added state pattern example

src/learn/simple_state/Context.java

@@ -0,0 +1,34 @@
+package learn.simple_state;
+
+public class Context {
+    private State onState;
+    private State offState;
+
+    private State currentState;
+
+    public Context() {
+        onState = new OnState(this);
+        offState = new OffState(this);
+        currentState = offState;
+    }
+
+    public void setCurrentState(State currentState) {
+        this.currentState = currentState;
+    }
+
+    public State getOffState() {
+        return offState;
+    }
+
+    public State getOnState() {
+        return onState;
+    }
+
+    public void on() {
+        currentState.on();
+    }
+
+    public void off() {
+        currentState.off();
+    }
+}

src/learn/simple_state/Main.java

@@ -0,0 +1,12 @@
+package learn.simple_state;
+
+public class Main {
+    public static void main(String[] args) {
+        Context context = new Context();
+        context.off();
+        context.off();
+        context.on();
+        context.on();
+        context.off();
+    }
+}

src/learn/simple_state/OffState.java

@@ -0,0 +1,20 @@
+package learn.simple_state;
+
+public class OffState implements State {
+    private Context context;
+
+    public OffState(Context context) {
+        this.context = context;
+    }
+
+    @Override
+    public void on() {
+        context.setCurrentState(context.getOnState());
+        System.out.println("Transition to on");
+    }
+
+    @Override
+    public void off() {
+        System.out.println("Already off");
+    }
+}

src/learn/simple_state/OnState.java

@@ -0,0 +1,20 @@
+package learn.simple_state;
+
+public class OnState implements State {
+    private Context context;
+
+    public OnState(Context context) {
+        this.context = context;
+    }
+
+    @Override
+    public void on() {
+        System.out.println("Already on");
+    }
+
+    @Override
+    public void off() {
+        context.setCurrentState(context.getOffState());
+        System.out.println("Transition to off");
+    }
+}

src/learn/simple_state/State.java

@@ -0,0 +1,6 @@
+package learn.simple_state;
+
+public interface State {
+    void on();
+    void off();
+}