How to create a simple state machine in java

Question

I am currently learning java and would like to know how to control state in a OO way. I implemented a Pong app. If I wanted multiple states like gameplay and menu, and each one of these states had to execute start, stop and run how would I achieve this and how would I switch between these states.

I know I could simply throw in a big switch statement but what's the best way to implement this?

I want to be able to switch to the menu state in the gameplay state and vice versa.

import java.applet.*;
import java.awt.*;
import java.awt.event.*;

public class Pong extends Applet implements Runnable, KeyListener{

    public void start ()
    {
        setSize(screen);
        setFocusable(true);
        Thread th = new Thread (this);
        th.start ();
    }

    public void stop()
    {
    }
    //Etc..
}

Martin Andersson · Accepted Answer · 2013-12-10 09:58:18Z

You can simulate a basic FSM (Finite State Machine) using enums:

public enum State {

    ONE {
        @Override
        public Set<State> possibleFollowUps() {
            return EnumSet.of(TWO, THREE);
        }
    },

    TWO {
        @Override
        public Set<State> possibleFollowUps() {
            return EnumSet.of(THREE);
        }
    },

    THREE // final state 

    ;
    public Set<State> possibleFollowUps() {
        return EnumSet.noneOf(State.class);
    }

}

While the code to generate this will be very verbose if things get more complicated, the nice part is that you get compile-time safety, thread-safety and high performance.

NOTE: when overriding methods in specific enums, their class is not the same. (in this case THREE.getClass() is State, and TWO.getClass() could be State$2) — Asaf, Nov 23 '11 at 10:52
This code enlightens your tough process especially in java. Thanks. — Trying, Jul 3 '13 at 14:20
@Sean: Noting Asaf's comment, I would like to know, if you would see any disadvantage passing the possible follow up states in a constructor to the different states instead of the overwritten methods? (likeONE(EnumSet.of(TWO, THREE)), TWO(EnumSet.of(THREE))... — Tom Fink, Jul 17 '13 at 12:34
Ok, figured it just out myself. Values of the enum obviously are not allowed inside constructors of the enum itself. :p — Tom Fink, Jul 17 '13 at 16:21
@TomFink exactly. otherwise your version would of course be preferable — Sean Patrick Floyd, Jul 17 '13 at 19:15

jk. · Answer 2 · 2013-03-22 16:52:26Z

up vote2down vote

There is a design pattern for this situation. State Pattern and here is simple implementation in java.

edited Mar 22 '13 at 16:52

jk.
1,715815

answered Apr 26 '11 at 12:32

Mojo Risin
5,12452141

So in my Pong app, how would I switch between the menu and gameplay state? – Sam152 Apr 26 '11 at 12:42

the implementation mentioned above is not for the state pattern. Downvoting. – javadeveloper Dec 20 '13 at 16:13

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Neigyl R. Noval · Answer 3 · 2011-04-26 12:31:35Z

These links may help you:

Designing high-performance State Machine in Java.

http://www.techrepublic.com/article/use-this-java-framework-for-finite-state-machines/1049528

http://javathings.blogspot.com/2006/01/state-machine.html

http://unimod.sourceforge.net/fsm-framework.html

I could have searched for these, none of them really help me. I would just like a little code sample of how to achieve something really basic. — Sam152, Apr 26 '11 at 12:40

Nargis · Answer 4 · 2012-11-05 08:37:43Z

Try this link http://doc.akka.io/docs/akka/2.0.3/java/fsm.html

this could help

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How to create a simple state machine in java

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