Factory Method (and other Design Patterns)

While talking about a method inside Verve (3D simulator for robotic control), my teammate said that it was similar to a “Factory”. At first, I thought he was making an analogy between an actual factory and the method. But it turned out that he was referring to a pattern called “Factory Method”, which was of course outlined in the seminal book “Design Patterns” by Gamma et.al that I conveniently forgot to read after impulsively buying it from Amazon.

Factory Method is a design pattern in object oriented program that allows an instantiation of an object in the subclass instead of the superclass. The superclass would be an interface that outlines the objects that need to be implemented. It is the job of a subclass that implements the interface to instantiate these objects outlined in the interface.

Here’s an example. Say you have a Shape super class. Inside the Shape’s constructor, it calls a “makeShape” function, which is supposed to instantiate and return a Shape. But the program won’t know which shape to make (in another words, “instantiate”) because the type of shape is defined in the subclasses. So the job of instantiation is passed to the subclasses of Shape super class, such as Circle. Circle Subclass would implement the “makeShape” method that instantiates the shape and returns it.

There is another pattern that is similar to Factory Method but does not instantiate each time but rather passes back the same object that was instantiated once for that class. It’s called a “Singleton”.