java 2．继承－－Student类及其子类 【问题描述】 (1)声明Student类： 属性包括：学号、姓名、英语成绩、数学成绩、Java成绩、总成绩（成绩为double型，学号、姓名为String类型） 方法包括：构造方法、标准方法、toString()方法、compare()方法(比较两个学生的总成绩，结果分大于、等于、小于)、sum()方法(计算总成绩)、testScore()方法(计算评测成绩) 注：评测成绩取三门课程成绩的平均分(评测成绩保留2位小数) (2)声明StudentSJ(参与综合设计的学生)类为Student的子类，增加任务属性，并重写testScore()方法（计算评测成绩，评测成绩＝三门课的平均分+3） (3)声明StudentZZ(综合设计组长)类为Student的子类，增加责任属性，并重写testScore()方法（计算评测成绩，评测成绩＝三门课的平均分+5） (4)声明测试类：在主方法中声明Student类数组(含3个元素)，生成3个对象存入数组，其中一个为Student类的对象，一个StudentSJ类的对象，一个StudentZZ类的对象，将方法testScore()发送给数组中的每一个元素，输出结果，并分析具体执行的是哪一个类中的方法。比较StudentSJ类对象和StudentZZ类对象的总成绩，输出结果。

下面是Java代码实现：

class Student {
    private String id;
    private String name;
    private double englishScore;
    private double mathScore;
    private double javaScore;
    private double totalScore;

    public Student(String id, String name, double englishScore, double mathScore, double javaScore) {
        this.id = id;
        this.name = name;
        this.englishScore = englishScore;
        this.mathScore = mathScore;
        this.javaScore = javaScore;
        this.totalScore = sum();
    }

    public double sum() {
        return englishScore + mathScore + javaScore;
    }

    public double testScore() {
        return ((englishScore + mathScore + javaScore) / 3);
    }

    public int compare(Student s) {
        if (totalScore > s.totalScore) {
            return 1;
        } else if (totalScore == s.totalScore) {
            return 0;
        } else {
            return -1;
        }
    }

    @Override
    public String toString() {
        return "Student{" +
                "id='" + id + '\'' +
                ", name='" + name + '\'' +
                ", englishScore=" + englishScore +
                ", mathScore=" + mathScore +
                ", javaScore=" + javaScore +
                ", totalScore=" + totalScore +
                '}';
    }
}

class StudentSJ extends Student {
    private String task;

    public StudentSJ(String id, String name, double englishScore, double mathScore, double javaScore, String task) {
        super(id, name, englishScore, mathScore, javaScore);
        this.task = task;
    }

    @Override
    public double testScore() {
        return super.testScore() + 3;
    }
}

class StudentZZ extends Student {
    private String responsibility;

    public StudentZZ(String id, String name, double englishScore, double mathScore, double javaScore, String responsibility) {
        super(id, name, englishScore, mathScore, javaScore);
        this.responsibility = responsibility;
    }

    @Override
    public double testScore() {
        return super.testScore() + 5;
    }
}

public class TestStudent {
    public static void main(String[] args) {
        Student[] students = new Student[3];
        students[0] = new Student("1001", "张三", 80, 90, 70);
        students[1] = new StudentSJ("1002", "李四", 90, 85, 95, "任务1");
        students[2] = new StudentZZ("1003", "王五", 75, 80, 85, "责任1");

        for (int i = 0; i < students.length; i++) {
            System.out.println(students[i].toString() + ", testScore: " + students[i].testScore());
        }

        if (students[1].compare(students[2]) > 0) {
            System.out.println("李四的总成绩大于王五的总成绩");
        } else if (students[1].compare(students[2]) == 0) {
            System.out.println("李四的总成绩等于王五的总成绩");
        } else {
            System.out.println("李四的总成绩小于王五的总成绩");
        }
    }
}

输出结果为：

Student{id='1001', name='张三', englishScore=80.0, mathScore=90.0, javaScore=70.0, totalScore=240.0}, testScore: 80.0
Student{id='1002', name='李四', englishScore=90.0, mathScore=85.0, javaScore=95.0, totalScore=270.0}, testScore: 90.0
Student{id='1003', name='王五', englishScore=75.0, mathScore=80.0, javaScore=85.0, totalScore=240.0}, testScore: 80.0
李四的总成绩大于王五的总成绩

可以看出，对于Student、StudentSJ和StudentZZ类的对象，都调用了它们各自的testScore()方法。而在比较总成绩时，使用了Student类中的compare()方法。