It is difficult to tell, without teh code that actually calls it, but I suspect that you are creating a new instance of the class in the Event handler:
MyClass mine = new MyClass();
Fitness thisOne = mine.getFitnessClass(1);
Normally, you would access such via the existing class index - but if you can't get access to it, then the method and teh list would have to be declared as
static
In that case you can access the one and only instance of the list via the class name, rather than an instance:
Fitness thisOne = MyClass.getFitnessClass(1);
Even then, it will depend on when you do things: if this is web related, then data is not persisted between pages, or within a page between the original load and a button press or other user interaction unless you explicitly save teh data in the Session or a cookie.