想要被排序的类需要实现Comparable接口并且重写compareTo方法。 this - 传进来的参数为升序。

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

public class Main {
    public static void main(String[] args) throws ParseException {

        List<User> list = new ArrayList<User>();
        list.add(new User(65, "小张"));
        list.add(new User(77, "小强"));
        list.add(new User(39, "小明"));
        list.add(new User(99, "小红"));
        Collections.sort(list);
        System.out.println(list);
    }

}

class User implements Comparable<User> {
    private int score;
    private String name;

    public User(int score, String name) {

        this.score = score;
        this.name = name;
    }

    public int getScore() {
        return score;
    }

    public void setScore(int score) {
        this.score = score;
    }

    public String getName() {
        return name;
    }

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

    @Override
    public String toString() {
        return "User{" +
                "score=" + score +
                ", name='" + name + '\'' +
                '}';
    }

    @Override
    public int compareTo(User o) {
        return this.getScore() - o.getScore();
    }
}