There are two common ways to parameterize a system by the classes of objects it creates. One way is to subclass the class that creates the objects; this corresponds to using the Factory Method (107) pattern. The main drawback of this approach is that it can require creating a new subclass just to change the class of the product. Such changes can cascade. For example, when the product creator is itself created by a factory method, then you have to override its creator as well.

The other way to parameterize a system relies more on object composition: Define an object that's responsible for knowing the class of the product objects, and make it a parameter of the system. This is a key aspect of the Abstract Factory (87), Builder (97), and Prototype (117) patterns. All three involve creating a new "factory object" whose responsibility is to create product objects. Abstract Factory has the factory object producing objects of several classes. Builder has the factory object building a complex product incrementally using a correspondingly complex protocol. Prototype has the factory object building a product by copying a prototype object. In this case, the factory object and the prototype are the same object, because the prototype is responsible for returning the product.

Consider the drawing editor framework described in the Prototype pattern. There are several ways to parameterize a GraphicTool by the class of product:

- By applying the Factory Method pattern, a subclass of GraphicTool will be created for each subclass of Graphic in the palette. GraphicTool will have a NewGraphic operation that each GraphicTool subclass will redefine.

- By applying the Abstract Factory pattern, there will be a class hierarchy of GraphicsFactories, one for each Graphic subclass. Each factory creates just one product in this case: CircleFactory will create Circles, LineFactory will create Lines, and so on. A GraphicTool will be parameterized with a factory for creating the appropriate kind of Graphics.

- By applying the Prototype pattern, each subclass of Graphics will implement the Clone operation, and a GraphicTool will be parameterized with a prototype of the Graphic it creates.

Which pattern is best depends on many factors. In our drawing editor framework, the Factory Method pattern is easiest to use at first. It's easy to define a new subclass of GraphicTool, and the instances of GraphicTool are created only when the palette is defined. The main disadvantage here is that GraphicTool subclasses proliferate, and none of them does very much.

Abstract Factory doesn't offer much of an improvement, because it requires an equally large GraphicsFactory class hierarchy. Abstract Factory would be preferable to Factory Method only if there were already a GraphicsFactory class hierarchy—either because the compiler provides it automatically (as in Smalltalk or Objective C) or because it's needed in another part of the system.

Overall, the Prototype pattern is probably the best for the drawing editor framework, because it only requires implementing a Clone operation on each Graphics class. That reduces the number of classes, and Clone can be used for purposes other than pure instantiation (e.g., a Duplicate menu operation).

Factory Method makes a design more customizable and only a little more complicated. Other design patterns require new classes, whereas Factory Method only requires a new operation. People often use Factory Method as the standard way to create objects, but it isn't necessary when the class that's instantiated never changes or when instantiation takes place in an operation that subclasses can easily override, such as an initialization operation.

Designs that use Abstract Factory, Prototype, or Builder are even more flexible than those that use Factory Method, but they're also more complex. Often, designs start out using Factory Method and evolve toward the other creational patterns as the designer discovers where more flexibility is needed. Knowing many design patterns gives you more choices when trading off one design criterion against another.

Structural Patterns
Singleton