Design Patterns Massimo Felici Massimo Felici Design Patterns - PowerPoint PPT Presentation

Design Patterns Massimo Felici Massimo Felici Design Patterns � 2011 c

1 Design Patterns • A design pattern is a standardised solution to a problem commonly encountered during object-oriented software development. • A pattern is not a piece of reusable code, but an overall approach that has proven to be useful in several different systems already. Massimo Felici Design Patterns � 2011 c

Slide 1: References • Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides, Design Patterns: Elements of Reusable Object-Oriented Software, Addison-Wesley, 1995. • Frank Buschmann, Eduardo Fernandez-Buglioni, Duane Hybertson, and Peter Sommerlad, Security Patterns: Integrating Security and Systems Engineering, John Wiley & Sons, 2005.

2 Contents of a Design Pattern Design patterns usually include: • A pattern name • A statement of the problem solved by the pattern • A description of the solution • A list of advantages and liabilities (good and bad consequences) Massimo Felici Design Patterns � 2011 c

3 Design Patterns and Large-Scale Development • For a large team, design patterns are useful in creating a shared vocabulary. • First, everyone agrees on a standard reference text (or set of them). • Informal discussions, class naming, etc. can then use the pattern names. • Large groups can develop and name their own patterns. Massimo Felici Design Patterns � 2011 c

4 Design Pattern Examples • Creational Patterns: – E.g. Abstract Factory, Factory Method, etc. • Structural Patterns: – E.g. Composite, Proxy, etc. • Behavioral Patterns: – E.g. Command, Visitor, etc. These are from the “Design Patterns” book (a.k.a. the Gang of Four book), but there are many other pattern collections Massimo Felici Design Patterns � 2011 c

5 Composite: Pattern • Composes objects into tree structures to represent part-whole hierarchies. • Lets clients treat individual objects and compositions of objects uniformly. Massimo Felici Design Patterns � 2011 c

6 Composite: Problem • User wants to be able to treat groups of things as a unit • Surrounding code would get complex if it were always conditional on whether an object was a group or a primitive • Want to support hierarchical containers of containers Massimo Felici Design Patterns � 2011 c

7 Composite: Solution Three classes: • Component: Shared interface between all, some shared implementation • Leaf: A primitive, implemented directly • Composite: for all children Components, do operation Massimo Felici Design Patterns � 2011 c

8 Composite: Example Massimo Felici Design Patterns � 2011 c

9 Composite: Advantages • Simple support for arbitrarily complex hierarchies • Clients can be simple — do not need to know about composition • New Composite and Leaf classes can be introduced without changing Component Massimo Felici Design Patterns � 2011 c

10 Composite: Liabilities • Hard for client to predict/restrict what components might be encountered • Hard to test that client works for all components • Often need to define operations on Components that make sense only for some Component types, e.g. Composites Massimo Felici Design Patterns � 2011 c

11 Summary • Many, many other patterns available • Design patterns help provide a library of solutions to common OO problems • Usually low level, but form vocabulary for a large team • Important to agree on definitions, apply consistently Massimo Felici Design Patterns � 2011 c

12 Required Readings • T. Winn, P. Calder. Is This a Pattern?. IEEE Software, 19(1):59-66, January/February, 2002. http://dx.doi.org/10.1109/52.976942 • F. Buschmann, K. Henney, D.C. Schmidt. Past, Present, and Future Trends in Software Patterns. IEEE Software, 24(4):31-37, July, 2007. http://dx.doi.org/10.1109/MS.2007. 115 • Erich Gamma, Richard Helm, Ralph E. Johnson, and John M. Vlissides. 1993. Design Patterns: Abstraction and Reuse of Object-Oriented Design. In Proceedings of the 7th European Conference on Object-Oriented Programming (ECOOP ’93), Oscar Nierstrasz (Ed.). Springer- Verlag, London, UK, UK, 406-431. http://dx.doi.org/10.1007/3-540-47910-4_21 Massimo Felici Design Patterns � 2011 c

13 Suggested Readings • D. Manolescu, W. Kozaczynski, A. Miller, J. Hogg. The Growing Divide in the Patterns World. IEEE Software, 24(4):61-67, July 2007. http://dx.doi.org/10.1109/MS.2007.120 • M.P. Cline. The pros and cons of adopting and applying design patterns in the real world. Communications of the ACM, 39(10):47-49, October, 1996. http://dx.doi.org/10.1145/ 236156.236167 • L. D’Adderio, R. Dewar, A. Lloyd, P. Stevens. Has the pattern emperor any clothes? A controversy in three acts. In ACM/SIGSOFT Software Engineering Notes, Jan/Feb 2002. http://dx.doi.org/10.1145/566493.1148026 Massimo Felici Design Patterns � 2011 c