Lecture 12 Introduction to Design Patterns

1. SWE 316: Software Design and Architecture Lecture 12Introduction to Design Patterns Ch 6 • Explain the purpose and the general form of design patterns. Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

2. 2/21 Motivation Design Patterns Characteristics of DP's DP's Forms: Delegation and Recursion Design Purposes • Reusability, Flexibility, and Maintainability • Reuse flexible designs • Keep code at a general level • Minimize dependency on other classes • Robustness • Reuse reliable designs • Reuse robust parts • Sufficiency / Correctness • Modularize design • Reuse trusted parts 6.1

3. 3/21 Motivation Design Patterns Characteristics of DP's DP's Forms: Delegation and Recursion Motivation • OOD methods emphasize design notations • Fine for specification, documentation • But OOD is more than just drawing diagrams • Good OO designers rely on lots of experience • Most powerful reuse is design reuse • Match problem to design experience Based on material produced by Prof. Douglas C. Schmidt

4. 4/21 Motivation Design Patterns Characteristics of DP's DP's Forms: Delegation and Recursion Why do we need Design Patterns? • There is a “designer” and there is a “good designer” • Knowing concepts like abstraction, inheritance, polymorphism, etc. does not “necessary” make you a good designer • A good designer knows how to combine these concepts to make flexible, maintainable, and reusable designs. • Using design patterns: • Some body already solved a similar problem • Problem:capturing, communicating, & applying this knowledge 1 2 3

5. 5/21 Motivation Design Patterns Characteristics of DP's DP's Forms: Delegation and Recursion Reasons for studying design patterns • Reuse existing, high-quality solutions to commonly recurring problems. • Establish common terminology to improve communications within teams. • Shift the level of thinking to a higher perspective. • Decide whether you have the right design, not just one that works. • Improve individual learning and team learning. • Improve the modifiability of code. • Facilitate adoption of improved design alternatives, even when patterns are not used explicitly. • Discover alternatives to large inheritance hierarchies.

6. 6/21 Motivation Design Patterns Characteristics of DP's DP's Forms: Delegation and Recursion Students activity (how to do this?) Design Goal: Flexibility

7. 7/21 Motivation Design Patterns Characteristics of DP's DP's Forms: Delegation and Recursion What are design patterns? • Design patterns are class combinations and accompanying algorithms that fulfill common design purposes • Design pattern expresses an idea • Another Definition “Design patterns are descriptions of communicating objects and classes that are customized to solve a general design problem in a particular context. 6.2

8. 8/21 Motivation Design Patterns Characteristics of DP's DP's Forms: Delegation and Recursion Design Patterns by Type 1- Creational patterns: used to create objects in flexible or constrained ways. 2 - Structural patterns: used to represent data structures such as trees, with uniform processing interfaces. 3 - Behavioral patterns: capturing behavior among a collection of objects. 6.3

9. 9/21 Motivation Design Patterns Characteristics of DP's DP's Forms: Delegation and Recursion Types of design patterns (cont.)

10. 10/21 Motivation Design Patterns Characteristics of DP's DP's Forms: Delegation and Recursion Creational design patterns • These patterns provide guidance on the creation of objects. • They help hide the details of the object instantiation from the code that uses those objects. • They make a system independent of how its objects are created, composed and represented. • This leads to low coupling between the users and the way the objects are created. • Patterns Factory Abstract Factory Singleton Builder Prototype

11. 11/21 Motivation Design Patterns Characteristics of DP's DP's Forms: Delegation and Recursion Structural design patterns • These patterns describe the organization of objects; that is, how classes and objects are composed to form larger structures. • Patterns -Adapter -Bridge -Composite -Decorator -Facade -Flyweight -Proxy

12. 12/21 Motivation Design Patterns Characteristics of DP's DP's Forms: Delegation and Recursion Behavioral design patterns • These patterns are concerned with organizing, managing and assigning responsibilities to objects during execution. • The focus is on communication between the objects involved during some task. • Typically, these patterns characterize complex control flows that are difficult to follow at run time. • They therefore help to shift the emphasis away from the low-level flow of control to the higher level object interactions. -Chain of Responsibility -Command -Interpreter -Iterator -Mediator -Memento-Observer -State -Strategy-Template -Visitor

13. 13/21 Motivation Design Patterns Characteristics of DP's DP's Forms: Delegation and Recursion Characteristics of Design Patterns 1 • Viewpoints – ways to describe patterns • Static: class model (building blocks) • Dynamic: sequence or state diagram (operation) • Levels – decomposition of patterns • Abstract level describes the core of the pattern • Concrete (= non abstract) level describes the particulars of this case • Roles –the “players” in pattern usage • Application of the design pattern itself • Clients of the design pattern application • Setup code initializes and controls

14. 14/21 Motivation Design Patterns Characteristics of DP's DP's Forms: Delegation and Recursion Characteristics of Design Patterns 2 (class or classes) 1. Client role 3. Role: Application of the design pattern A. Static viewpoint B. Dynamic viewpoint A B (i) Abstract level (ii) Concrete level C D (sequence or state diagram) (class model) 2. Setup role : Reference direction (class or classes)

15. 15/21 Motivation Design Patterns Characteristics of DP's DP's Forms: Delegation and Recursion Viewpoints & Levels • Two viewpoints • We consider design patterns from the static viewpoint (what they are made from) and the dynamic viewpoint (how they function). • Levels • Design patterns usually have an abstract level and a non-abstract (“concrete”) level.

16. 16/21 Motivation Design Patterns Characteristics of DP's DP's Forms: Delegation and Recursion Design Patterns Forms • Delegation • Recursion 6.5

17. 17/21 Motivation Design Patterns Characteristics of DP's DP's Forms: Delegation and Recursion Basic Idea of Delegation … clientFunction( … ) { … intermediaryFunction( … ) … } … intermediaryFunction( … ) { … requiredFunction() … } Client clientFunction() intermediaryFunction()  replace requiredFunction()

18. 18/21 Motivation Design Patterns Characteristics of DP's DP's Forms: Delegation and Recursion Basic Design Pattern Form #1: Delegation Client … interfaceMethod( … ) { … doerObject.doIt() … } doerObject DPInterface interfaceMethod() DoerBase doIt() ConcreteDoer1 doIt() ConcreteDoer2 doIt() . . .

19. 19/21 Motivation Design Patterns Characteristics of DP's DP's Forms: Delegation and Recursion Basic Design Pattern Form #2: Recursion RecursionBase doOperation() Client aggregate NonrecursiveClass doOperation() RecursiveClass doOperation() void doOperation( … ) { … aggregate … }

20. 20/21 Motivation Design Patterns Characteristics of DP's DP's Forms: Delegation and Recursion The Recursion Form Applied to an Organization Chart Employee printOrganization() Client supervisees IndividualContributor printOrganization() Supervisor printOrganization() void printOrganization( … ) { … supervisees.printOrganization() … }

21. 21/21 Motivation Design Patterns Characteristics of DP's DP's Forms: Delegation and Recursion Summary of This Chapter • Design Patterns are recurring designs satisfying recurring design purposes • Classified as Creational, Structural, or Behavioral • Described by Static and Dynamic Viewpoints • Typically class models and sequence diagrams respectively • Design patterns Forms usually Delegation or Recursion