Add all 23 GoF design pattern implementations (2026-06-13)

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/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"; }
+}