[Mr. Lim]

Patrick's suggestion above is a good start.

Also it's possible you are trying to create more than one instance of your Sweeper.

Check your code to ensure you are NEVER trying to create an instance of the Sweeper OUTSIDE of your getInstance() method.

If you have:

Sweeper sweep = new Sweeper();

anywhere else in your code, you need to replace it with:

Sweeper sweep = Sweeper.getInstance();

If you try and create multiple Sweepers by running new Sweeper() more than once, you will get the error error you described. The getInstance() method should prevent that from ever happening.