What Is the Factory Method Pattern? | 67To add a new product, we’ll have to modify thesimpleFactory( )method and add
another IF clause. This goes against the open-closed principle in OOP where code
such as classes and methods should beopen for extension,butclosed for modifica-
tion. One of the primary advantages of the factory method pattern is indeed the
extensibility it affords to accommodate change. Let’s look at the class diagram of the
classic factory method pattern.
The creator and the product are both defined asinterfacesand not concrete classes
(interfaces are indicated by italicized class names in class diagrams). Interfaces define
the type and method signatures for classes. Classes that implement an interface have
to implement the methods declared in the interface. A pure interface does not pro-
vide any implementation for declared methods. However, there is a special kind of
interface called anabstract interface. Abstract interfaces can provide default imple-
mentations for methods. They’re also calledabstract classes, and cannot be instanti-
ated, but can be extended by other classes.
TheCreatorclass in Figure 2-2 is an abstract class. It declares afactory method
(calledfactoryMethod( )in this case). This is where most of the action takes place.
The factory method is defined as anabstract methodwithout any implementation.
This puts the onus on subclasses (classes that extend the abstract class such as
ConcreteCreator) to provide the implementation details for the factory method. The
Creatorclass also defines and implements a public method (calledoperation( )in
this case) that calls the factory method.
operation( )
{
..
product = FactoryMethod( )
..
}Theoperation( ) method callsFactoryMethod( ) to create product objects.
factoryMethod( ) {
..
return new ConcreteProduct
}Figure 2-2. Class diagram of the factory method pattern
Client Creator
operation()
factoryMethod()operation() {
..
product=factoryMethod()
..
Product }ConcreteCreator
factoryMethod()FactoryMethod() {
..
return new ConcreteProduct
}ConcreteProduct