java/ShapeTest.java

class Shape {
    public void draw() {
        System.out.println("绘制一个图形");
    }
}

class Circle extends Shape {
    @Override
    public void draw() {
        System.out.println("绘制一个圆形 ○");
    }
}

class Rectangle extends Shape {
    @Override
    public void draw() {
        System.out.println("绘制一个矩形 ▭");
    }
}

// 主类必须是 public，名字和文件名完全一致
public class ShapeTest {
    public static void drawShape(Shape s) {
        s.draw();
    }

    public static void main(String[] args) {
        Shape circle = new Circle();
        Shape rectangle = new Rectangle();
        Shape shape = new Shape();

        drawShape(shape);
        drawShape(circle);
        drawShape(rectangle);
    }
}