Java Design Patterns: Overview

Design patterns are reusable solutions to common software design problems. They provide proven approaches that improve code maintainability, readability, and scalability, while establishing a shared vocabulary for developers to communicate design decisions.

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The Three Categories

Category	Purpose	Patterns Covered
Creational	Control how objects are created	Singleton, Factory Method, Abstract Factory, Builder, Prototype
Structural	Manage object composition & relationships	Adapter, Bridge, Composite, Decorator, Facade, Proxy
Behavioral	Define how objects communicate & share responsibility	Chain of Responsibility, Observer, Strategy, Command, Template Method

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Creational Patterns at a Glance

Pattern	Intent	Key Mechanism
Singleton	One instance globally	Private constructor + static accessor
Factory Method	Delegate creation to subclasses	Factory method returns interface type
Abstract Factory	Create families of related objects	Factory of factories
Builder	Step-by-step complex construction	Fluent builder with build()
Prototype	Clone existing objects	clone() method

Structural Patterns at a Glance

Pattern	Intent	Key Mechanism
Adapter	Make incompatible interfaces compatible	Wraps adaptee, implements target
Bridge	Decouple abstraction from implementation	Composition linking two hierarchies
Composite	Tree structures with uniform interface	Component interface for leaf + composite
Decorator	Add behavior dynamically	Wraps object, extends same interface
Facade	Simplify complex subsystem	High-level wrapper methods
Proxy	Control access to an object	Same interface, intercepts requests

Behavioral Patterns at a Glance

Pattern	Intent	Key Mechanism
Chain of Responsibility	Pass request along handler chain	Linked handlers with next reference
Observer	Notify dependents of state change	Subject maintains observer list
Strategy	Swap algorithms at runtime	Composition with strategy interface
Command	Encapsulate request as object	Command object with execute()/undo()
Template Method	Fixed algorithm, customizable steps	Abstract class with final template method

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Design Patterns vs Design Principles

Concept	What It Is	Examples
Design Principle	General guideline for writing good code	SOLID, DRY, KISS, YAGNI
Design Pattern	Specific, proven solution template for a recurring problem	Singleton, Factory, Observer

Patterns often implement one or more principles — for example, the Strategy pattern applies the Open/Closed Principle and Dependency Inversion.

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When to Use Design Patterns

• DO use patterns when you recognize a recurring design problem they solve
• DO use patterns to communicate intent clearly with your team
• DON'T force patterns into every problem — simplicity beats cleverness
• DON'T over-engineer with patterns when a straightforward solution works

"Design patterns should be used to simplify code, not to complicate it."