Introduction to Design Patterns:
Building Reusable Object-Oriented Software

Overview

• Design OO software is not easy, but design reusable object-oriented software is even harder.
• You are almost bound to mistake when you first try out your OO design
• However, experienced object-oriented designers do make good designs
• Design patterns are the well documented experience of OO designer.
• These patterns solve specific design problems and make object-oriented designs more flexible, elegant, and ultimately reusable.
• Once you know the pattern, a lot of design decisions follow automatically.
• Expressing proven techniques as design patterns makes them more accessible to developers of new systems.

What is a Design Pattern

There are four essential elements:

1. Pattern name: to capture a design problem, its solutions and consequences in a word or two
2. Problem: to describe when to apply the pattern
3. Solution: to describe the elements that make up the design, their relationships, responsibilities, and collaborations.
4. Consequences: describe the results and trade-offs of applying the pattern.

Although design patterns supposingly describe object-oriented designs, they are based on practical solutions that have been implemented using OOP such as C++, Smalltalk and (Java).

Syntax

We will be using a consistent format to describe a particular Design Patterns

• Pattern Name and Classification

• Intent:
  A short statement that states what does the design pattern do, rationale and intent, etc.

• Aka (Also Known As):
  The alias or alternative name

• Motivations:
  A real scenario that exercise the class and object structure to solve a problem

• Applicability:
  The specific domain that the pattern will apply

• Structure:
  Graphical representation using CRC cards, OMT or Booch to describe classes and interactions within the pattern

• Participants:
  Listings of classes and / or objects participating in the design pattern and their responsibilities

• Collaborations:
  How the participants collaborate to carry out their responsibilities.

• Consequences:
  The results and the trade-offs of using the patterns.

• Implementation:
  A checklist of how to build a solution using the design pattern. Listings of pitfalls, hints or techniques.

• Sample Code

• Known Uses

• Related Patterns:
  Design patterns usually work together to solve a problem. List all the related patterns in this section.

The Catalog of Design Patterns

• Abstract Factory
• Adapter
• Bridge
• Builder
• Chain of Responsibility
• Command
• Composite
• Decorator
• Facade
• Factory Method
• Flyweight
• Interpreter
• Iterator
• Mediator
• Memento
• Observer
• Prototype
• Proxy
• Singleton
• State
• Strategy
• Template Method
• Visitor

Design Pattern Space

How to Select a Design Pattern

Consider how design patterns solve design problems

Scan Intent sections

Study how patterns interrelate

Study patterns of like purpose

Examine a cause of redesign

Consider what should be variable in your design

How to Use a Design Pattern

Read the pattern once through for an overview.

Go back and study the Structure, Participants, and Collaborations sections.

Look at the Sample Code section to see a concrete example of the pattern in code.

Choose names for pattern participants that are meaningful in the application context.

Define the classes.

Define application-specific names for operations in the pattern.

Implement the operations to carry out the responsibilities and collaborations in the pattern.