256 lines
10 KiB
Markdown
256 lines
10 KiB
Markdown
# GoF Design Patterns in Java
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Complete runnable Java 25 implementations of all 23 Gang of Four design patterns.
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Each pattern has a dedicated article at [ankurm.com](https://ankurm.com/gof-design-patterns-java/)
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with a UML diagram, step-by-step explanation, and console output.
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## Structure
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```
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design-patterns/
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├── 01-creational/
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│ ├── factory-method/
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│ ├── abstract-factory/
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│ ├── builder/
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│ ├── prototype/
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│ └── singleton/
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├── 02-structural/
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│ ├── adapter/
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│ ├── bridge/
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│ ├── composite/
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│ ├── decorator/
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│ ├── facade/
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│ ├── flyweight/
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│ └── proxy/
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└── 03-behavioral/
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├── chain-of-responsibility/
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├── command/
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├── interpreter/
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├── iterator/
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├── mediator/
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├── memento/
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├── observer/
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├── state/
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├── strategy/
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├── template-method/
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└── visitor/
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```
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## Prerequisites
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- Java 25+ (Eclipse Temurin recommended)
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- No build tool required — plain `javac` / `java`
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## Run any pattern
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```bash
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# Linux / macOS / Git Bash (Windows)
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javac 03-behavioral/strategy/*.java -d out/strategy
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java -cp out/strategy strategy.Main
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```
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```cmd
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REM Windows Command Prompt
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javac 03-behavioral\strategy\*.java -d out\strategy
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java -cp out\strategy strategy.Main
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```
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## Articles
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| Pattern | Category | Article |
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|---------|----------|---------|
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| Factory Method | Creational | https://ankurm.com/factory-method-design-pattern-java/ |
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| Abstract Factory | Creational | https://ankurm.com/abstract-factory-design-pattern-java/ |
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| Builder | Creational | https://ankurm.com/builder-design-pattern-java/ |
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| Prototype | Creational | https://ankurm.com/prototype-design-pattern-java/ |
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| Singleton | Creational | https://ankurm.com/singleton-design-pattern-java/ |
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| Adapter | Structural | https://ankurm.com/adapter-design-pattern-java/ |
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| Bridge | Structural | https://ankurm.com/bridge-design-pattern-java/ |
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| Composite | Structural | https://ankurm.com/composite-design-pattern-java/ |
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| Decorator | Structural | https://ankurm.com/decorator-design-pattern-java/ |
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| Facade | Structural | https://ankurm.com/facade-design-pattern-java/ |
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| Flyweight | Structural | https://ankurm.com/flyweight-design-pattern-java/ |
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| Proxy | Structural | https://ankurm.com/proxy-design-pattern-java/ |
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| Chain of Responsibility | Behavioral | https://ankurm.com/chain-of-responsibility-design-pattern-java/ |
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| Command | Behavioral | https://ankurm.com/command-design-pattern-java/ |
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| Interpreter | Behavioral | https://ankurm.com/interpreter-design-pattern-java/ |
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| Iterator | Behavioral | https://ankurm.com/iterator-design-pattern-java/ |
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| Mediator | Behavioral | https://ankurm.com/mediator-design-pattern-java/ |
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| Memento | Behavioral | https://ankurm.com/memento-design-pattern-java/ |
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| Observer | Behavioral | https://ankurm.com/observer-design-pattern-java/ |
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| State | Behavioral | https://ankurm.com/state-design-pattern-java/ |
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| Strategy | Behavioral | https://ankurm.com/strategy-design-pattern-java/ |
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| Template Method | Behavioral | https://ankurm.com/template-method-design-pattern-java/ |
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| Visitor | Behavioral | https://ankurm.com/visitor-design-pattern-java/ |
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## Post ↔ Code Mapping
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Each article is split into multiple runnable parts. This section maps every post section to the exact file(s) in this repository, so you can jump straight from "what I just read" to "the file that implements it." Updated pattern-by-pattern as each article is verified against Java 25.
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### Factory Method (`01-creational/factory-method/`)
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| Post Section | File(s) |
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| The Problem: Object Creation Leaks Into Business Logic | `NotificationService.java` (anti-pattern shown for contrast — not called by `Main`) |
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| Part 1 — The Notification Interface | `Notification.java` |
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| Part 2 — The Concrete Notification Classes | `EmailNotification.java`, `SmsNotification.java`, `PushNotification.java` |
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| Part 3 — The NotificationSender Class | `NotificationSender.java` |
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| Part 4 — The Concrete Sender Subclasses | `EmailSender.java`, `SmsSender.java`, `PushSender.java` |
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| Part 5 — Running It: Client Code and Output | `Main.java` |
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| Adding a New Channel (Slack) | `SlackNotification.java`, `SlackSender.java` |
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Run it:
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```bash
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javac 01-creational/factory-method/*.java -d out/factory-method
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java -cp out/factory-method factorymethod.Main
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```
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### Abstract Factory (`01-creational/abstract-factory/`)
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| Post Section | File(s) |
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| Part 1 — The Abstract Products: Button and TextField | `Button.java`, `TextField.java` |
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| Part 2 — The Concrete Products (Light family) | `LightButton.java`, `LightTextField.java` |
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| Part 2 — The Concrete Products (Dark family) | `DarkButton.java`, `DarkTextField.java` |
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| Part 3 — The Abstract Factory: UIFactory Interface | `UIFactory.java` |
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| Part 4 — The Concrete Factories | `LightThemeFactory.java`, `DarkThemeFactory.java` |
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| Part 5 — The Client: Application | `Application.java`, `Main.java` |
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Run it:
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```bash
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javac 01-creational/abstract-factory/*.java -d out/abstract-factory
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java -cp out/abstract-factory abstractfactory.Main
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# or, for the dark theme family:
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java -Dapp.theme=dark -cp out/abstract-factory abstractfactory.Main
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```
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### Builder (`01-creational/builder/`)
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| Post Section | File(s) |
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| Part 1 — The Product: HttpRequest (Immutable) | `HttpRequest.java` |
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| Part 2 — The Director: Encapsulating Common Configurations | `HttpClientConfig.java` |
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| Part 3 — Using the Builder: Client Code | `Main.java` |
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Note: the Lombok `@Builder` example and the JDK standard-library snippets (`HttpRequest.newBuilder()`, `StringBuilder`, `ProcessBuilder`) are illustrative only — they are not part of this repository's runnable example.
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Run it:
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```bash
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javac 01-creational/builder/*.java -d out/builder
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java -cp out/builder builder.Main
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```
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### Prototype (`01-creational/prototype/`)
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| Post Section | File(s) |
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| Part 1 — The Prototype Interface: Copyable | `Copyable.java` |
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| Part 2 — The Nested Object: DocumentMetadata | `DocumentMetadata.java` |
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| Part 3 — The Concrete Prototype: Document | `Document.java` |
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| Part 4 — The Prototype Registry: DocumentRegistry | `DocumentRegistry.java` |
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| Part 5 — Using the Prototype: Client Code | `Main.java` |
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Note: the `ShallowVsDeepDemo` snippet (shallow vs. deep copy) and the `Cloneable`-based example are illustrative only — they are not part of this repository's runnable example.
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Run it:
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```bash
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javac 01-creational/prototype/*.java -d out/prototype
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java -cp out/prototype prototype.Main
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```
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### Singleton (`01-creational/singleton/`)
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| Post Section | File(s) |
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| Implementation 1 — Eager Initialization | `AppConfigEager.java` |
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| Implementation 2 — Naïve Lazy Initialization | `AppConfigNaiveLazy.java` |
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| Implementation 3 — Synchronized Method | `AppConfigSynchronized.java` |
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| Implementation 4 — Double-Checked Locking | `AppConfigDCL.java` |
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| Implementation 5 — Initialization-on-Demand Holder | `AppConfigHolder.java` |
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| Implementation 6 — Enum Singleton | `AppConfigEnum.java` |
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| Running All Six Implementations Together | `Main.java` |
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Each implementation is renamed to its own class (e.g. `AppConfigEager`, `AppConfigHolder`) so all six can be compiled together in one package and compared directly. `Main.java` exercises all six in one run. The "Breaking via Reflection," "Breaking via Serialization," and Spring `@Component` snippets are illustrative only — they are not part of this repository's runnable example.
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Run it:
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```bash
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javac 01-creational/singleton/*.java -d out/singleton
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java -cp out/singleton singleton.Main
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```
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### Adapter (`02-structural/adapter/`)
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| Post Section | File(s) |
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| Step 1 — Define the Target Interface | `PaymentGateway.java` |
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| Step 2 — The Adaptee (What You're Wrapping) | `StripeClient.java` |
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| Step 3 — The Object Adapter | `StripePaymentAdapter.java` |
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| Step 4 — The Client (Knows Nothing About Stripe) | `OrderService.java` |
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| Putting It Together: The Main Demo | `Main.java` |
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| Object Adapter vs Class Adapter | `StripePaymentClassAdapter.java` |
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Note: the `InputStreamReader` JDK-adapter snippet is illustrative only — it is not part of this repository's runnable example.
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Run it:
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```bash
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javac 02-structural/adapter/*.java -d out/adapter
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java -cp out/adapter adapter.Main
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```
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### Bridge (`02-structural/bridge/`)
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| Post Section | File(s) |
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| Step 1 — The Implementor Interface | `Device.java` |
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| Step 2 — Concrete Implementors (TV and Radio) | `TV.java`, `Radio.java` |
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| Step 3 — The Abstraction (RemoteControl) | `RemoteControl.java` |
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| Step 4 — Refined Abstraction (AdvancedRemote) | `AdvancedRemote.java` |
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| Wiring It Together: Main Demo | `Main.java` |
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Note: the "Class Explosion Problem — Visualised" pseudocode and the JDBC/SLF4J snippets are illustrative only — they are not part of this repository's runnable example.
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Run it:
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```bash
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javac 02-structural/bridge/*.java -d out/bridge
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java -cp out/bridge bridge.Main
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```
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### Composite (`02-structural/composite/`)
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| Post Section | File(s) |
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| The Problem: Treating Leaves and Containers Uniformly | illustrative only — not part of this repository's runnable example |
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| Step 1 — The Component Interface | `FileSystemItem.java` |
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| Step 2 — The Leaf (File) | `File.java` |
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| Step 3 — The Composite (Directory) | `Directory.java` |
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| Building and Using the Tree | `Main.java` |
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Run it:
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```bash
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javac 02-structural/composite/*.java -d out/composite
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java -cp out/composite composite.Main
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```
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### Decorator (`02-structural/decorator/`)
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| Post Section | File(s) |
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| Step 1 — Component Interface | `TextProcessor.java` |
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| Step 2 — Concrete Component | `PlainTextProcessor.java` |
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| Step 3 — Base Decorator | `TextDecorator.java` |
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| Step 4 — Concrete Decorators | `UpperCaseDecorator.java`, `TrimDecorator.java`, `ProfanityFilterDecorator.java` |
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| Stacking the Decorators — Order Matters | `Main.java` |
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Note: the JDK `InputStream`/`BufferedInputStream`/`GZIPInputStream` snippet is illustrative only — it is not part of this repository's runnable example.
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Run it:
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```bash
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javac 02-structural/decorator/*.java -d out/decorator
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java -cp out/decorator decorator.Main
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```
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## Reference
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- *Design Patterns: Elements of Reusable Object-Oriented Software* — Gamma, Helm, Johnson, Vlissides
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- https://refactoring.guru/design-patterns
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