Nov R McFadyen1 Design Patterns (GoF) contains the creational patterns: Abstract factory Builder Factory method (section 23.3 has a Simple Factory) 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 GoF Patterns (Aside)

Nov R McFadyen2 Simple Factory (23.3) Problem: who should be responsible for creating objects when there are special considerations such as complex creational logic, … Solution: create a Pure Fabrication object called a Factory that handles the creation. (The factory creates objects.) E.g. Which adapters should exist when NextGenPOS is running? The text presents a class that determines which adapters will be instantiated. Simple Factory Pattern

Nov R McFadyen3 Textbook example The NextGen POS system supports several kinds of 3 rd 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 text is concerned with determining the class that should create the required adapters. Choosing a class in the domain model will lower the cohesion of that class. Instead we apply pure fabrication, and using the Simple Factory Pattern, we have a Factory class that has the responsibility to create such adapter objects. Simple Factory Pattern

Nov R McFadyen4 Textbook example ServicesFactory is a class that is responsible for creating the various adapters required in NextGenPOS. It is a factory. Simple Factory Pattern ServicesFactory accountingAdapter:… inventoryAdapter:… taxCalculatorAdapter:… getAccountingAdapter():… getInventoryAdapter():… getTaxCalculatorAdapter():…

Nov R McFadyen5 Textbook example Suppose an external source contains the names of the adapters that are to be used. This source is available at runtime. ServicesFactory accesses this source and instantiates the correct adapter objects. Different adapters will be instantiated for different NextGenPOS systems. Simple Factory Pattern ServicesFactory accountingAdapter:… inventoryAdapter:… taxCalculatorAdapter:… getAccountingAdapter():… getInventoryAdapter():… getTaxCalculatorAdapter():…

Nov R McFadyen6 Singleton (23.4) Problem: Exactly one instance of a certain object is required (this object is called a singleton). We must ensure a class has only one instance. Solution: use a static method of the class to return the singleton Factories and Facades are usually singletons. Textbook problem: how do we control the instantiation of the ServicesFactory? Singleton Pattern

Nov R McFadyen7 Singleton (23.4) In Java, you would have code such as: Public static synchronized ServicesFactory getInstance() { If (instance == null) Instance := new ServicesFactory() Return instance } Singleton Pattern 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.

Nov R McFadyen8 Singleton (23.4) In Java, you would have code such as: Public static synchronized ServicesFactory getInstance() { If (instance == null) Instance := new ServicesFactory() Return instance } Singleton Pattern The “synchronized” keyword ensures that only one “thread” runs this code at a time. The text refers to a “critical section”.

Nov R McFadyen9 Singleton Pattern Suppose an instance of the ServicesFactory is created and, later on, a message, getTaxCalculatorAdapter(), is passed to this instance. (Note: on page 350 the text makes reference to the situation illustrated below as “uninteresting”) :Register :ServicesFactory getInstance() ServicesFactory create() getTaxCalculatorAdapter()

Nov R McFadyen10 Singleton Pattern Figure 23.6 :Register :AccountingAdapter getAccountingAdapter() > :ServicesFactory create() post() By applying the > stereotype to this object, they are implying that the getInstance method was used to instantiate it.

Nov R McFadyen11 Singleton Pattern The text makes reference to Lazy Instantiation – The ServicesFactory object was created when it was needed, and not before :Register :AccountingAdapter getAccountingAdapter() > :ServicesFactory create() post()

Nov R McFadyen12 Patterns Quote from text, p 352: “To handle the problem of varying interfaces for external services, let’s use Adapters generated from a Singleton Factory” :Register :AccountingAdapter getAccountingAdapter() > :ServicesFactory create()...