Moved to "Processing with Other Languages" section because it concerns Processing.js.

The main problem is that you are trying to use java classes/methods. However, Processing.js has only ported the Processing commands, not the whole of java. See the reference and then select "Javascript (Processing.js)" to see which methods are available.

I'm not sure why you would want to point to a file on your computer for js code that runs inside a browser. My guess is that you will need a different kind of solution in a web-based javascript context. I don't have experience with that, so can't help you futher, but maybe someone else knows...

Indeed, JavaScript cannot see the files on the user's system. Otherwise, that would be a security breach.
Beside, your path wouldn't work on my computer, for example, so, as said, such operation isn't useful in a Web context.

Hey, sorry if I wasn't clear. I'm not expecting it to interface with the user's computer, it is meant to read log files stashed on the server and display data after some analysis on it. I guess I didn't realize it is actually trying to load from the client. Is there some way to do what I was expecting?

Yes, but you need a script on the server that would read the files and send the list when the sketch applet asks them (GET or POST request, or Ajax).