2. Pertemuan 08 Design Patterns & Anti-Patterns
Mata kuliah : T0144 – Advanced Topics in Software Engineering
Tahun : 2010
Pertemuan 08 Design Patterns & Anti-Patterns

3. Learning Outcomes Pada akhir pertemuan ini, diharapkan:
Mahasiswa dapat mendesain software dengan menerapkan Design Pattern dan menghindari Anti- Pattern 3

4. Outline Material Design Patterns Definition Design Patterns Examples
Singleton
Adapter
Decorator
Proxy
Observer
Façade
How to use Design Pattern
Anti-Patterns 4

5. What is a Design Pattern?
In software engineering, a design pattern is a general reusable solution to a commonly occurring problem in software design.
A design pattern is not a finished design that can be transformed directly into code. It is a description or template for how to solve a problem that can be used in many different situations.
Object-oriented design patterns typically show relationships and interactions between classes or objects, without specifying the final application classes or objects that are involved.

6. Example1 : Singleton
Singleton pattern is a design pattern that is used to restrict instantiation of a class to one object.
This is useful when exactly one object is needed to coordinate actions across the system.

7. Example2: Adapter
Adapter design pattern (often referred to as the wrapper pattern or simply a wrapper) translates one interface for a class into a compatible interface.
An adapter allows classes to work together that normally could not because of incompatible interfaces, by providing its interface to clients while using the original interface.
The adapter translates calls to its interface into calls to the original interface, and the amount of code necessary to do this is typically small.

8. Example3: Decorator
In object-oriented programming, the decorator pattern is a design pattern that allows new/additional behaviour to be added to an existing object dynamically.

9. Example4: Proxy
A proxy is a class functioning as an interface to something else.
The proxy could interface to anything: a network connection, a large object in memory, a file, or some other resource that is expensive or impossible to duplicate.

10. Example5: Observer
The observer pattern is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods.
It is mainly used to implement distributed event handling systems.

11. Example6: Façade
A facade is an object that provides a simplified interface to a larger body of code, such as a class library.

12. Façade Pattern Benefits
A facade can:
make a software library easier to use and understand, since the facade has convenient methods for common tasks;
make code that uses the library more readable, for the same reason;
reduce dependencies of outside code on the inner workings of a library, since most code uses the facade, thus allowing more flexibility in developing the system;
wrap a poorly-designed collection of APIs with a single well-designed API (as per task needs).

13. Source Code Demo

14. How to use Design Patterns
Each pattern describe a particular design problem, with specific constraints and context.
The “pattern” is a well proven generic solution scheme.
Aims to decouple strongly interdependent objects
Strong Coupling == more interdependency == bad
Distribution of methods/data to other components
Avoids the ‘God object’ syndrome.

15. Anti-Patterns
Anti-Pattern is a design pattern that may be commonly used but is ineffective and/or counterproductive in practice
Symptoms:
Some repeated pattern of action, process or structure that initially appears to be beneficial, but ultimately produces more bad consequences than beneficial results
A refactored solution exists that is clearly documented, proven in actual practice and repeatable.
By formally describing repeated mistakes, one can recognize the forces that lead to their repetition and learn how others have refactored themselves out of these broken patterns.

16. Anti-Pattern Categories
Organizational anti-patterns
Project management anti-patterns
Analysis anti-patterns
Software design anti-patterns
Programming anti-patterns
Methodological anti-patterns
Configuration management anti-patterns

17. Example 1: Copy&Paste Programming
Copy and paste programming is a pejorative term to describe highly repetitive computer programming code apparently produced by copy and paste operations.
It is frequently symptomatic of a lack of programming competence, or an insufficiently expressive development environment, as subroutines or libraries would normally be used instead.
In certain contexts it has legitimate value, if used with care.

18. Example 2: God Object
In object-oriented programming, a God object is an object that knows too much or does too much.
Why is this bad ?

19. References
Design Patterns: Elements of Reusable Object-Oriented Software. Gamma, Erich; Richard Helm, Ralph Johnson, and John Vlissides (1995). Addison-Wesley. ISBN
AntiPatterns: Refactoring Software, Architectures, and Projects in Crisis. Brown, William J.; Raphael C. Malveau, Hays W. "Skip" McCormick, Thomas J. Mowbray, Theresa Hudson (ed) (1998). John Wiley & Sons, ltd. ISBN
Anti-pattern on Wikipedia 19