Intro to Design Patterns

What are design patterns and why use them

Overview

Design patterns are reusable solutions to commonly occurring problems in software design. They are not finished code but templates for how to solve problems that can be used in many different situations.

The Gang of Four (GoF) book introduced 23 classic patterns categorized into three types: Creational, Structural, and Behavioral.

Key Concepts

Patterns are proven solutions to common problems

Three categories: Creational, Structural, Behavioral

Creational: Object creation mechanisms

Structural: Class and object composition

Behavioral: Object interaction and responsibility

Code Example

java

1/*
223 Gang of Four Design Patterns:
3
4CREATIONAL (5) - How objects are created
5├── Singleton      - One instance only
6├── Factory Method - Create objects via method
7├── Abstract Factory - Create families of objects
8├── Builder        - Construct complex objects step by step
9└── Prototype      - Clone existing objects
10
11STRUCTURAL (7) - How classes/objects are composed
12├── Adapter        - Convert interface to another
13├── Bridge         - Separate abstraction from implementation
14├── Composite      - Tree structures of objects
15├── Decorator      - Add responsibilities dynamically
16├── Facade         - Simplified interface to subsystem
17├── Flyweight      - Share common state efficiently
18└── Proxy          - Placeholder for another object
19
20BEHAVIORAL (11) - How objects interact
21├── Chain of Responsibility - Pass request along chain
22├── Command        - Encapsulate request as object
23├── Iterator       - Sequential access to elements
24├── Mediator       - Centralize complex communications
25├── Memento        - Capture and restore state
26├── Observer       - Notify dependents of changes
27├── State          - Change behavior based on state
28├── Strategy       - Interchangeable algorithms
29├── Template Method - Define algorithm skeleton
30├── Visitor        - Add operations without changing classes
31└── Interpreter    - Grammar and interpretation (rarely used)
32*/
33
34// Pattern Selection Guide
35// Need to create objects? → Creational
36// Need to compose structures? → Structural
37// Need to manage behavior/communication? → Behavioral

Overview of all 23 GoF design patterns organized by category.

Best Practices

1.
Don't force patterns - let them emerge from needs
2.
Learn the problem each pattern solves
3.
Understand trade-offs of each pattern
4.
Start with simple solutions, refactor to patterns if needed
5.
Patterns can be combined

💡 Interview Tips

Know all 23 GoF patterns by name and category
Deep-dive on 5-7 most common patterns
Be able to explain when to use each
Practice implementing: Singleton, Factory, Strategy, Observer, Decorator
Know real-world examples (Java Collections, Spring Framework)