Though that only works if you already know the name of the file you're looking for. If you're looking for any new files in a directory, you can use listFiles() (in the File class) to see all the files in a given directory, then check the lastModified() method of each one to see if it's been recently changed (since the last time you checked that directory).
On some systems (Unix I think) you can shorcut this process by
testing the lastModified() time of the directory itself - if new files have been added to the directory, the lastModified() time of the directory will show the time at whic files were added or deleted. So if that time has not changed, you don't need to check each individual file.
However this doesn't work the same on all systems. So only rely on this if you've tested carefully on your system and you're sure the probram won't need to be run on a different system later. Hmmm, truth is, that's probably a dangerous assumption, so maybe
you should just forget this paragraph unless it turns out that you need to detect new files
really quickly and performance of the listFiles() method is a problem. Which is pretty unlikely I'd guess, but you never know.