類不抽象，不覆蓋抽象方法問題

Question

我努力做到了。 但是我收到一條錯誤消息“Student 不是抽象的，並且不會覆蓋 Comparable 類 Student extends Person { 中的抽象方法 compareTo(Object)”

abstract class Person implements Comparable {
    private String name;

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

    public String getName() {
        return name;
    }
}

class Student extends Person {
    private int id;

    public Student(String name, int id) {
        super(name);

        this.id = id;
    }

    public String toString() {
        return Integer.toString(id);
    }

    public int getId() {
        return this.id;
    }

    @Override
    public int compareTo(Student s) {
        if (this.id < s.getId()) {
            return -1;
        }else if (this.id > s.getId()) {
            return 1;
        }
        return 0;
    }
} 

@Override
    public int compareTo(Student s) {
        if (this.id < s.getId()) {
            return -1;
        }else if (this.id > s.getId()) {
            return 1;
        }
        return 0;
    }

這就是我認為有問題的地方......

Answer 1

正如@Andreas 已經解釋過的，讓Student implements Comparable ，而不是Person 。

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

abstract class Person {
    private String name;

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

    public String getName() {
        return name;
    }
}

class Student extends Person implements Comparable<Student> {
    private int id;

    public Student(String name, int id) {
        super(name);

        this.id = id;
    }

    public String toString() {
        return Integer.toString(id);
    }

    public int getId() {
        return this.id;
    }

    @Override
    public int compareTo(Student o) {
        return Integer.compare(this.id, o.id);
    }
}

演示

public class Main {
    public static void main(String[] args) {
        List<Student> list = new ArrayList<Student>();
        list.add(new Student("Abc", 321));
        list.add(new Student("Xyz", 12));
        list.add(new Student("Mnp", 123));
        Collections.sort(list);
        System.out.println(list);
    }
}

輸出：

[12, 123, 321]