A simple state machine implementation in Java using just an enum.

SimpleStateMachine.java

import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public enum State {
	
    Ready(Event.Deposit, Event.Quit),
    Waiting(Event.Select, Event.Refund),
    Dispensing(Event.Remove),
    Refunding(Event.Refunding),
    Exiting(Event.Quiting);

    State(Event... event) {
        events = Arrays.asList(event);
    }

    State next(Event event, State current) {
    	return events.contains(event) ? map.getOrDefault(event, current) : current;
    }

    final List<Event> events;
    final static Map<Event, State> map = new HashMap<>();

    static {
        map.put(Event.Deposit, State.Waiting);
        map.put(Event.Quit, State.Exiting);
        map.put(Event.Select, State.Dispensing);
        map.put(Event.Refund, State.Refunding);
        map.put(Event.Remove, State.Ready);
        map.put(Event.Refunding, State.Ready);
    }
    
    public static enum Event {
        Deposit, Quit, Select, Refund, Remove, Refunding, Quiting
    }
}