Question

I have been having a hard time understanding how to use a singleton to share a common variable. I am trying to make a blackberry app which has two entry points which need to share a common variable, iconCount. I have been advised to use a singleton with the RunTimeStore API by someone on a forum. Googling around eventually leads to:

http://docs.blackberry.com/en/developers/deliverables/17952/CS_creating_a_singleton_by_using_rutnime_store_1554335_11.jsp

I have been a few pages deep in Google but I still can`t understand what this does and how to implement it. My current understanding is that a singleton will create a “global variable” somehow through the code:

class MySingleton {
   private static MySingleton _instance;
   private static final long GUID = 0xab4dd61c5d004c18L;

   // constructor
   MySingleton() {}

   public static MySingleton getInstance() {
      if (_instance == null) {
         _instance = (MySingleton)RuntimeStore.getRuntimeStore().get(GUID);
      if (_instance == null) {
         MySingleton singleton = new MySingleton();

         RuntimeStore.getRuntimeStore().put(GUID, singleton);
         _instance = singleton;
         }
      }
      return _instance;
   }
} 

And another question would be how would I create a variable from this singleton? I need to declare variable iconCount = 0 at the beginning and then be able to use it. Would declaring it be something like

Integer iconCount = (Integer) RuntimeStore.getInstance(); 

? This is very new to me as I have just started Java so if anyone could explain this keeping in mind you’re communicating with a novice I would be very grateful. Thanks in advance!

Answer 1

They mean please make sure that user initializing MySingleton class just onetime so you will not have problem with multiple instances and initialize two count in the same time. I mean from multiple instance something like below:

Mysingleton single = new Mysingleton();
Mysingleton single2 = new Mysingleton();

Because both initilaization can have diffetent count. You need something like this:

public class IconManager {
    private static iconManager _instance;
    private static final long GUID = 0xab4dd61c5d004c18L;
    private static int count = 0;

    // constructor
    IconManager() {
    }

    public static IconManager getInstance() {
        if (_instance == null) {
            _instance = (IconManager) RuntimeStore.getRuntimeStore().get(GUID);
            if (_instance == null) {
                IconManager singleton = new IconManager();

                RuntimeStore.getRuntimeStore().put(GUID, singleton);
                _instance = singleton;
            }
        }
        return _instance;
    }

    public static int getCount() {
        return count;
    }

    public static void setCount(int count) {
        this.count = count;
    }

}

and after you can create an instance for the class:

public static void main(String[] args){

    IconManager iconManager = IconManager.getInstance();
    iconManager.setCount(iconmanager.getCount() + 1);

}

So application will do first validation, if there is already an instance it will update existing one, if not than it will create new one.