Collections.sort的使用

import java.util.*;

public class Test {

    public static void main(String[] args) {
        ArrayList<Fruit> list = new ArrayList<>();
        list.add(new Fruit(4));
        list.add(new Fruit(1));
        list.add(new Fruit(7));
        Collections.sort(list);
        System.out.println(list);
    }
}

class Apple extends Fruit{

}

class Fruit implements Comparable<Fruit>{
    public int x;
    public int compareTo(Fruit f) {
//        return f.x - x;
        return x - f.x;
    }
    public Fruit(int x) {
        this.x = x;
    }
    public Fruit(){
        x = 0;
    }
    @Override
    public String toString() {
        return "fruit: " + x;
    }
}