The Robert C. Martin Clean Code Collection (Collection) (Robert C. Martin Series)

by Robert C. Martin

public class Square { public Point topLeft; public double side; } public class Rectangle { public Point topLeft; public double height; public double width; } public class Circle { public Point center; public double radius; } public class Geometry { public final double PI = 3.141592653589793; public double area(Object shape) throws NoSuchShapeException { if (shape instanceof Square) { Square s = (Square)shape; return s.side * s.side; } else if (shape instanceof Rectangle) { Rectangle r = (Rectangle)shape; return r.height * r.width; } else if (shape instanceof Circle) { Circle c = (Circle)shape; return PI * c.radius * c.radius; } throw new NoSuchShapeException(); } } Object-oriented programmers might wrinkle their noses at this and complain that it is procedural—and they’d be right. But the sneer may not be warranted. Consider what would happen if a perimeter() function were added to Geometry. The shape classes would be unaffected! Any other classes that depended upon the shapes would also be unaffected! On the other hand, if I add a new shape, I must change all the functions in Geometry to deal with it. Again, read that over. Notice that the two conditions are diametrically opposed.