Java Programming

I'm creating a Java class which will execute other applications or shell scripts.
It works fine for batch processes and applications which just writes stuff on the screen.

But how can I accomplish interaction with a an application in case it requests user input? I know I can use Process.getInputStream() and Process.getOutputStream(), but how can I tell if the sub-process is requesting a user input, and then print out the output again, and so on, until the end of the process?

I don't know if I made myself clear, I only have code to execute and output the sub-process.

Thanks in advance,
Christian