Programming in the abstract can sometimes be entertaining, but without the ability to get data into and back out of your program, they are pretty much useless.
There are a few core concepts that you need to understand well to do Input/Output in Java. The first of these is exceptions which, fortunately, we've just covered 1. With all IO things can (and do) go wrong. You also need to do know about streams and filters; in Java IO, you will spend lots of time putting on stream inside another. And you'll need to think about performance; IO can be very slow. In general, you work around this by "buffering".
Please read the IO Trail and attempt both exercises.
1. You would almost think it was planned!