Add all 23 GoF design pattern implementations

03-behavioral/state/TrafficLight.java

@@ -0,0 +1,21 @@
+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(); }
+}