1. Plab – Tirgul 12 Design Patterns.

2. What is a Design Pattern ?
Invented by Christopher Alexander, Architect.
“Each pattern describes a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it the same way twice”, Christopher Alexander.
Originally referred to buildings and towns, applies to OOD as well.

3. What is a Design Pattern ?
A pattern has 4 elements:
Name: Naming a pattern immediately increases our design vocabulary.
Problem: When to apply the pattern: the problem and the context.
Solution: Describes the elements that make up the design, their relationships, responsibilities, and collaborations. Not a specific implementation.
Consequence: The results and trade-offs of applying the pattern. Allows evaluation between different patterns.

4. Why Design Patterns ? Common terminology. Solves practical problem.
Reuse of design and experience.
Improves the documentation and maintenance of existing systems.

5. Design Patterns – The book
Written by the “Gang Of four” (Go4).

6. Design Patterns classification
Creational
Singleton
Abstract Factory
Factory Method
Structural
Composite
Behavioral
Strategy
Command
Iterator

7. Singleton
Ensures that at most a single instance exists, allows to access it.
Applications:
Printer spooler.
An accounting system will be dedicated to serving one company.
Problem: How do you keep track of number of instances ?

8. Singleton Structure Note that the constructor is hidden (private)
static getInstance() method1() method2() static uniqueInstance
return uniqueInstance

9. Decorator
Attaching and removing additional features to an object dynamically (run time) and independently.
Avoid complicating the API of the base class.
Examples:
Word processor: Create Bold, underline, or BOTH characters.
Windows system: windows with scroll bar, border, header.

10. Notation Part of the UML (Unified Modeling Language) Inheritance
Association
Aggregation
Composite aggregation

11. Decorator Structure Component operation() Decorator operation()
ConcreteComponent operation()
component->operation()
component
ConcreteDecoratorA operation()
AddedBehavior()
ConcreteDecoratorB operation() AddedBehavior()
Decorator::operation() AddedBehavior()

12. Strategy
Encapsulates a family of algorithms, making them interchangeable.
Strategy lets the algorithm vary independently from clients that use it.
Used when:
many related classes differ only in their behavior. Strategies provide a way to configure a class with one of many behaviors.
you need different variants of an algorithm (space/time trade-off).
an algorithm uses data that clients shouldn't know about.
a class defines many behaviors, and these appear as multiple conditional statements in it's operations. Instead of many conditionals, move related conditional branches into their own Strategy class.
Examples:
Comparator for data structure.
Modeling behavior of computer games characters.

13. Strategy Structure Strategy Algorithm() Context StrategyA Algorithm()
StrategyB Algorithm()

14. Abstract Factory
Provides an interface for creating objects without specifying their concrete classes.
When to use:
a system should be independent of how its products are created, composed, and represented.
a system should be configured with one of multiple families of products.
a family of related product objects is designed to be used together, and you need to enforce this constraint.
you want to provide a class library of products, and you want to reveal just their interfaces, not their implementations
Applications:
Implementation of different ‘look and feel’ interface for windowing system.
Levels in computer games.
Improves portability across operating systems.

15. Abstract Factory Structure
createA()
createB()
Client
AbstractProductA
ProdA2
ProdA1
Creates
ConcreteFactory1
createA()
createB()
ConcreteFactory2
createA()
createB()
AbstractProductB
Creates
ProdB2
ProdB1

16. Consequences (Abstract Factory)
It isolates concrete classes – client not even aware of concrete class name !
It makes exchanging product families easy.
It promotes consistency among products.
Supporting new kinds of products is difficult.

17. Factory Method
Define an interface for creating an object, but let subclasses decide which class to instantiate
Example:
Used in Abstract Factory, but also can be used independently.

18. Factory Method Structure
Product
Creator
method1() method2() FactoryMethod()
ConcreteCreator
FactoryMethod()
Creates
ConcreteProduct

19. More on Design Patterns
Links:
Object Oriented Design course (67615) very recommended !!