Design Patterns - State Pattern

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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

 package com.tutorialspoint; public interface State { public void doAction(Context context); } 

Step 2

Create concrete classes implementing the same interface.

StartState.java

 package com.tutorialspoint; 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

 package com.tutorialspoint; 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

 package com.tutorialspoint; public class Context { private State state; public Context(){ state = null; } public void setState(State state){ this.state = state; } public State getState(){ return state; } } 

Example - Usage of State Pattern

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

StatePatternDemo.java

 package com.tutorialspoint; 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()); } } interface State { public void doAction(Context context); } 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"; } } 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"; } } class Context { private State state; public Context(){ state = null; } public void setState(State state){ this.state = state; } public State getState(){ return state; } } 

Output

Verify the output.

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