Design Patterns

Design Patterns - Elements of Reusable Object-Oriented Software. This is a classical book. The authors won ACM 2010 SIGSOFT outstanding research award for their contribution to software engineering for. The four authors are classed the GoF (Gang of Four). The design patterns in their book is called the GoF patterns.

Design patterns are solutions abstracted from repeatedly occurring design problems and can be reused in similar situations. Each has a pattern name, associated problem, solution and consequence.

Some design patterns are bounded to languages features, so is easier to implement in some languages than the others. For example, the Template pattern is easy to do in C++ and Java, since C++ provides template and Java provides generics.

In this book, designed patterns are divided into 3 categories based on purpose. The following notes are extracted from the book.

A. Creational

Class:

1. Factory method
Define an interface for creating an object, but let subclasses decide which class to instantiate. It lets a class defer instantiation to subclasses.

Object:

2. Abstract Factory
Provide an interface for creating families of related or dependent objects w/o specifying their concrete classes.

Isn't this the interface concept in C++/Java? Obviously it's related to polymorphism.

3. Builder
Separate the construction of a complex object from its representation so that the same construction process can create different representations.

4. Prototype
Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.

5. Singleton
Ensure a class only has one instance, and provide a global point of access to it.

B. Structural

Class:

6. Adapter (class)
Convert the interface of a class into another interface clients expect. It lets classes work together that couldn't otherwise because of incompatible interfaces.

Object:

7. Adapter (object)

8. Bridge
Decouple an abstraction from its implementation so that the two can vary independently.

This is the abstraction and encapsulation principles of OOP.

9. Composite
Compose objects into tree structures to represent part-whole hierarchies. It lets clients treat individual objects and compositions of objects uniformly.

OOP uses inheritance for IS-A relationship, and uses composition for HAS-A relationship.

10. Decorator
Attach additional responsibilities to an object dynamically. It provides a flexible alternative to subclassing for extending functionality.

11. Facade
Provide a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use.

12. Flyweight
Use sharing to support large numbers of fine-grained objects efficiently.

13. Proxy
Provide a surrogate or placeholder for another object to control access to it.

C. Behavioral

Class:

14. Interpreter
Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language.

Sounds related to compiler/interpreter.

15. Template method
Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. Template method lets subclasses redefine certain steps of an algorithm w/o changing the algorithm's structure.

Object:

16. Chain of Responsibility
Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it.

One example to this is to catch a series of exceptions in C++/Java.

17. Command
Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.

18. Iterator
Provide a way to access the elements of an aggregate object sequentially w/o exposing its underlying representation.

This occurs abundantly in C++/Java.

19. Mediator
Define an object that encapsulates how a set of objects interact. It promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently.

20. Memento
W/o violating encapsulation, capture and externalize an object's internal state so that the object can be restored to this state later.

21. Observer
Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

For example, Java uses wait(), notify() and notifyAll() methods for threads communication.

22. State
Allow an object to alter its behavior when its internal state changes. It will appear to change it class.

One example for this is the workflow state management as in my work.

23. Strategy
Define a family of algorithms, encapsulate each one, and make them interchangeable. It lets the algorithm vary independently from clients that use it.

24. Visitor
Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation w/o changing the classes of the elements on which it operates.

The famous MVC is not included in list because it's a combination of multiple patterns. Use Smalltalk MVC as an example. The VC relationship uses the Strategy pattern. MVC also uses Factory method to specify the default controller class for a view, and Decorator pattern to add scrolling to a view.

A new comer at OOD can start with the simplest and most common patterns:
Creational: Abstract Factory, Factory
Structural: Adapter, Composite, Decorator
Behavioral: Observer, Strategy, Template(! Yeah, this is behavioral, not structural)

Seems like I already had experience with at least these design patterns:
Creational: Singleton, Factory
Structural: Adapter, Bridge, Composite
Behavioral: Interpreter, Template, Chain of Responsibility, Iterator, Observer, State

[1] Amazon: Design patterns. By Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides. QA 76.64 .D47 1994.
[2] Wiki - design patterns. Short but comprehensive list of design patterns from different sources.
[3] Design pattern implementation in C# and VB.NET. Good link with real code examples. E.g., load balancer using the Singleton pattern.