enum state machine

State.java

interface IState {
    public void GuiBookOn() throws InvalidStateException;
    public void GuiBookOff() throws InvalidStateException;
    public void NetBookedOn() throws InvalidStateException;
    public void NetBookedOff() throws InvalidStateException;
}

public enum State implements IState {
    BOOKED_OFF {
        @Override
        public void GuiBookOn() throws InvalidStateException {
            // send book on message
            changeState(BOOKING_ON);
        }
    }, BOOKING_ON {
        @Override
        public void NetBookedOn() throws InvalidStateException {
            changeState(DESKTOP);
        }
    }, DESKTOP {
        @Override
        public void GuiBookOn() throws InvalidStateException {
            // send book off message
            changeState(BOOKING_OFF);
        }
    }, BOOKING_OFF {
        @Override
        public void NetBookedOn() throws  InvalidStateException{
            changeState(BOOKED_OFF);
        }
    };

    public static State state = BOOKED_OFF;

    private static void changeState(State newState) {
        // inform listeners, maybe emit intents?
        state = newState;
    }
    
    public void GuiBookOn() throws InvalidStateException { throw new InvalidStateException(); };
    public void GuiBookOff() throws InvalidStateException { throw new InvalidStateException(); };
    public void NetBookedOn() throws InvalidStateException { throw new InvalidStateException(); };
    public void NetBookedOff() throws InvalidStateException { throw new InvalidStateException(); };
}