1. © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 1 Patterns in Software Design

2. © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2 Objectives  To explain why design patterns are important  To define software design patterns  To present a taxonomy of design patterns based on their granularity  To discuss pattern catalogs

3. © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3 Topics  Why design patterns are important  Christopher Alexander  Software design patterns  Pattern granularity  Pattern catalogs

4. © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 4 Why Design Patterns?  Expert designers behave differently from novices—what do experts know that novices do not?  Among other things, experts have a store of successful design patterns from past experience that they apply to new problems.

5. © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 5 More Pattern Advantages  Promoting communication—Pattern names and knowledge of advantages and disadvantages speeds communication  Streamlining documentation—Pattern form and behavior need not be elaborated  Increasing efficiency—Tool support for patterns makes development faster  Supporting reuse—Patterns and their implementations can be reused extensively  Providing ideas—Patterns can be the starting point for design or a basis for improvements

6. © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 6 Design Patterns in Building Architecture  In the 1970s architect Christopher Alexander introduced a new approach to building design based on design patterns.  Alexander argued that patterns are consequences of human anatomy, psychology, physiology, sociology, and politics; great architecture has always relied on patterns, but they have never been studied systematically; and anyone can make great buildings once the patterns are known and understood.

7. © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7 Examples of Alexander’s Patterns  Scattered Work—Use laws and incentives to encourage work places throughout a city.  Four-Story Limit—In an urban area, keep most buildings four stories high or less.  South Facing Outdoors—Always build structures north of the outdoor spaces that go with them.  Warm Colors—Choose light and surface colors to achieve a yellow-red light tone in a room.

8. © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 8 Results of Alexander’s Work  Alexander’s work has not been widely accepted among building architects, and his efforts to get non-professionals to design great buildings have often failed, as he himself admits.  But Alexander has inspired a community of software developers to generate work in design patterns.

9. © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 9 Design Patterns Defined A pattern is a model proposed for imitation. A software design pattern is a model proposed for imitation in solving a software design problem. The pattern community generally prefers a more complex definition of patterns involving context, resolved forces, etc.

10. © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 10 Design Pattern Granularity Software design patterns have no inherent granularity. Architectural styles or patterns are for entire systems and sub-systems. Design patterns proper involve several interacting functions or classes. Data structures & algorithms are low-level patterns. Idioms are ways of doing things in particular programming languages.

11. © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 11 Pattern Catalogs  Realization of the importance of design patterns has spurred creation of catalogs of patterns.  These are much like the pattern books used in building architecture or interior design and the handbooks used in engineering.  We will consider a small collection of patterns, presenting our own catalog.

12. © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 12 Summary  Design patterns capture expertise and make it available to novices and experts, providing many advantages.  Design patterns are models propose for imitation in solving software design problems.  Patterns come in various granularities including architectural styles, mid-level design patterns, data structures and algorithms, and programming language idioms.