GoF Patterns – Ch 23

1. GoF Patterns – Ch 23 • GoF (Gang of Four): Gamma, Johnson, Helm & Vlissides • Book: • Design Patterns: Elements of Reusable Object-Oriented Software • 1995 91.3913 R McFadyen

2. GoF Patterns • Design Patterns contains the GoF patterns including: • Abstract factory (23.3) • Builder • Factory method • Prototype • Singleton (23.4) • These are creational patterns for the instantiation process • Help to make a system independent of how its objects are created • They hide how instances of classes are created 91.3913 R McFadyen

3. Factory Pattern • Factory (23.3) • Problem: who should be responsible for creating objects when there are special considerations such complex creational logic, … • Solution: create a Pure Fabrication object called a Factory that handles the creation 91.3913 R McFadyen

4. Factory Pattern • Textbook example • The NextGen POS system supports several kinds of 3rd party services. For example: tax calculators such as TaxMaster and Good As Gold, account packages such as SAP and Great Northern. The support is provided through “adapters”. • The concern here is the choice of class to create the required adapters. • Choosing a class in the domain model will lower the cohesion of that class. • Applying the Factory Pattern, we would then have a Factory object that is given the responsibility to create such adapter objects. 91.3913 R McFadyen

5. SingletonPattern • Singleton (23.4) • Problem: Exactly one instance of a certain object is required (this object is called a singleton). Ensure a class has only one instance. (Factories and Facades are usually singletons). • Solution: use a static method of the class to return the singleton 91.3913 R McFadyen

6. SingletonPattern • Singleton (23.4) • In Java, you would have code such as: • Public static synchronized ServicesFactory getInstance() • { • If (instance == null) • Instance := new ServicesFactory() • Return instance • } The point of the above is that only one instance is ever created. If one already exists, then the create is bypassed and the reference to the existing object is returned. 91.3913 R McFadyen

7. SingletonPattern Collaboration diagram: suppose an instance of the ServicesFactory is created and, later on, a message is passed to this instance. (Note: on page 350 the text makes reference to the situation illustrated below as “uninteresting”) :Register ServicesFactory getInstance() create() :ServicesFactory doIt() 91.3913 R McFadyen

8. SingletonPattern Figure 23.6 By applying the <<singleton>> stereotype to this object, they are implying that the getInstance method was used to instantiate it. <<Singleton>> :ServicesFactory :Register getAccountingAdapter() create() :AccountingAdapter doIt() 91.3913 R McFadyen

9. SingletonPattern The text makes reference to Lazy Instantiation – The ServicesFactory object was created when it was needed, and not before <<Singleton>> :ServicesFactory :Register getAccountingAdapter() create() :AccountingAdapter doIt() 91.3913 R McFadyen

10. SingletonPattern • Example. 256-262 of Design Patterns Explained • (Shalloway & Trott) 91.3913 R McFadyen