Design Patterns

1. Design Patterns Pepper

2. Find Patterns Gang of Four created 23 Siemens published another good set • http://www.blackwasp.co.uk/GofPatterns.aspx • http://www.javacamp.org/designPattern/ Just Singleton pattern: • http://www.oodesign.com/singleton-pattern.html • http://www.blackwasp.co.uk/Singleton.aspx

3. Singleton • When there should only be one shared instance in a system - global variable • Ensure that only one instance of a class is created. • Provide a global point of access to the object.

4. Code Guide • Create the class • Use the class Singleton.getInstance().doSomething();

5. More Efficient If we see that the singleton object is already created we just have to return it without using any synchronized block.

6. How can your project use this? • Log file class • Audit file class

7. Design patterns • A design pattern is a way of reusing abstract knowledge about a problem and its solution. • A pattern is a description of the problem and the essence of its solution. • It should be sufficiently abstract to be reused in different settings. • Pattern descriptions usually make use of object-oriented characteristics such as inheritance and polymorphism. Chapter 7 Design and implementation

8. Pattern elements • Name • A meaningful pattern identifier. • Problem description. • Solution description. • Not a concrete design but a template for a design solution that can be instantiated in different ways. • Consequences • The results and trade-offs of applying the pattern. Chapter 7 Design and implementation

9. The Observer pattern • Name • Observer. • Description • Separates the display of object state from the object itself. • Problem description • Used when multiple displays of state are needed. • Solution description • See slide with UML description. • Consequences • Optimisations to enhance display performance are impractical. Chapter 7 Design and implementation

10. The Observer pattern (1) Chapter 7 Design and implementation

11. The Observer pattern (2) Chapter 7 Design and implementation

12. Multiple displays using the Observer pattern Chapter 7 Design and implementation

13. A UML model of the Observer pattern Chapter 7 Design and implementation

14. Good Examples of Use • Very Simple use example: http://javarevisited.blogspot.com/2011/12/observer-design-pattern-java-example.html • MVC example: http://www.javaworld.com/article/2077258/learn-java/observer-and-observable.html • Example using listener: http://www.vogella.com/tutorials/DesignPatternObserver/article.html • Good description: • http://www.blackwasp.co.uk/Observer.aspx

15. Design problems • To use patterns in your design, you need to recognize that any design problem you are facing may have an associated pattern that can be applied. • Tell several objects that the state of some other object has changed (Observer pattern). • Tidy up the interfaces to a number of related objects that have often been developed incrementally (Façade pattern). • Provide a standard way of accessing the elements in a collection, irrespective of how that collection is implemented (Iterator pattern). • Allow for the possibility of extending the functionality of an existing class at run-time (Decorator pattern). Chapter 7 Design and implementation