2.定义抽象类Shape,抽象方法为showArea(),求出面积并显示,定义矩形类Rectangle,正方形类Square,圆类 Circle,根据各自的属性,用showArea方法求出各自的面积,在main方法中构造3个对象,调用showArea方法。程序

// Shape.java
public abstract class Shape {
    public abstract void showArea();
}

// Rectangle.java
public class Rectangle extends Shape {
    private double length;
    private double width;

    public Rectangle(double length, double width) {
        this.length = length;
        this.width = width;
    }

    @Override
    public void showArea() {
        double area = length * width;
        System.out.println("The area of the rectangle is " + area);
    }
}

// Square.java
public class Square extends Shape {
    private double side;

    public Square(double side) {
        this.side = side;
    }

    @Override
    public void showArea() {
        double area = side * side;
        System.out.println("The area of the square is " + area);
    }
}

// Circle.java
public class Circle extends Shape {
    private double radius;

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

    @Override
    public void showArea() {
        double area = Math.PI * radius * radius;
        System.out.println("The area of the circle is " + area);
    }
}

// Main.java
public class Main {
    public static void main(String[] args) {
        Shape rect = new Rectangle(10, 20);
        Shape square = new Square(5);
        Shape circle = new Circle(3);

        rect.showArea();
        square.showArea();
        circle.showArea();
    }
}

输出结果：

The area of the rectangle is 200.0
The area of the square is 25.0
The area of the circle is 28.274333882308138