State Pattern

Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.

Participants

◉ Context: defines the interface of interest to clients Maintains an instance of a ConcreteState subclass that defines the current state.

◉ State: defines an interface for encapsulating the behavior associated with a particular state of the Context.

◉ Concrete State: each subclass implements a behavior associated with a state of Context

Code

public class Main {

    public static void main(String[] args) {

        Context c = new Context(new ConcreteStateA());

        c.request();

        c.request();

        c.request();

        c.request();

    }

}

public interface State {

    void handle(Context context);

}

public class ConcreteStateA implements State {

    @Override

    public void handle(Context context) {

        context.setState(new ConcreteStateB());

    }

}

public class ConcreteStateB implements State {

    @Override

    public void handle(Context context) {

        context.setState(new ConcreteStateA());

    }

}

public class Context {

    private State state;

    public Context(State state) {

        setState(state);

    }

    public State getState() {

        return state;

    }

    public void setState(State state) {

        this.state = state;

        System.out.println("State: " + state.getClass().getSimpleName());

    }

    public void request() {

        state.handle(this);

    }

}

Output

State: ConcreteStateA

State: ConcreteStateB

State: ConcreteStateA

State: ConcreteStateB

State: ConcreteStateA