设计图形Shape接口及其子类圆Circle和矩形Rectangle类。接口中声明求图形面积的方法getArea，并两个子类中实现。再定义测试类Test，实现圆和矩形面积的输出。（接口）

下面是Java代码实现：

// Shape接口
public interface Shape {
    double getArea();
}

// 圆类
public class Circle implements Shape {
    private double radius;
    
    public Circle(double radius) {
        this.radius = radius;
    }
    
    public double getRadius() {
        return radius;
    }
    
    public void setRadius(double radius) {
        this.radius = radius;
    }
    
    @Override
    public double getArea() {
        return Math.PI * radius * radius;
    }
}

// 矩形类
public class Rectangle implements Shape {
    private double length;
    private double width;
    
    public Rectangle(double length, double width) {
        this.length = length;
        this.width = width;
    }
    
    public double getLength() {
        return length;
    }
    
    public void setLength(double length) {
        this.length = length;
    }
    
    public double getWidth() {
        return width;
    }
    
    public void setWidth(double width) {
        this.width = width;
    }
    
    @Override
    public double getArea() {
        return length * width;
    }
}

// 测试类
public class Test {
    public static void main(String[] args) {
        Circle c = new Circle(5);
        System.out.println("圆的面积为：" + c.getArea());
        
        Rectangle r = new Rectangle(3, 4);
        System.out.println("矩形的面积为：" + r.getArea());
    }
}