The Factory Method Design Pattern

Transcription

1 The Factory Method Design Pattern Some common applications A factory method is used to create objects, whose subclasses can then override to specify the derived type of product that will be created More generally, the term factory method is often used to refer to any method whose main purpose is creation of objects. Factory methods are common in toolkits and frameworks where library code needs to create objects of types which may be subclassed by applications using the framework. Parallel class hierarchies often require objects from one hierarchy to be able to create appropriate objects from another. In the GoF classification this is a creational design pattern

2 Acknowledgements Materials were borrowed from Design Patterns in Java by Steven Metsker and William Wake (textbook for course) Head First Design Patterns by Elisabeth Freeman, Eric Freeman, Bert Bates, and Kathy Sierra

3 A Classic Example: Iterators Iterators are an example of the iterator design pattern we will study later in the course Iterators with Java collections are created using a factory method associated with the Collections interface This isolates each caller to iterator() from knowing which class is actually created Challenge 16.1 What is the actual class for the iterator object for this specific code?

4 Solution 16.1

5 Pizzas, yummy, yummy The following 16 slides are based on an example from Head First Design Patterns This easy to understand example involves making pizzas in pizza stores We will develop this example first as factory method (this chapter) and then add an abstract factory (next chapter) We first evolve the need for a design pattern by developing specific code and eventually abstracting it to a design pattern

6 Ordering Pizzas

7 The Need for Frequent Changes Fast food joints and pizza joints frequently have to modify their food products to keep attracting customers We need software that can accommodate frequent changes

8 A Simple Pizza Factory

9 Implementation

10 Reworking the Pizza Store

11 Class Diagram This is a programming idiom that is commonly mistaken for a true design pattern; the problem is we want to share common methods when possible

12 Franchising Pizza Stores We want different factories for different regions but the factory should only control how the pizza is created and not the other operations

13 A Framework for a Pizza Store We make the pizzastore abstract; the methods for prepare, bake, cut and box can be shared The method for creating the pizza must be implemented by the subclasses

14 Allowing the Subclasses to Decide

15 A Remake of the Pizza Store

16 The Factory Method The factory method handles the creation of an object The parameters allow variation in the product created The method is abstract so the subclasses themselves are required to handle object creation

17 The Pizza Class Itself

18 The Concrete Subclasses

19 The Driver Program

20 Parallel Class Hierarchies

21 The Factory Method Design Pattern

22 Another Type of Factory We will now introduce the abstract factory pattern by extending our pizza example Notice that the ingredients form product families (dough, sauce, cheese, vegetables, meat)

23 The Abstract Factory Design Pattern Some common applications Used to encapsulate a group of individual factories that have a common theme Example an abstract factory class DocumentCreator that provides interfaces to create a number of products (eg. createletter() and createresume()) The factory determines the actual concrete type of object to be created This insulates client code from object creation by having clients ask a factory object to create an object of the desired abstract type and to return an abstract pointer to the object In the GoF classification this is a creational design pattern

24 We will use an abstract factory to vary the ingredients for each family and allow for future expansion Pizzas here

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27 Building the New York Ingredients Factory

28 Pizza is abstract Every subclass must implement prepare Default implementations of the other methods are provided

29 Here is the code for creating the sauce

30 Here is the cheese pizza

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33 Applying the factory pattern in the pizza store

34 Comparing the Factory Method and the Abstract Factory

35 Comparing the Factory Method and the Abstract Factory

36 Factories and Factory Methods Work Together