +
                ‘}‘;
    }

}

2、編寫測試代碼

package com.abcd;

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

public class PersonTest {
    public static void main(String[] args){
        List<Person> people = new ArrayList<>();
        people.add(new Person("AAA",20,100));
        people.add(new Person("BBB",18,109));
        people.add(new Person("CCC",30,58));

        System.out.println(people);
　　　　　//排序規則 salary降序
        Collections.sort(people, new Comparator<Person>() {
            @Override
            public int compare(Person o1, Person o2) {
                return o2.getSalary()- o1.getSalary();
            }
        });
        System.out.println(people);
    }
}

3、運行結果

[Person{name=‘AAA‘, age=20, salary=100}, Person{name=‘BBB‘, age=18, salary=109}, Person{name=‘CCC‘, age=30, salary=58}]
[Person{name=‘BBB‘, age=18, salary=109}, Person{name=‘AAA‘, age=20, salary=100}, Person{name=‘CCC‘, age=30, salary=58}]

十三、實現Comparable接口和new Comparator<T>(){ }排序的實現過程