[Mr. Lim]

Instead of:

Sweeper sweep;

It's best to have:

Sweeper sweep = Sweeper.getInstance();

Looking through your code, you don't have any getInstance() methods in Sweeper, which means you may not everything you need to have a "singleton class."

A singleton is a class that ensure that only one instance of it is ever created. The getInstance() method should check to see if a Sweeper has already been created. If it has, return that instance, if not, create a new one.

There are examples of how to make a getInstance() method to ensure your subsystems classes are all singletons. These examples can be found in the WPILib Cookbook.