Add all 23 GoF design pattern implementations

01-creational/abstract-factory/Application.java

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+package abstractfactory;
+
+public class Application {
+
+    // The client holds references to the Abstract Factory and Abstract Products.
+    // It never imports LightButton, DarkButton, or any concrete class.
+    private final UIFactory factory;
+    private Button submitButton;
+    private TextField usernameField;
+
+    public Application(UIFactory factory) {
+        this.factory = factory;  // theme is injected; client doesn't decide
+    }
+
+    public void buildUI() {
+        // Ask the factory — get consistent products from the same family
+        submitButton = factory.createButton();
+        usernameField = factory.createTextField();
+    }
+
+    public void render() {
+        System.out.println("Rendering login screen:");
+        usernameField.render();
+        submitButton.render();
+    }
+
+    public void simulateSubmit() {
+        submitButton.onClick();
+    }
+}

01-creational/abstract-factory/Button.java

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+package abstractfactory;
+
+// Every button, regardless of theme, must be able to render and handle clicks
+public interface Button {
+    void render();   // draw on screen
+    void onClick();  // respond to a click event
+}

01-creational/abstract-factory/DarkButton.java

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+package abstractfactory;
+public class DarkButton implements Button {
+    @Override
+    public void render() {
+        System.out.println("[Dark Button] Charcoal background, white text, no border");
+    }
+    @Override
+    public void onClick() {
+        System.out.println("[Dark Button] Bright highlight ripple effect");
+    }
+}

01-creational/abstract-factory/DarkTextField.java

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+package abstractfactory;
+public class DarkTextField implements TextField {
+    private String value = "";
+    @Override
+    public void render() {
+        System.out.println("[Dark TextField] Dark gray background, light placeholder text");
+    }
+    @Override
+    public String getValue() { return value; }
+}

01-creational/abstract-factory/DarkThemeFactory.java

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+package abstractfactory;
+
+public class DarkThemeFactory implements UIFactory {
+
+    @Override
+    public Button createButton() {
+        return new DarkButton();        // Only Dark products come from this factory
+    }
+
+    @Override
+    public TextField createTextField() {
+        return new DarkTextField();
+    }
+}

01-creational/abstract-factory/LightButton.java

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+package abstractfactory;
+public class LightButton implements Button {
+    @Override
+    public void render() {
+        System.out.println("[Light Button] White background, dark text, 1px gray border");
+    }
+    @Override
+    public void onClick() {
+        System.out.println("[Light Button] Subtle gray ripple effect");
+    }
+}

01-creational/abstract-factory/LightTextField.java

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+package abstractfactory;
+public class LightTextField implements TextField {
+    private String value = "";
+    @Override
+    public void render() {
+        System.out.println("[Light TextField] White background, dark placeholder text");
+    }
+    @Override
+    public String getValue() { return value; }
+}

01-creational/abstract-factory/LightThemeFactory.java

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+package abstractfactory;
+
+public class LightThemeFactory implements UIFactory {
+
+    @Override
+    public Button createButton() {
+        return new LightButton();       // Only Light products come from this factory
+    }
+
+    @Override
+    public TextField createTextField() {
+        return new LightTextField();
+    }
+}

01-creational/abstract-factory/Main.java

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+package abstractfactory;
+
+// Entry point: the ONLY place that knows the theme
+public class Main {
+    public static void main(String[] args) {
+
+        // Production: read theme from config or OS preference
+        String theme = System.getProperty("app.theme", "light");
+        UIFactory factory = "dark".equals(theme) ? new DarkThemeFactory() : new LightThemeFactory();
+
+        Application app = new Application(factory);
+        app.buildUI();
+        app.render();
+        app.simulateSubmit();
+    }
+}

01-creational/abstract-factory/README.md

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+# Abstract Factory Design Pattern — Java Example
+
+**Pattern:** Creational → Abstract Factory
+**Article:** https://ankurm.com/abstract-factory-design-pattern-java/
+
+## What this example shows
+
+A UI toolkit that produces families of related widgets — light theme or dark theme — without the client ever naming a concrete class. `Button` and `TextField` are abstract products; `LightButton`/`LightTextField` and `DarkButton`/`DarkTextField` are the two concrete product families. `UIFactory` declares the creation methods for the whole family; `LightThemeFactory` and `DarkThemeFactory` each produce one consistent family. `Application` is the client — it only depends on `UIFactory` and the abstract products, so swapping the entire theme is a one-line change. `Main` selects the factory via a system property.
+
+## How to run
+
+```bash
+javac 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
+```
+
+Requires Java 25.
+
+## Post Section ↔ File Mapping
+
+| 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` |
+
+Article: https://ankurm.com/abstract-factory-design-pattern-java/
+All patterns: https://ankurm.com/design-patterns-java/

01-creational/abstract-factory/TextField.java

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+package abstractfactory;
+
+// Every text field must be able to render and return its value
+public interface TextField {
+    void render();
+    String getValue();
+}

01-creational/abstract-factory/UIFactory.java

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+package abstractfactory;
+
+public interface UIFactory {
+
+    // Factory method for buttons — returns the abstract type, not LightButton or DarkButton
+    Button createButton();
+
+    // Factory method for text fields — same principle
+    TextField createTextField();
+
+    // Adding a new product type (e.g., Checkbox) means adding a method here
+    // AND implementing it in every Concrete Factory. This is the one cost of the pattern.
+}