27 — Design Patterns

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▶️ pkg8patterns/patterns1SingletonPattern.java → patterns23VisitorPattern.java

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GoF categories

Category	Purpose	Examples
Creational	Object creation	Singleton, Factory, Builder, Prototype
Structural	Composition	Adapter, Decorator, Facade, Proxy
Behavioral	Communication	Strategy, Observer, Command, Template Method

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Must-know patterns (interview)

Pattern	One-line	When
Singleton	One instance	Config, connection pool
Factory	Delegate creation	Hide concrete types
Builder	Step-by-step construction	Many optional params
Strategy	Swappable algorithms	Payment methods, sorting
Observer	Publish/subscribe	Event listeners
Decorator	Add behavior dynamically	Java I/O streams
Proxy	Control access	Lazy load, security, caching
Template Method	Fixed skeleton, variable steps	Abstract process()

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Study method

For each of the 23 patterns in pkg8patterns:

1. Run the demo
2. Draw a class diagram on paper
3. Name a real framework use (Spring AOP = Proxy, InputStreamReader = Adapter)
4. State trade-offs

References → 04-reference/03-DesignPatterns.md · 03-interview/02-OopAndSolid.md · 03-interview/13-DesignPatterns.md

Next → 28 I/O & NIO