State Pattern

A State Pattern says that "the class behavior changes based on its state". In State Pattern, we create objects which represent various states and a context object whose behavior varies as its state object changes.

The State Pattern is also known as Objects for States.

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Benefits:

• It keeps the state-specific behavior.
• It makes any state transitions explicit.

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Usage:

• When the behavior of object depends on its state and it must be able to change its behavior at runtime according to the new state.
• It is used when the operations have large, multipart conditional statements that depend on the state of an object.

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