Sarah's Lab: Design Patterns

I was asked to do a presentation about design patterns and after the presentation people had asked me to post it, so here is a copy of my PPT :

What are Design Patterns?

In software engineering, a design pattern is a general repeatable solution to a commonly occurring problem in software design.

Not a finished design that can be transformed directly into code

A description of how to solve a problem

Algorithms are not thought of as design patterns, since they solve computational problems rather than design problems.

Show relationships and interactions between classes or objects, without specifying the final application classes or objects that are involved.

Why Design Patterns?

Can speed up the development process.

Prevent issues that can cause major problems.

Improves code readability.

Provide general solutions.

Design reuse, which is more powerful than code reuse.

Classifications of Design Patterns:

Creational Patterns:
–Abstract Factory Creates an instance of several families of classes
–Builder Separates object construction from its representation
–Factory Method Creates an instance of several derived classes
–Prototype A fully initialized instance to be copied or cloned
–Singleton A class of which only a single instance can exist

Structural Patterns
–Adapter Match interfaces of different classes
–Bridge Separates an object’s interface from its implementation
–Composite A tree structure of simple and composite objects
–Decorator Add responsibilities to objects dynamically
–Facade A single class that represents an entire subsystem
–Flyweight A fine-grained instance used for efficient sharing
–Proxy An object representing another object

Behavioral Patterns
–Chain of Resp. A way of passing a request between a chain of objects
–Command Encapsulate a command request as an object
–Interpreter A way to include language elements in a program
–Iterator Sequentially access the elements of a collection
–Mediator Defines simplified communication between classes
–Memento Capture and restore an object's internal state
–Observer A way of notifying change to a number of classes
–State Alter an object's behavior when its state changes
–Strategy Encapsulates an algorithm inside a class
–Template Method Defer the exact steps of an algorithm to a subclass Visitor Defines a new operation to a class without change

A Sample of a Design Pattern Document:

Pattern Name and Classification:

Intent: goal

Also Known As: Other names for the pattern.

Motivation (Forces): A scenario in which this pattern can be used.

Applicability: Situations in which this pattern is usable; the context for the pattern.

Structure: A graphical representation of the pattern. (Class diagrams, Interaction diagrams)

Participants: A listing of the classes and objects used in the pattern.

Collaboration: A description of how classes and objects used in the pattern interact

Consequences: A description of the results, side effects, and trade offs.

Implementation: A description of an implementation

Sample Code:

Known Uses: Examples of real usages of the pattern. Related Patterns: Other patterns that have some relationship with the pattern; discussion of the differences between the pattern and similar patterns.

Example?

Pattern Name and Classification: Singleton, creational pattern

Intent: Ensure a class has only one instance and provide a global point of access to it.

Also Known As: Single Instance .

Motivation (Forces): Sometimes it's appropriate to have exactly one instance of a class: window managers.

Applicability: Singleton can be used in a data repository or data collection where creation of more than one object can be resource wastage. Hence each client is given a reference to a single shared object to operate on. While the second case Singleton can be used in locking mechanisms. Once a client has got the object, no other client can have an object..

Structure:

Participants: The classes and/or objects participating in this pattern are:

–Singleton:defines an Instance operation that lets clients access its unique instance. Instance is a class operation.

responsible for creating and maintaining its own unique instance.

Collaboration: A description of how classes and objects used in the pattern interact

Consequences: A description of the results, side effects, and trade offs.

Implementation: A description of an implementation

Sample Code:
– class Singleton { private static Singleton instance; // Note: Constructor is 'protected' protected Singleton() { } public static Singleton Instance() { // Use 'Lazy initialization' if (instance == null) { instance = new Singleton(); } return instance; } }
–// .NET Singleton
–sealed class Singleton
–{
– private Singleton() {}
– public static readonly Singleton Instance = new Singleton();
–}

Known Uses: Connection Pooling

Related Patterns: Other patterns that have some relationship with the pattern; discussion of the differences between the pattern and similar patterns

References: