How to check if index of an array is empty ?

I don't quite understand what you mean... Why do you say checking for array[i] == null doesn't work?

Both of those conditions make sense and work perfectly fine so there must be something else wrong if you think they don't work. Or else I'm not understanding what the actual problem is.

If your array stores value types ins$$anonymous$$d of reference types, the array can't contain nulls, so you have to check against the default value of that type or establish some other rule for when an index is "empty" (like you have done with int there)