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

by Robert C. Martin

Now consider the object-oriented solution in Listing 6-6. Here the area() method is polymorphic. No Geometry class is necessary. So if I add a new shape, none of the existing functions are affected, but if I add a new function all of the shapes must be changed!1 Listing 6-6 Polymorphic Shapes public class Square implements Shape { private Point topLeft; private double side; public double area() { return side*side; } } public class Rectangle implements Shape { private Point topLeft; private double height; private double width; public double area() { return height * width; } } public class Circle implements Shape { private Point center; private double radius; public final double PI = 3.141592653589793; public double area() { return PI * radius * radius; } } Again, we see the complimentary nature of these two definitions; they are virtual opposites! This exposes the fundamental dichotomy between objects and data structures: Procedural code (code using data structures) makes it easy to add new functions without changing the existing data structures. OO code, on the other hand, makes it easy to add new classes without changing existing functions. The complement is also true: Procedural code makes it hard to add new data structures because all the functions must change. OO code makes it hard to add new functions because all the classes must change.