Design Patterns - State Pattern

In State pattern a class behavior changes based on its state. This type of design pattern comes under behavior pattern.
In State pattern, we create objects which represent various states and a context object whose behavior varies as its state object changes.

Implementation

We are going to create a State interface defining an action and concrete state classes implementing the State interface. Context is a class which carries a State.
StatePatternDemo, our demo class, will use Context and state objects to demonstrate change in Context behavior based on type of state it is in.

Step 1

Create an interface.
State.java

public interface State {
   public void doAction(Context context);
}

Step 2

Create concrete classes implementing the same interface.
StartState.java

public class StartState implements State {

   public void doAction(Context context) {
      System.out.println("Player is in start state");
      context.setState(this); 
   }

   public String toString(){
      return "Start State";
   }
}

StopState.java

public class StopState implements State {

   public void doAction(Context context) {
      System.out.println("Player is in stop state");
      context.setState(this); 
   }

   public String toString(){
      return "Stop State";
   }
}

Step 3

Create Context Class.
Context.java

public class Context {
   private State state;

   public Context(){
      state = null;
   }

   public void setState(State state){
      this.state = state;  
   }

   public State getState(){
      return state;
   }
}

Step 4

Use the Context to see change in behaviour when State changes.
StatePatternDemo.java

public class StatePatternDemo {
   public static void main(String[] args) {
      Context context = new Context();

      StartState startState = new StartState();
      startState.doAction(context);

      System.out.println(context.getState().toString());

      StopState stopState = new StopState();
      stopState.doAction(context);

      System.out.println(context.getState().toString());
   }
}

Step 5

Verify the output.

Player is in start state
Start State
Player is in stop state
Stop State