03-Factory Method for design patterns.pdf
- 1. Design Patterns CSE354 Dr. Muhammad Farhan Assistant Professor, CUI Sahiwal farhan@cuisahiwal.edu.pk +92-3324428802
- 3. Factory Method Factory Method is a creational design pattern that provides an interface for creating objects in a superclass, but allows subclasses to alter the type of objects that will be created.
- 4. Problem (Cont…) Imagine that you’re creating a logistics management application. The first version of your app can only handle transportation by trucks, so the bulk of your code lives inside the Truck class. After a while, your app becomes pretty popular. Each day you receive dozens of requests from sea transportation companies to incorporate sea logistics into the app.
- 5. Adding a new class to the program isn’t that simple if the rest of the code is already coupled to existing classes
- 6. Problem Great news, right? But how about the code? At present, most of your code is coupled to the Truck class. Adding Ships into the app would require making changes to the entire codebase. Moreover, if later you decide to add another type of transportation to the app, you will probably need to make all of these changes again. As a result, you will end up with pretty nasty code, riddled with conditionals that switch the app’s behavior depending on the class of transportation objects.
- 7. Solution (Cont…) The Factory Method pattern suggests that you replace direct object construction calls (using the new operator) with calls to a special factory method. Don’t worry: the objects are still created via the new operator, but it’s being called from within the factory method. Objects returned by a factory method are often referred to as “products.
- 8. Solution (Cont…) At first glance, this change may look pointless: we just moved the constructor call from one part of the program to another. However, consider this: now you can override the factory method in a subclass and change the class of products being created by the method. There’s a slight limitation though: subclasses may return different types of products only if these products have a common base class or interface. Also, the factory method in the base class should have its return type declared as this interface.
- 9. Solution (Cont...) For example, both Truck and Ship classes should implement the Transport interface, which declares a method called deliver . Each class implements this method differently: trucks deliver cargo by land; ships deliver cargo by sea. The factory method in the RoadLogistics class returns truck objects, whereas the factory method in the SeaLogistics class returns ships.
- 10. Solution (Cont…) The code that uses the factory method (often called the client code) doesn’t see a difference between the actual products returned by various subclasses. The client treats all the products as abstract Transport .
- 11. Solution The client knows that all transport objects are supposed to have the deliver method, but exactly how it works isn’t important to the client.
- 13. Structure (Cont…) The Product declares the interface, which is common to all objects that can be produced by the creator and its subclasses. Concrete Products are different implementations of the product interface. The Creator class declares the factory method that returns new product objects. It’s important that the return type of this method matches the product interface. You can declare the factory method as abstract to force all subclasses to implement their own versions of the method. As an alternative, the base factory method can return some default product type
- 14. Structure (Cont…) Note, despite its name, product creation is not the primary responsibility of the creator. Usually, the creator class already has some core business logic related to products. The factory method helps to decouple this logic from the concrete product classes. Here is an analogy: a large software development company can have a training department for programmers. However, the primary function of the company as a whole is still writing code, not producing programmers.
- 15. Structure Concrete Creators override the base factory method, so it returns a different type of product. 76 Creational Design Patterns / Factory Method Note that the factory method doesn’t have to create new instances all the time. It can also return existing objects from a cache, an object pool, or another source.