SourceMaking

// 1. Create a "wrapper" class that models the state machine

class FSM {

   private State[] states     = { new A(), new B(), new C() }; // 2. states

   private int[][] transition = { {2,1,0}, {0,2,1}, {1,2,2} }; // 4. transitions

   private int     current    = 0;                             // 3. current

   private void next( int msg ) { current = transition[current][msg]; }

 

   // 5. All client requests are simply delegated to the current state object

   public void on()  { states[current].on();   next( 0 ); }

   public void off() { states[current].off();  next( 1 ); }

   public void ack() { states[current].ack();  next( 2 ); }

}

 

// 6. Create a state base class that makes the concrete states interchangeable

abstract class State {

   public void on()  { System.out.println( "error" ); }  // 7. The State base 

   public void off() { System.out.println( "error" ); }  //    class specifies 

   public void ack() { System.out.println( "error" ); }  //    default behavior

}

 

class A extends State {

   public void on()  { System.out.println( "A + on  = C" ); }

   public void off() { System.out.println( "A + off = B" ); }

   public void ack() { System.out.println( "A + ack = A" ); }

}

 

class B extends State {

   public void on()  { System.out.println( "B + on  = A" ); }

   public void off() { System.out.println( "B + off = C" ); }

}

 

class C extends State {

   // 8. The State derived classes only override the messages they need to

   public void on()  { System.out.println( "C + on  = B" ); }

}

 

public class StateDemo {

   public static void main( String[] args ) {

      FSM   fsm  = new FSM();

      int[] msgs = { 2, 1, 2, 1, 0, 2, 0, 0 };

      for (int i=0; i < msgs.length; i++)

         if      (msgs[i] == 0) fsm.on();

         else if (msgs[i] == 1) fsm.off();

         else if (msgs[i] == 2) fsm.ack();

}  }