State in Java

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         State design pattern - an FSM with two states and one event
(distributed transition logic - logic in the derived state classes)

Create a "wrapper" class that models the state machine
The wrapper class maintains a "current" state object
All client requests are simply delegated to the current state object and the wrapper object's "this" pointer is passed
Create a state base class that makes the concrete states interchangeable
The State base class specifies any useful "default" behavior
The State derived classes only override the messages they need to o/r
The State methods will change the "current" state in the "wrapper"

import java.io.*;

                                                   //  Event - push

///// 1. The "wrapper" class /////                 //  State   ----

class Button {                                     //   ON      OFF

   ///// 2. The "current" state object /////       //   OFF     ON

   private State current;

 

   public Button()                   { current = OFF.instance(); }

   public void setCurrent( State s ) { current = s; }

   ///// 3. The "wrapper" always delegates to the "wrappee" /////

   public void push()                { current.push( this ); }

}

 

///// 4. The "wrappee" hierarchy /////

class State {

   public void push( Button b )  {              // 5. Default behavior

      b.setCurrent( OFF.instance() );           // can go in the base

      System.out.println( "   turning OFF" );   // class

}  }

 

class ON extends State {

   private static ON inst = new ON();

   private ON() { }

 

   public static State instance() { return inst; }

}

 

class OFF extends State {

   private static OFF inst = new OFF();

   private OFF() { }

 

   public static State instance() { return inst; }

   ///// 6. Override only the necessary methods /////

   public void push( Button b ) {

      ///// 7. The "wrappee" may callback to the "wrapper" /////

      b.setCurrent( ON.instance() );

      System.out.println( "   turning ON" ); 

}  }

 

public class StateToggle {

   public static void main( String[] args ) throws IOException {

      InputStreamReader is = new InputStreamReader( System.in );

      int ch;

      Button btn = new Button();

      while (true) {

         System.out.print( "Press 'Enter'" );

         ch = is.read();    ch = is.read();

         btn.push();

}  }  }

D:\Java> java StateToggle Press 'Enter' turning ON Press 'Enter' turning OFF Press 'Enter' turning ON Press 'Enter' turning OFF

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