Forward slash or backslash?

Using forward slashes will make it system independent. I'd stick to that for simplicity.

Consider using java.io.File.separator if you ever display the path to the user. You'd rather not surprise those Windows users. They're a jumpy lot.

I've never found it documented anywhere, but the JDK classes let you use slashes regardless of whether you're on Windows or not. (You can see this in the JDK source, where it explicitly converts path separators for you.)

Officially — and certainly in any UI you're doing — you should use the file.separator system property, which is available via System.getProperty(the list of standard system properties is documented in the docs for System.getProperties):

String sep = System.getProperty("file.separator");

...and also via the static fields They're also available as File.separator (and File.separatorChar).

You can also use the various features of the java.io.File class for combining and splitting paths, and/or the various features of the interfaces and classes in java.nio.file.