Assuming you can be happy with "reasonably quick" notification, say once a minute, its easy.
Better approach is to have a daemon
thread sleep, look for file, and if its there, immediately rename it to another directory.
Then release semaphore or other sync technique, to have the "worker" thread do the processing while the "checking" thread takes a nap and does the next lookup.
Its only 30 or so lines of code.