There are underscores in front of all your variables. So there is no x, only a _x.

If you want to use underscores you'll probably want to use 'em like so...

1. class Ball {
2.   int x;
3.   Ball(int _x) {
4.    x = _x;
5.   }
6. }

And personally I prefer to use identical names and just use this. to get 'em where they need to go.

1. class Ball {
2.   int x;
3.   Ball(int x) {
4.    this.x = x;
5.   }
6. }