I'm afraid I haven't a clue with the filesystem structure in the android apps. I somehow doubt that they would change it so that it would be horrendously difficult though, so I would presume that its either a linux structure, or even better you may have an environment path for it to write to.
Java in particular I find is very easy to read/write files from. Here's a quick code block I found for using the ObjectOutputStream. Depending on what this data is depends on how you want to write it; object output stream relies on serializable data, so it works great for reading and writing complete objects or even collections of objects that are serializable to file in their entire representation. The con of course its its a lot larger than just a flat file of text: http://www.javadb.com/writing-object...ctoutputstream
Neither flat file or object stored data will hinder what you want to do. Choose whichever for simplicity (I'd go Object route since then I don't have to ATOI to type again).