Add all 23 GoF design pattern implementations

03-behavioral/state/GreenState.java

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+package state;
+
+public class GreenState implements TrafficLightState {
+    @Override public void onEnter(TrafficLight light) {
+        System.out.println("  [GREEN] Go. Traffic flows.");
+    }
+    @Override public void next(TrafficLight light) {
+        light.setState(new YellowState());
+    }
+    @Override public String getColor() { return "GREEN"; }
+}

03-behavioral/state/Main.java

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+package state;
+
+/**
+ * State Design Pattern — Runnable Demo
+ * Run: javac state/*.java -d out/state && java -cp out/state state.Main
+ * Article: https://ankurm.com/state-design-pattern-java/
+ */
+public class Main {
+    public static void main(String[] args) {
+        System.out.println("=== State Design Pattern Demo ===\n");
+
+        TrafficLight light = new TrafficLight();
+        System.out.println("Starting state: " + light.getColor());
+
+        System.out.println("\n-- Cycling through states --");
+        for (int i = 0; i < 6; i++) {
+            light.next();
+        }
+
+        System.out.println("\n-- Without State pattern: a single class with if/else --");
+        System.out.println("   Every new state adds to every method's if-else chain.");
+        System.out.println("   With State: add one new class, touch nothing else.");
+
+        System.out.println("\n=== Demo complete ===");
+    }
+}

03-behavioral/state/README.md

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+# State Design Pattern — Java Example
+
+**Pattern:** Behavioral → State
+**Article:** https://ankurm.com/state-design-pattern-java/
+
+## What this example shows
+
+A traffic light with zero if-else state machines. `TrafficLightState` declares what every state must do: handle entry (`onEnter`), handle the transition trigger (`next`), and report its own color. `TrafficLight` is the context — it holds a reference to the current state and delegates every call to it. `RedState`, `GreenState`, and `YellowState` each know only their own entry message and their own next state (Red→Green→Yellow→Red); none of them knows about the full cycle. `Main` creates the light, cycles through six transitions, and prints why this beats a single class full of if-else branches.
+
+## How to run
+
+```bash
+javac state/*.java -d out/state
+java -cp out/state state.Main
+```
+
+Requires Java 25.
+
+## Post Section ↔ File Mapping
+
+| 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 (the fragile `TrafficLight` with a `String state` field) is illustrative only — it is not part of this repository's runnable example.
+
+Article: https://ankurm.com/state-design-pattern-java/
+All patterns: https://ankurm.com/design-patterns-java/

03-behavioral/state/RedState.java

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+package state;
+
+public class RedState implements TrafficLightState {
+    @Override public void onEnter(TrafficLight light) {
+        System.out.println("  [RED]   Stop. Pedestrians cross.");
+    }
+    @Override public void next(TrafficLight light) {
+        light.setState(new GreenState());
+    }
+    @Override public String getColor() { return "RED"; }
+}

03-behavioral/state/TrafficLight.java

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+package state;
+
+/** Context — holds the current state and delegates behaviour to it */
+public class TrafficLight {
+    private TrafficLightState state;
+
+    public TrafficLight() {
+        setState(new RedState());
+    }
+
+    public void setState(TrafficLightState newState) {
+        this.state = newState;
+        state.onEnter(this);
+    }
+
+    public void next() {
+        state.next(this);
+    }
+
+    public String getColor() { return state.getColor(); }
+}

03-behavioral/state/TrafficLightState.java

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+package state;
+
+/** State interface — each concrete state handles events differently */
+public interface TrafficLightState {
+    void onEnter(TrafficLight light);
+    void next(TrafficLight light);
+    String getColor();
+}

03-behavioral/state/YellowState.java

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+package state;
+
+public class YellowState implements TrafficLightState {
+    @Override public void onEnter(TrafficLight light) {
+        System.out.println("  [YELLOW] Slow down. Prepare to stop.");
+    }
+    @Override public void next(TrafficLight light) {
+        light.setState(new RedState());
+    }
+    @Override public String getColor() { return "YELLOW"; }
+}