Creational

1. Behavioral Paradigm Shift, Inc. Software Factory Creational Structural Lesson 1:Introduction Object-Oriented Patterns Design

2. Lesson Objectives • Be able to use patterns to define good design • Understand why design patterns are important • Discuss the process of discovering design patterns • Present other common techniques for reuse • Learn the answers of the following questions: • What is a pattern? • Where can we look to understand more about patterns? • What are some examples of what might be considered patterns? and how can we catalog them? • What is the process of discovering patterns? • What is the potential impact of patterns?

3. Introduction • Object-oriented design patterns and frameworks help to improve key software quality factors • e.g., reuse, extensibility, modularity, performance • OO design patterns support reuse of software architecture • Design patterns capture the static and dynamic structures and collaborations of successful solutions to problems that arise when building applications in a particular domain. • Frameworks support reuse of design and code • A framework is a set of cooperating classes that make up a reusable design for a specific domain

4. Design Patterns: Properties & Definitions • [Webster] A pattern is a fully realized form, original, or model accepted, or proposed for imitation. • [Webster] A pattern is something regarded as normative example to be copied; archetype ; exemplar . • [Alexander 79] A pattern is a solution to a problem in a context. • [Alexander 79] A pattern has three parts: • Problem(s) • Context • Solution • A pattern offers a workable solutions. • Patterns are rules of thumbs that can be used again and again -- useful, practical “how-to” guideline.

5. Design Patterns: More Properties & Definitions • [Gamma 95.] Design patterns identify, name, and describe common and recurring designs appearing frequently in object-oriented systems. • [Gamma 95] You can think of a design pattern as a micro architecture that contributes to overall system architecture. • Each design pattern tends to be relatively small in size and scope. • [Coplien 92] Patterns are a way of describing, documenting, and creating system architectures for software. • Patterns tend not to be domain specific. • Patterns are one of the primary mechanisms that people use for passing on expertise to others.

6. Patterns of Learning • Solutions to most challenging areas of human endeavor are deeply rooted in patterns • An important goal of education and training is to transmit patterns from generation to generation. • We will use the analogy of learning to play chess to explore the use of design patterns • In many ways, learning to develop good software is similar to learning to play good chess • Though the consequences of failure are often less dramatic

7. Becoming a Chess Master • First learn rules and physical requirements • e.g., names of the pieces, legal movements, chess board geometry and orientation, etc. • Then learn principles • e.g., relative value of certain pieces, strategic value of center squares, power of a thread, etc. • Study the games of other masters • These games contain patterns that must be understood, memorized, and applied repeatedly • There are thousands upon thousands of these patterns

8. Becoming a Master Software Designer • First learn rules and physical requirements • e.g., the algorithms, data structures, software methods and techniques, analysis guidelines , etc. • Then learn principles of software engineering • e.g., modularity, information-hiding, OO analysis, design, and programming, etc. • Study the designs of other masters • These designs contain patterns that must be understood, memorized, and applied repeatedly • There are thousands upon thousands of these patterns

9. Design Patterns • Design Patterns represent solutions to problems that arise when developing software within a particular context • Patterns == problem/solution pairs in a context • Design Patterns capture the static and dynamic structure and collaboration among key participants in software designs • Design patterns are particularly useful for articulating how and why to solve non-functional forces • Design Patterns facilitate reuse of successful software architecture and design

10. Abstract Factory Pattern’s Name: Design Problem: Solution: Consequences: A Design Pattern Has Four Essential Elements (1) • Pattern’s Name • A handle • Part of Design Vocabulary • Makes it easier to think about design and to communicate them and their trade-offs to others • Finding good names is hard • Design Problem • Describes when to apply the pattern • Explains the problem and its context • Might describe a class or object structures that are symptomatic of an inflexible design • Might describe assumptions or a list of conditions before applying the pattern Design Pattern’s Description Form

11. Abstract Factory Pattern’s Name: Design Problem: Solution: Consequences: Design pattern descriptions are often independent of programming languages or implementation details A Design Pattern Has Four Essential Elements (2) • Solution • Describes the elements that make up the design, their relationships, responsibilities, and collaborations • Doesn’t describe a particular concrete design because a pattern is like a template that can be applied in many different situations • Consequences • The result and trade-offs of applying the pattern. • Critical for evaluating design alternatives and understanding the cost and the benefits of applying the pattern • Includes the pattern’s impact on a system’s flexibility, extensibility, or portability. Design Pattern’s Description Form

12. Design Patterns Catalog Design Pattern Space • Creational Patterns • Deal with initializing and configuring classes and objects • Structural Patterns • Deal with decoupling interface and implementation of classes and objects • Behavioral Patterns • Deal with dynamic interactions among societies of classes and objects

13. Design Patterns Catalog Samples of Creational Patterns • Abstract Factory • Factory for building related objects • Builder • Factory for building complex objects incrementally • Factory Method • Method in a derived class creates associates • Prototype • Factory for cloning new instances from a prototype • Singleton • Factory for a singular (Sole) instance

14. Design Patterns Catalog Samples of Structural Patterns • Adapter • Translator adapts a server interface for a client • Bridge • Abstraction for binding one of many implementations • Composite • Structure for building recursive aggregations • Decorator • Decorator extends an object transparently • Flyweight • Many fine-grained objects shared efficiently

15. Design Patterns Catalog Samples of Behavioral Patterns (1) • Chain of Responsibility • Request delegated to the responsible service provider • Command • Request as First-Class object • Mediator • Mediator coordinates interactions between its associates • Memento • Snapshot captures and restores object states privately

16. Design Patterns Catalog Samples of Behavioral Patterns (2) • Interpreter • Language interpreter for a small grammar • Iterator • Aggregate elements are accessed sequentially • Strategy • Abstraction for selecting one of any algorithms • Observer • Dependents update automatically when a subject changes • State • Object whose behavior depends on its state

17. The Process of Discovering Patterns • Experience • Do it yourself • Building things and watch yourself as you build them • Watch others doing it • Watch others performing or building things. This is called Protocol Analysis • Catalog what you have done • Teach and learn

18. The Potential impact of Patterns H U G E

19. Design Patterns Make Some of the Chaotic Problems Simple. Why? Patterns provide: • A basis for architectural consistency • A higher-level (design) approach for software problems • A higher-level style convention • A teaching vehicle

20. Why are Design Patterns Important? • Achieving Software Design Reuse • A reusable Base of Experience • A Documentation and Learning Aid • A Target for Refactoring • An Adjunct to Existing Methodologies • Architectural Elements • Delegation and Double Dispatch • Enhancing Communication • Enhancing Maintainability