Design Patterns in Java Chapter 1 Introduction

1. Design Patterns in JavaChapter 1Introduction Summary prepared by Kirk Scott

2. Intended Audience, Coverage, etc. • The classic book, Design Patterns, was written by Gamma, Helm, Johnson, and Vlissides • This book covers the same set of 23 patterns • The book is intended for software programmers/developers who know Java and want to improve their skills as designers

3. What is a Pattern? • A pattern is a way of doing something • It can be thought of as a technique for accomplishing some goal • This book dwells on the intent of patterns as an organizing characteristic

4. Where do Patterns Come From? • Software development is repetitive: Quite often, different programmers have to solve the same problem • Experienced programmers have compared notes and discovered that they arrived at common solutions to the same problem • Over time, these common solutions, have been documented as the best know approach to solving a given problem

5. Why Design Patterns? • Observe that in school, you learn to program first and learn about design later • When you compare your solution to a problem with someone else’s solution, you may realize that the other solution is simpler, more efficient, or more desirable in some other way • In practice, it is helpful to have knowledge of design before trying to code a solution to a problem

6. Why Design Patterns? Cont’d. • Design patterns are one step up from straight code writing • They represent recognized software solutions in terms of (small) collections of classes related in a certain way and containing certain sets of methods • It is important to keep in mind that the implementation is not the pattern—a pattern is the abstract design which embodies the intent

7. Why Design Patterns? Cont’d. • Design patterns have been developed most extensively in the object-oriented realm • The authors point out that there are currently over 100 documented design patterns • The 23 covered in the book have been chosen for historic reasons • They are certainly worth knowing, whether they are absolutely the most important

8. Why Design Patterns? Cont’d. • In particular, whether the most important or not, the 23 chosen patterns provide a broad introduction to and illustration of what patterns are and what good they are • The authors assert that if a person wants to become a good Java programmer, it is necessary to study design patterns • Design patterns are the first step out of the swamp of syntax that an introduction to programming is

9. Why Java? • Java is a consolidation language • This means that it is a kitchen sink language in the footsteps of C and C++, and for this reason, if no other, it is popular • The object-oriented programming is based on classes and instances, and this paradigm is currently the most popular approach to implementing graphical user interfaces, for example • Java’s characteristics make it likely that future popular languages will follow in Java’s footsteps

10. UML • Implementations of patterns are done in code • However, the pattern itself consists of the relationships among classes and so on • UML is one of several notations for diagramming notations, but it is certainly a popular one • The point is that some abstract representation of patterns is desirable, rather than appealing always to the code of a particular implementation

11. Challenges • The book contains questions • These may be short answer questions • They may also be programming questions based on the book’s example code • This course will not give these challenges as assignments • Instead, the challenges and the authors’ solutions will be examined to see what can be gained from them

12. The Organization of the Book • The patterns in a book of this kind could be given in the same order as the original design patterns book • They could also be grouped together according to similarity in structure • This book organizes the patterns according to the intent which the authors ascribe to them

13. The Organization of This Book, cont’d. • This raises the question of how to classify intent • The authors suggest that categories of patterns can be recognized according to how the patterns make it possible to go beyond the basic characteristics built into the structure of Java • They use as an initial example the idea of interfaces

14. The Organization of This Book, cont’d. • Suppose you have a class/method which requires an object of a class which implements a given interface • Suppose that you would like to apply the above to an object of a class which does not implement the required interface • Short of rewriting the given class, what approach could be taken?

15. The Organization of This Book, cont’d. • The answer is that there is a pattern known as the Adapter pattern (not to be confused with adapter classes in Java) which accomplishes this goal • The Adapter pattern, and others with similar intents, are classified in an intent category known as Interfaces • To recapitulate, these patterns help the programmer accomplish things using interfaces, going beyond the syntax provided in Java

16. The Organization of This Book, cont’d. • The book classifies patterns into these five categories: • Interfaces • Responsibility • Construction • Operations • Extensions

17. The Organization of This Book, cont’d. • The book, as a consequence is organized in the same way • The authors admit that various patterns may fit more than one category • In such cases, the pattern is given in detail in the first category where it fits, and is mentioned briefly in subsequent sections • The classification of patterns is not an absolute, but some classification is useful and necessary, and is a starting point for suggesting alternative classifications

18. The Organization of This Book, cont’d. • What follows is a complete preview of the contents of the book • Intent Category: Interfaces. Patterns: Adapter, Façade, Composite, Bridge • Intent Category: Responsibility. Patterns: Singleton, Observer, Mediator, Proxy, Chain of Responsibility, Flyweight

19. The Organization of This Book, cont’d. • Intent Category: Construction. Patterns: Builder, Factory Method, Abstract Factory, Prototype, Memento • Intent Category: Operations. Patterns: Template Method, State, Strategy, Command, Interpreter • Intent Category: Extensions. Patterns: Decorator, Iterator, Visitor

20. Welcome to Oozinoz! • The book’s examples are based on an integrated code base which is collectively known as Oozinoz (oohs and ahs), which deals with software needed for the production of fireworks • The authors’ examples will be studied, but as noted already, the challenges will not be given as assignments

21. Summary • This is the book’s summary of the chapter, verbatim: • Patterns are distillations of accumulated wisdom that provides a standard jargon, naming the concepts that experienced practitioners apply. • The patterns in the classic book Design Patterns are among the most useful class-level patterns and are certainly worth learning.

22. Summary, cont’d. • This book explains the same patterns as those documented in Design Patterns but uses Java and its libraries for its examples and challenges. • By working through the challenges in this book, you will learn to recognize and apply an important part of the wisdom of the software community.

23. The End