State Design Pattern

State Design Pattern

State design pattern is used when an Object change its behaviour based on its internal state.

State design pattern provides a mechanism to change behavior of an object based on the object’s state.

Suppose you press the remote on/off button following cases are possible:

• If TV is off it will turn ON
•  If TV is ON it will turn OFF

public class TVBasic {

      private String currentState="OFF";

      public String getCurrentState() {

            return currentState;

      }

      public void setCurrentState(String currentState) {

            this.currentState = currentState;

      }

      public void pressPowerButton(){

            if(currentState.equalsIgnoreCase("ON")){

                  System.out.println("TV  turned OFF");

                  currentState="OFF";

            }else if(currentState.equalsIgnoreCase("OFF")){

                  System.out.println("TV turned ON");

                  currentState="ON";

            }

      }

      public static void main(String args[]){

            TVBasic tvBasic = new TVBasic();

            tvBasic.pressPowerButton();

            tvBasic.pressPowerButton();

            }

}

Meaning that when same function is executed TV will behave differently based on the current state of TV.

Let this function be pressing the power button on TV object

      public static void main(String[] args) {

            TV tv = new TV();

            tv.pressPowerButton();

TV will behave differently based on the present state of TV

The different states may be:

• TV OFF  à TvOffState.Java
• TV ON   à TvOnState.Java

ie in these two states when the power button is pressed will behave in differently

We create a TV class having pressPowerButton() function.

We create two state objects also having pressPowerButton() function so that they can react differently.

This TV class will delegate the call to its current state Object.

public interface State {

      public void pressPowerButton();

}

public class TvOnState implements State {

      TV tv;

      public TvOnState(TV tv) {

            this.tv = tv;

      }

      @Override

      /*

       * In TvOnState if you press power button the TV will turned OFF

       * and its current state will be TvOffState

       *

       */

      public void pressPowerButton() {

            System.out.println("TV is turned OFF");

            tv.setCurrentState(tv.getTvOffState());

      }

}

public class TvOffState implements State{

      TV tv;

     

      public TvOffState(TV tv) {

            this.tv = tv;

      }

     

      @Override

      /*

       * In TvOffState if you press power button the TV will turned ON

       * and its current state will be set to  TvOnState

       */

      public void pressPowerButton() {

             System.out.println("TV is turned ON");

           tv.setCurrentState(tv.getTvOnState());

      }

}

public class TV {

      State tvOnState;

      State tvOffState;

      State currentState ;

     

      public TV() {

            tvOffState = new TvOffState(this);

            tvOnState = new TvOnState(this);

            //assuming the TV is off

            currentState = tvOffState;

      }

      /*

       * When somebody presses the power button the TV simply

       * deligates the request to its current state

       */

      public void pressPowerButton() {

            currentState.pressPowerButton();

      }

     

     

      public State getTvOnState() {

            return tvOnState;

      }

      public State getTvOffState() {

            return tvOffState;

      }

      public void setCurrentState(State state) {

            this.currentState = state;

      }

}

public class TestTV {

      public static void main(String[] args) {

            TV tv = new TV();

            tv.pressPowerButton();

            tv.pressPowerButton();

           

      }

}

Suppose you are on ground floor and you press the lift button following cases are possible:

• Lift will open immediately
• The LED will show down arrow and then lift will open after some time
• The LED will not down arrow and shows some number and after some time LED will show down arrow and then lift will open

Meaning that when same function is executed lift will behave differently based on the present state of lift

The different states may be:

• Lift Idle
• Lift moving up
• Lift moving down
• Lift waiting to move up
• Lift waiting to move down

ie, Lift will open immediately if lift is idle and standing on same floor and

The LED will show down arrow and then lift will open after some time if lift is idle and standing on different floor

So the idea is to make the different state Objects and have a same set methods in it but these methods will behave differently.

Also there will be one central class which will create the objects of all the states and invoke the method of the state. Also there will be one initial/current state of  central class . Also state object have a control over changing the state of current state of  central class.