003 java | 排序 Comparator

简介

首先要明白comparator是升序排序的。
简单来说就是return o1 - o2时，是升序排序的。
return -(o1-o2) 就是逆序排序的。
comparator返回负值时，o1会排在o2前面。
返回正值时，o1会排在o2后面。

逆序代码

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

public class SortTest {
    public static void main(String[] args) {
        List<Integer> arr = new ArrayList<>();
        arr.add(5);
        arr.add(9);
        arr.add(3);
        System.out.println(arr);
        Collections.sort(arr, new Comparator<Integer>() {
            @Override
            public int compare(Integer o1, Integer o2) {
                return - (o1 - o2);
            }
        });
        System.out.println(arr);
    }
}

代码示例

package edu.sjtu.ist.comutil;  

import java.util.Comparator;  

class Student {  
    private String name;  
    private int age;  
    private float score;  

    public Student(String name, int age, float score) {  
        this.name = name;  
        this.age = age;  
        this.score = score;  
    }  

    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 float getScore() {  
        return score;  
    }  
    public void setScore(float score) {  
        this.score = score;  
    }  

    public String toString()  
    {  
        return name+"\t\t"+age+"\t\t"+score;  
    }  

}  

class StudentComparator implements Comparator<Student>{  

    @Override  
    public int compare(Student o1, Student o2) {  
        // TODO Auto-generated method stub  
        if(o1.getScore()>o2.getScore())  
            return -1;  
        else if(o1.getScore()<o2.getScore())  
            return 1;  
        else{  
            if(o1.getAge()>o2.getAge())  
                return 1;  
            else if(o1.getAge()<o2.getAge())  
                return -1;  
            else   
                return 0;  
        }  
    }  

}  


public class ComparableDemo02 {  

    /** 
     * @param args 
     */  
    public static void main(String[] args) {  
        // TODO Auto-generated method stub  

        Student stu[]={new Student("zhangsan",20,90.0f),  
                new Student("lisi",22,90.0f),  
                new Student("wangwu",20,99.0f),  
                new Student("sunliu",22,100.0f)};  
        java.util.Arrays.sort(stu,new StudentComparator());  
        for(Student s:stu)  
        {  
            System.out.println(s);  
        }  
    }  

}