3.1.设计一个Calculable接口和它的实现类Shape以及Shape的子类Square和Circle。要求如下： （1）Calculable接口中有一个抽象方法calArea()，方法无参数，返回一个double类型的结果。 （2）Shape类作为抽象类定义，有一个name成员变量 （3）Square和Circle重写父类中的方法calArea() （4）编写应用程序，创建Square和Circle对象，计算边长为2的正方形的面积和半径为3的圆形面积。 3.2学校中有老师和学生两类人，而在职研究生既是老师又是学生，对学生的管理和对教师的管理在他们身上都有体现。

3.1.

public interface Calculable {
    double calArea();
}

public abstract class Shape {
    protected String name;
}

public class Square extends Shape implements Calculable {
    private double length;

    public Square(double length) {
        this.name = "正方形";
        this.length = length;
    }

    @Override
    public double calArea() {
        return length * length;
    }
}

public class Circle extends Shape implements Calculable {
    private double radius;

    public Circle(double radius) {
        this.name = "圆形";
        this.radius = radius;
    }

    @Override
    public double calArea() {
        return Math.PI * radius * radius;
    }
}

public class Main {
    public static void main(String[] args) {
        Square square = new Square(2);
        System.out.println(square.name + "的面积为：" + square.calArea());

        Circle circle = new Circle(3);
        System.out.println(circle.name + "的面积为：" + circle.calArea());
    }
}

输出结果为：

正方形的面积为：4.0
圆形的面积为：28.274333882308138

3.2.

public abstract class Person {
    protected String name;

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

    public abstract void work();
}

public class Teacher extends Person {
    public Teacher(String name) {
        super(name);
    }

    @Override
    public void work() {
        System.out.println(name + "老师正在教课");
    }
}

public class Student extends Person {
    public Student(String name) {
        super(name);
    }

    @Override
    public void work() {
        System.out.println(name + "学生正在学习");
    }
}

public class GraduateStudent extends Person {
    public GraduateStudent(String name) {
        super(name);
    }

    @Override
    public void work() {
        System.out.println(name + "研究生正在教课和学习");
    }
}

public class Main {
    public static void main(String[] args) {
        Person teacher = new Teacher("张三");
        teacher.work();

        Person student = new Student("李四");
        student.work();

        Person graduateStudent = new GraduateStudent("王五");
        graduateStudent.work();
    }
}

输出结果为：

张三老师正在教课
李四学生正在学习
王五研究生正在教课和学习