InputStream or Reader wrapper for progress reporting

Question

So, I'm feeding file data to an API that takes a Reader, and I'd like a way to report progress.

It seems like it should be straightforward to write a FilterInputStream implementation that wraps the FileInputStream, keeps track of the number of bytes read vs. the total file size, and fires some event (or, calls some update() method) to report fractional progress.

(Alternatively, it could report absolute bytes read, and somebody else could do the math -- maybe more generally useful in the case of other streaming situations.)

I know I've seen this before and I may even have done it before, but I can't find the code and I'm lazy. Has anyone got it laying around? Or can someone suggest a better approach?

---

One year (and a bit) later...

I implemented a solution based on Adamski's answer below, and it worked, but after some months of usage I wouldn't recommend it. When you have a lot of updates, firing/handling unnecessary progress events becomes a huge cost. The basic counting mechanism is fine, but much better to have whoever cares about the progress poll for it, rather than pushing it to them.

(If you know the total size, you can try only firing an event every > 1% change or whatever, but it's not really worth the trouble. And often, you don't.)

Answer 1

The answer by Adamski works but there is a small bug. The overridden read(byte[] b) method calls the read(byte[] b, int off, int len) method trough the super class.
So updateProgress(long numBytesRead) is called twice for every read action and you end up with a numBytesRead that is twice the size of the file after the total file has been read.

Not overriding read(byte[] b) method solves the problem.