java對list進行排序

使用Colletions.sort方法

import java.util.*;

public class Main {
    public static void main(String[] args) {
        List<Student> listStu = new ArrayList<>();
        listStu.add(new Student(98));
        listStu.add(new Student(50));
        listStu.add(new Student(70));
        listStu.add(new Student(46));
        listStu.add(new Student(98));
        listStu.add(new Student(100));
        listStu.add(new Student(68));
        System.out.println(listStu);
        Collections.sort(listStu,new Student());
        System.out.println(listStu);
    }
}

class Student implements Comparator<Student>{
    int grade;//成績
    public Student(int grade){
        this.grade = grade;
    }
    public Student(){}
    public String toString(){
        return String.valueOf(grade);
    }

    @Override
    public int compare(Student o1, Student o2) {
        if(o1.grade<o2.grade)
            return -1;
        else if(o1.grade>o2.grade)
            return 1;
        else
            return 0;
    }
}

　java對map進行排序

先將map轉化成Map.Entry的列表，再對該列表進行排序，與上面的對list排序一致。

package com.company;

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Map<String,Student> map = new HashMap<>();
        map.put("小明",new Student(90));
        map.put("小花",new Student(100));
        map.put("小君",new Student(90));
        map.put("小李",new Student(83));
        map.put("小蘭",new Student(75));

        System.out.println(map);
        List<Map.Entry<String,Student>> listStu = new ArrayList<>(map.entrySet());
        Collections.sort(listStu, new Comparator<Map.Entry<String, Student>>() {
            @Override
            public int compare(Map.Entry<String, Student> o1, Map.Entry<String, Student> o2) {
                if(o1.getValue().grade<o2.getValue().grade)
                    return -1;
                else if(o1.getValue().grade>o2.getValue().grade)
                    return 1;
                else
                    return 0;
            }
        });
        System.out.println(listStu);
    }
}

class Student{
    int grade;//成績
    public Student(int grade){
        this.grade = grade;
    }
    public Student(){}
    public String toString(){
        return String.valueOf(grade);
    }
}

　　輸出如下：

ps，若需要倒排序時，可以傳入Collections.reverseOrder()這個比較器，或者在重寫compare方法時按照實際的需求重寫即可。