Java Comparator和Comparable的区别

Comparable & Comparator 都是用来实现集合中元素的比较、排序的;
Comparable 是在集合内部定义的方法实现的排序，
Comparator 是在集合外部实现的排序，
所以，如想实现排序，就需要在集合外定义 Comparator 接口的方法或在集合内实现 Comparable 接口的方法。

package java.lang;
import java.util.*;

public interface Comparable {
    public int compareTo(T o);
}

package java.util;

public interface Comparator {

    int compare(T o1, T o2);
    
    boolean equals(Object obj);

}

若一个类要实现Comparator接口：它一定要实现compare(T o1, T o2) 函数，但可以不实现equals(Object obj) 函数。

上述的compareTo和compare函数是自定义排序的方法，当这两个函数返回正数，则交换两个数的位置，解释如下：

x.compareTo(y): 如果return x-y; 则表示升序排列，因为数组中x在前，y在后，x-y如果是负数，则保持原序，即升序；如果x-y是正数，则交换两个数位置，使之保持升序。

compare(T o1, T o2): 如果return o1-o2; 则表示升序排列，原因如上

package thread.comparable;

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

public class Person implements Comparable {
    private int age;
    private String name;

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

    @Override
    public int compareTo(Object o) {
        if (o instanceof Person) {
            Person person = (Person) o;
            int result;
            result = getAge() - person.getAge();
            //result =person.getAge() -  getAge();
            return result;
        }
        return 0;
    }

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

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }


    public static void main(String[] args) {
        ArrayList list = new ArrayList();
        list.add(new Person("ccc", 20));
        list.add(new Person("AAA", 30));
        list.add(new Person("bbb", 10));
        list.add(new Person("ddd", 40));
        // 打印list的原始序列
        System.out.printf("Original sort, list:%sn", list);
        Collections.sort(list);
        System.out.printf("Original sort, list:%sn", list);
    }
}

import java.util.ArrayList;
import java.util.Comparator;

public class Person {
    private int age;
    private String name;

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

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

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

}

class AseAgePerson implements Comparator {


    @Override
    public int compare(Person o1, Person o2) {
        int result;
        result = o1.getAge() - o2.getAge();
        return result;
    }


}

class DescAgePerson implements Comparator {


    @Override
    public int compare(Person o1, Person o2) {
        int result;
        result = o2.getAge() - o1.getAge();
        return result;
    }


}

class Test {

    public static void main(String[] args) {
        ArrayList list = new ArrayList<>();
        list.add(new Person("ccc", 20));
        list.add(new Person("AAA", 30));
        list.add(new Person("bbb", 10));
        list.add(new Person("ddd", 40));
        // 打印list的原始序列
        System.out.println(list);
        list.sort(new AseAgePerson());
        //list.sort(new DescAgePerson());
        System.out.println(list);
    }
}

参考：https://www.jianshu.com/p/6f564e298918