# GoF Design Patterns in Java

Complete runnable Java 25 implementations of all 23 Gang of Four design patterns.
Each pattern has a dedicated article at [ankurm.com](https://ankurm.com/gof-design-patterns-java/)
with a UML diagram, step-by-step explanation, and console output.

## Structure

```
design-patterns/
├── 01-creational/
│   ├── factory-method/
│   ├── abstract-factory/
│   ├── builder/
│   ├── prototype/
│   └── singleton/
├── 02-structural/
│   ├── adapter/
│   ├── bridge/
│   ├── composite/
│   ├── decorator/
│   ├── facade/
│   ├── flyweight/
│   └── proxy/
└── 03-behavioral/
    ├── chain-of-responsibility/
    ├── command/
    ├── interpreter/
    ├── iterator/
    ├── mediator/
    ├── memento/
    ├── observer/
    ├── state/
    ├── strategy/
    ├── template-method/
    └── visitor/
```

## Prerequisites

- Java 25+ (Eclipse Temurin recommended)
- No build tool required — plain `javac` / `java`

## Run any pattern

```bash
# Linux / macOS / Git Bash (Windows)
javac 03-behavioral/strategy/*.java -d out/strategy
java -cp out/strategy strategy.Main
```

```cmd
REM Windows Command Prompt
javac 03-behavioral\strategy\*.java -d out\strategy
java -cp out\strategy strategy.Main
```

## Articles

| Pattern | Category | Article |
|---------|----------|---------|
| Factory Method | Creational | https://ankurm.com/factory-method-design-pattern-java/ |
| Abstract Factory | Creational | https://ankurm.com/abstract-factory-design-pattern-java/ |
| Builder | Creational | https://ankurm.com/builder-design-pattern-java/ |
| Prototype | Creational | https://ankurm.com/prototype-design-pattern-java/ |
| Singleton | Creational | https://ankurm.com/singleton-design-pattern-java/ |
| Adapter | Structural | https://ankurm.com/adapter-design-pattern-java/ |
| Bridge | Structural | https://ankurm.com/bridge-design-pattern-java/ |
| Composite | Structural | https://ankurm.com/composite-design-pattern-java/ |
| Decorator | Structural | https://ankurm.com/decorator-design-pattern-java/ |
| Facade | Structural | https://ankurm.com/facade-design-pattern-java/ |
| Flyweight | Structural | https://ankurm.com/flyweight-design-pattern-java/ |
| Proxy | Structural | https://ankurm.com/proxy-design-pattern-java/ |
| Chain of Responsibility | Behavioral | https://ankurm.com/chain-of-responsibility-design-pattern-java/ |
| Command | Behavioral | https://ankurm.com/command-design-pattern-java/ |
| Interpreter | Behavioral | https://ankurm.com/interpreter-design-pattern-java/ |
| Iterator | Behavioral | https://ankurm.com/iterator-design-pattern-java/ |
| Mediator | Behavioral | https://ankurm.com/mediator-design-pattern-java/ |
| Memento | Behavioral | https://ankurm.com/memento-design-pattern-java/ |
| Observer | Behavioral | https://ankurm.com/observer-design-pattern-java/ |
| State | Behavioral | https://ankurm.com/state-design-pattern-java/ |
| Strategy | Behavioral | https://ankurm.com/strategy-design-pattern-java/ |
| Template Method | Behavioral | https://ankurm.com/template-method-design-pattern-java/ |
| Visitor | Behavioral | https://ankurm.com/visitor-design-pattern-java/ |

## Post ↔ Code Mapping

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.

### Factory Method (`01-creational/factory-method/`)

| Post Section | File(s) |
|---|---|
| The Problem: Object Creation Leaks Into Business Logic | `NotificationService.java` (anti-pattern shown for contrast — not called by `Main`) |
| Part 1 — The Notification Interface | `Notification.java` |
| Part 2 — The Concrete Notification Classes | `EmailNotification.java`, `SmsNotification.java`, `PushNotification.java` |
| Part 3 — The NotificationSender Class | `NotificationSender.java` |
| Part 4 — The Concrete Sender Subclasses | `EmailSender.java`, `SmsSender.java`, `PushSender.java` |
| Part 5 — Running It: Client Code and Output | `Main.java` |
| Adding a New Channel (Slack) | `SlackNotification.java`, `SlackSender.java` |

Run it:
```bash
javac 01-creational/factory-method/*.java -d out/factory-method
java -cp out/factory-method factorymethod.Main
```

### Abstract Factory (`01-creational/abstract-factory/`)

| Post Section | File(s) |
|---|---|
| Part 1 — The Abstract Products: Button and TextField | `Button.java`, `TextField.java` |
| Part 2 — The Concrete Products (Light family) | `LightButton.java`, `LightTextField.java` |
| Part 2 — The Concrete Products (Dark family) | `DarkButton.java`, `DarkTextField.java` |
| Part 3 — The Abstract Factory: UIFactory Interface | `UIFactory.java` |
| Part 4 — The Concrete Factories | `LightThemeFactory.java`, `DarkThemeFactory.java` |
| Part 5 — The Client: Application | `Application.java`, `Main.java` |

Run it:
```bash
javac 01-creational/abstract-factory/*.java -d out/abstract-factory
java -cp out/abstract-factory abstractfactory.Main
# or, for the dark theme family:
java -Dapp.theme=dark -cp out/abstract-factory abstractfactory.Main
```

### Builder (`01-creational/builder/`)

| Post Section | File(s) |
|---|---|
| Part 1 — The Product: HttpRequest (Immutable) | `HttpRequest.java` |
| Part 2 — The Director: Encapsulating Common Configurations | `HttpClientConfig.java` |
| Part 3 — Using the Builder: Client Code | `Main.java` |

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.

Run it:
```bash
javac 01-creational/builder/*.java -d out/builder
java -cp out/builder builder.Main
```

### Prototype (`01-creational/prototype/`)

| Post Section | File(s) |
|---|---|
| Part 1 — The Prototype Interface: Copyable | `Copyable.java` |
| Part 2 — The Nested Object: DocumentMetadata | `DocumentMetadata.java` |
| Part 3 — The Concrete Prototype: Document | `Document.java` |
| Part 4 — The Prototype Registry: DocumentRegistry | `DocumentRegistry.java` |
| Part 5 — Using the Prototype: Client Code | `Main.java` |

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.

Run it:
```bash
javac 01-creational/prototype/*.java -d out/prototype
java -cp out/prototype prototype.Main
```

### Singleton (`01-creational/singleton/`)

| Post Section | File(s) |
|---|---|
| Implementation 1 — Eager Initialization | `AppConfigEager.java` |
| Implementation 2 — Naïve Lazy Initialization | `AppConfigNaiveLazy.java` |
| Implementation 3 — Synchronized Method | `AppConfigSynchronized.java` |
| Implementation 4 — Double-Checked Locking | `AppConfigDCL.java` |
| Implementation 5 — Initialization-on-Demand Holder | `AppConfigHolder.java` |
| Implementation 6 — Enum Singleton | `AppConfigEnum.java` |
| Running All Six Implementations Together | `Main.java` |

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.

Run it:
```bash
javac 01-creational/singleton/*.java -d out/singleton
java -cp out/singleton singleton.Main
```

### Adapter (`02-structural/adapter/`)

| Post Section | File(s) |
|---|---|
| Step 1 — Define the Target Interface | `PaymentGateway.java` |
| Step 2 — The Adaptee (What You're Wrapping) | `StripeClient.java` |
| Step 3 — The Object Adapter | `StripePaymentAdapter.java` |
| Step 4 — The Client (Knows Nothing About Stripe) | `OrderService.java` |
| Putting It Together: The Main Demo | `Main.java` |
| Object Adapter vs Class Adapter | `StripePaymentClassAdapter.java` |

Note: the `InputStreamReader` JDK-adapter snippet is illustrative only — it is not part of this repository's runnable example.

Run it:
```bash
javac 02-structural/adapter/*.java -d out/adapter
java -cp out/adapter adapter.Main
```

### Bridge (`02-structural/bridge/`)

| Post Section | File(s) |
|---|---|
| Step 1 — The Implementor Interface | `Device.java` |
| Step 2 — Concrete Implementors (TV and Radio) | `TV.java`, `Radio.java` |
| Step 3 — The Abstraction (RemoteControl) | `RemoteControl.java` |
| Step 4 — Refined Abstraction (AdvancedRemote) | `AdvancedRemote.java` |
| Wiring It Together: Main Demo | `Main.java` |

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.

Run it:
```bash
javac 02-structural/bridge/*.java -d out/bridge
java -cp out/bridge bridge.Main
```

### Composite (`02-structural/composite/`)

| Post Section | File(s) |
|---|---|
| The Problem: Treating Leaves and Containers Uniformly | illustrative only — not part of this repository's runnable example |
| Step 1 — The Component Interface | `FileSystemItem.java` |
| Step 2 — The Leaf (File) | `File.java` |
| Step 3 — The Composite (Directory) | `Directory.java` |
| Building and Using the Tree | `Main.java` |

Run it:
```bash
javac 02-structural/composite/*.java -d out/composite
java -cp out/composite composite.Main
```

### Decorator (`02-structural/decorator/`)

| Post Section | File(s) |
|---|---|
| Step 1 — Component Interface | `TextProcessor.java` |
| Step 2 — Concrete Component | `PlainTextProcessor.java` |
| Step 3 — Base Decorator | `TextDecorator.java` |
| Step 4 — Concrete Decorators | `UpperCaseDecorator.java`, `TrimDecorator.java`, `ProfanityFilterDecorator.java` |
| Stacking the Decorators — Order Matters | `Main.java` |

Note: the JDK `InputStream`/`BufferedInputStream`/`GZIPInputStream` snippet is illustrative only — it is not part of this repository's runnable example.

Run it:
```bash
javac 02-structural/decorator/*.java -d out/decorator
java -cp out/decorator decorator.Main
```

### Facade (`02-structural/facade/`)

| Post Section | File(s) |
|---|---|
| The Subsystem (Complex, But Unchanged) — VideoFile | `VideoFile.java` |
| The Subsystem (Complex, But Unchanged) — Codec interface | `Codec.java` |
| The Subsystem (Complex, But Unchanged) — Concrete Codecs | `MPEG4CompressionCodec.java`, `OggCompressionCodec.java` |
| The Subsystem (Complex, But Unchanged) — CodecFactory | `CodecFactory.java` |
| The Subsystem (Complex, But Unchanged) — BitrateReader | `BitrateReader.java` |
| The Subsystem (Complex, But Unchanged) — AudioMixer | `AudioMixer.java` |
| The Facade | `VideoConversionFacade.java` |
| Client Code: One Line | `Main.java` |

Note: the SLF4J `LoggerFactory`, JDBC `DriverManager`, and Spring `JdbcTemplate` snippets under "Facade in the JDK and Real Frameworks" are illustrative only — they are not part of this repository's runnable example.

Run it:
```bash
javac 02-structural/facade/*.java -d out/facade
java -cp out/facade facade.Main
```

### Flyweight (`02-structural/flyweight/`)

| Post Section | File(s) |
|---|---|
| Step 1 — The Flyweight (TreeType) | `TreeType.java` |
| Step 2 — The Factory (TreeFactory) | `TreeFactory.java` |
| Step 3 — The Context (Tree) | `Tree.java` |
| Putting the Forest Together | `Main.java` |

Note: the "Flyweight in the JDK: String Pool and Integer Cache" and "Measuring the Benefit" snippets are illustrative only — they are not part of this repository's runnable example.

Run it:
```bash
javac 02-structural/flyweight/*.java -d out/flyweight
java -cp out/flyweight flyweight.Main
```

### Proxy (`02-structural/proxy/`)

| Post Section | File(s) |
|---|---|
| The Subject Interface | `DatabaseConnection.java` |
| The Real Subject | `RealDatabaseConnection.java` |
| Virtual Proxy: Lazy Initialisation | `LazyConnectionProxy.java` |
| Logging Proxy: Cross-Cutting Concerns | `LoggingProxy.java` |
| Wiring It Together: Proxy Chaining | `Main.java` |

Note: the "Dynamic Proxy with java.lang.reflect.Proxy" snippet is illustrative only — it is not part of this repository's runnable example.

Run it:
```bash
javac 02-structural/proxy/*.java -d out/proxy
java -cp out/proxy proxy.Main
```

### Chain of Responsibility (`03-behavioral/chain-of-responsibility/`)

| Post Section | File(s) |
|---|---|
| The Problem: Rigid Routing Logic | illustrative only — not part of this repository's runnable example |
| Implementation: Support Ticket Escalation — base handler | `SupportHandler.java` |
| Implementation: Support Ticket Escalation — the ticket | `SupportTicket.java` |
| Each concrete handler checks only its own responsibility — Level 1 & 2 | `Level1Support.java`, `Level2Support.java` |
| Level3Support and CriticalIncidentTeam follow exactly the same shape | `Level3Support.java`, `CriticalIncidentTeam.java` |
| The chain is assembled in one place | `Main.java` |

Run it:
```bash
javac 03-behavioral/chain-of-responsibility/*.java -d out/chain
java -cp out/chain chain.Main
```

### Command (`03-behavioral/command/`)

| Post Section | File(s) |
|---|---|
| Step 1 — Command Interface | `Command.java` |
| Step 2 — The Receiver (TextEditor) | `TextEditor.java` |
| Step 3 — Concrete Commands | `InsertCommand.java`, `DeleteCommand.java` |
| Step 4 — The Invoker (CommandHistory) | `CommandHistory.java` |
| Demo | `Main.java` |

Run it:
```bash
javac 03-behavioral/command/*.java -d out/command
java -cp out/command command.Main
```

### Iterator (`03-behavioral/iterator/`)

| Post Section | File(s) |
|---|---|
| Custom Iterator Implementation — the Iterator interface | `BookIterator.java` |
| Custom Iterator Implementation — the element type | `Book.java` |
| Custom Iterator Implementation — the Aggregate (BookShelf, ForwardIterator, DecadeIterator) | `BookShelf.java` |
| Custom Iterator Implementation — wiring it together | `Main.java` |

Note: the "How Java's For-Each Loop Uses Iterator" snippet (`Iterable<Book>` example) is illustrative only — it is not part of this repository's runnable example.

Run it:
```bash
javac 03-behavioral/iterator/*.java -d out/iterator
java -cp out/iterator iterator.Main
```

### Mediator (`03-behavioral/mediator/`)

| Post Section | File(s) |
|---|---|
| Implementation: Chat Room — the Mediator interface | `ChatMediator.java` |
| Implementation: Chat Room — the Colleague | `User.java` |
| Implementation: Chat Room — the Concrete Mediator | `ChatRoom.java` |
| Implementation: Chat Room — wiring it together | `Main.java` |

Note: the "Mediator in Practice: MVC and Spring Events" snippet (`ApplicationEventPublisher`/`@EventListener` example) is illustrative only — it is not part of this repository's runnable example.

Run it:
```bash
javac 03-behavioral/mediator/*.java -d out/mediator
java -cp out/mediator mediator.Main
```

### Memento (`03-behavioral/memento/`)

| Post Section | File(s) |
|---|---|
| The Memento: Immutable and Opaque | `EditorMemento.java` |
| The Originator (Editor) | `Editor.java` |
| The Caretaker (History) | `History.java` |
| Wiring it together (Main demo) | `Main.java` |

Note: the "Serialization as Memento" snippet (`ObjectOutputStream`/`ObjectInputStream` example) is illustrative only — it is not part of this repository's runnable example.

Run it:
```bash
javac 03-behavioral/memento/*.java -d out/memento
java -cp out/memento memento.Main
```

### Observer (`03-behavioral/observer/`)

| Post Section | File(s) |
|---|---|
| Implementation: Stock Price Monitoring — the Observer interface | `StockObserver.java` |
| Implementation: Stock Price Monitoring — the Subject | `StockMarket.java` |
| Implementation: Stock Price Monitoring — AlertObserver | `AlertObserver.java` |
| Implementation: Stock Price Monitoring — PortfolioObserver | `PortfolioObserver.java` |
| Implementation: Stock Price Monitoring — wiring it together | `Main.java` |

Note: the "Push vs Pull Notification Models" pull-model snippet and the "Thread Safety" `CopyOnWriteArrayList` snippet are illustrative only — they are not part of this repository's runnable example.

Run it:
```bash
javac 03-behavioral/observer/*.java -d out/observer
java -cp out/observer observer.Main
```

### State (`03-behavioral/state/`)

| Post Section | File(s) |
|---|---|
| With State: Traffic Light — the State interface | `TrafficLightState.java` |
| With State: Traffic Light — the Context | `TrafficLight.java` |
| With State: Traffic Light — RedState | `RedState.java` |
| With State: Traffic Light — GreenState | `GreenState.java` |
| With State: Traffic Light — YellowState | `YellowState.java` |
| With State: Traffic Light — wiring it together | `Main.java` |

Note: the "Without State: The if-else Machine" snippet is illustrative only — it is not part of this repository's runnable example.

Run it:
```bash
javac 03-behavioral/state/*.java -d out/state
java -cp out/state state.Main
```

### Strategy (`03-behavioral/strategy/`)

| Post Section | File(s) |
|---|---|
| Implementation: Pluggable Sorting — the Strategy interface | `SortStrategy.java` |
| Implementation: Pluggable Sorting — BubbleSort | `BubbleSort.java` |
| Implementation: Pluggable Sorting — MergeSort | `MergeSort.java` |
| Implementation: Pluggable Sorting — QuickSort | `QuickSort.java` |
| Implementation: Pluggable Sorting — the Context (Sorter) | `Sorter.java` |
| Implementation: Pluggable Sorting — wiring it together | `Main.java` |

Note: the "Strategy in Java 8+: Lambdas as Strategies" snippet is illustrative only — it is not part of this repository's runnable example.

Run it:
```bash
javac 03-behavioral/strategy/*.java -d out/strategy
java -cp out/strategy strategy.Main
```

### Template Method (`03-behavioral/template-method/`)

| Post Section | File(s) |
|---|---|
| Implementation: Data Migration Pipeline — the abstract class & template method | `DataMigration.java` |
| Implementation: Data Migration Pipeline — CsvMigration | `CsvMigration.java` |
| Implementation: Data Migration Pipeline — ApiMigration | `ApiMigration.java` |
| Implementation: Data Migration Pipeline — wiring it together | `Main.java` |

Run it:
```bash
javac 03-behavioral/template-method/*.java -d out/template-method
java -cp out/template-method template.Main
```

### Visitor (`03-behavioral/visitor/`)

| Post Section | File(s) |
|---|---|
| Implementation: Shape Geometry Operations — the Visitor interface | `ShapeVisitor.java` |
| Implementation: Shape Geometry Operations — the Element interface | `Shape.java` |
| Implementation: Shape Geometry Operations — Circle | `Circle.java` |
| Implementation: Shape Geometry Operations — Rectangle | `Rectangle.java` |
| Implementation: Shape Geometry Operations — Triangle | `Triangle.java` |
| Implementation: Shape Geometry Operations — AreaCalculator | `AreaCalculator.java` |
| Implementation: Shape Geometry Operations — PerimeterCalculator | `PerimeterCalculator.java` |
| Implementation: Shape Geometry Operations — wiring it together | `Main.java` |

Run it:
```bash
javac 03-behavioral/visitor/*.java -d out/visitor
java -cp out/visitor visitor.Main
```

## Reference

- *Design Patterns: Elements of Reusable Object-Oriented Software* — Gamma, Helm, Johnson, Vlissides
- https://refactoring.guru/design-patterns