Skip to content

01. Introduction

(From Design Patterns: Elements of Reusable Object-Oriented Software)

1.1 What Is a Design Pattern?

Definition

A design pattern is a general, reusable solution to a commonly occurring problem in software design within a given context.

Key Characteristics

A design pattern:

  • Is not code, but a template/blueprint

  • Solves a design problem, not an implementation detail

  • Is language-independent

  • Is proven through repeated use

  • Improves communication among developers

What a Design Pattern Is NOT

  • Not an algorithm

  • Not a finished design

  • Not a framework

  • Not a library

Why Design Patterns Matter

  • Promote reusability

  • Improve maintainability

  • Reduce coupling

  • Increase flexibility

  • Provide a shared vocabulary (e.g., “use Observer here”)

1.2 Design Patterns in Smalltalk MVC

MVC (Model–View–Controller)

MVC is an early example of a design pattern system used in Smalltalk.

Components

  • Model

    • Represents core data and business logic

  • View

    • Displays data to the user

  • Controller

    • Handles user input and updates the model

Design Patterns Inside MVC

MVC itself is not a single pattern but a combination of patterns:

  • Observer

    • Views observe the Model for changes

  • Strategy

    • Controllers act as interchangeable strategies

  • Composite

    • Views can be nested

Why MVC Is Important

  • Demonstrates how multiple patterns collaborate

  • Encourages separation of concerns

  • Influenced modern frameworks (Spring MVC, Django, Rails)

1.3 Describing Design Patterns

GoF uses a standard template to describe every pattern.

Pattern Description Elements

  1. Pattern Name

    • A handle to communicate design ideas

  2. Intent

    • What the pattern does and why

  3. Motivation

    • Problem scenario explaining need

  4. Applicability

    • When to use the pattern

  5. Structure

    • UML-style class relationships

  6. Participants

    • Classes/objects involved

  7. Collaborations

    • How participants interact

  8. Consequences

    • Trade-offs (flexibility vs complexity)

  9. Implementation

    • Pitfalls and tips

  10. Sample Code

  11. Known Uses

  12. Related Patterns

Interview Tip

If asked to explain a pattern, Intent + Applicability + Consequences is usually sufficient.

1.4 The Catalog of Design Patterns

The GoF catalog contains 23 design patterns.

Purpose of the Catalog

  • Provide standard solutions

  • Avoid reinventing the wheel

  • Encourage best practices

Pattern Scope

Patterns differ by:

  • Purpose (what they do)

  • Scope (class vs object)

1.5 Organizing the Catalog

Classification by Purpose

  1. Creational Patterns

    • Object creation mechanisms

    • Example: Factory, Singleton

  2. Structural Patterns

    • Class/object composition

    • Example: Adapter, Decorator

  3. Behavioral Patterns

    • Object interaction and responsibility

    • Example: Observer, Strategy

Classification by Scope

Scope Meaning
Class Uses inheritance
Object Uses composition

Why This Organization Helps

  • Makes pattern selection easier

  • Highlights design intent

  • Clarifies trade-offs

1.6 How Design Patterns Solve Design Problems

Core Design Problems Addressed

  1. Finding Appropriate Objects

    • Identify responsibilities, not classes

  2. Determining Object Granularity

    • Avoid God objects

  3. Specifying Object Interfaces

    • Program to interfaces, not implementations

  4. Managing Dependencies

    • Reduce tight coupling

  5. Making Code Reusable

    • Favor composition over inheritance

Fundamental OO Principles Used

  • Encapsulation

  • Abstraction

  • Polymorphism

  • Composition over inheritance

  • Loose coupling

1.7 How to Select a Design Pattern

Steps to Choose a Pattern

  1. Understand the problem

  2. Study pattern intents

  3. Consider causes of redesign

  4. Examine patterns with similar intent

  5. Evaluate consequences

  6. Consider scalability and flexibility

Interview Rule of Thumb

Do not force patterns.
Use them only when the problem context fits.

1.8 How to Use a Design Pattern

Steps to Apply a Pattern

  1. Read the pattern completely

  2. Understand its intent and structure

  3. Identify participants in your design

  4. Adapt names and structure

  5. Implement incrementally

  6. Refactor if needed

Common Mistakes

  • Overengineering

  • Blindly applying patterns

  • Using patterns without understanding trade-offs

Quick Interview Summary (1-Minute Revision)

  • Design patterns are reusable design solutions

  • GoF catalog contains 23 patterns

  • MVC demonstrates pattern collaboration

  • Patterns are described using a standard template

  • Classified into Creational, Structural, Behavioral

  • Patterns improve flexibility, maintainability, communication

  • Use patterns judiciously, not everywhere