simulating/automating the user of interactive console program

assume we have an interactive console program - you know, not a unixy filter but something that asks the user questions and waits for answers. this program doesn't provide any sort of API, just the interactive interface. what would be the best (and preferably most portable) way of writing a program that communicates with it in a bidirectional way (so it can't be simply piped to its stdin, as we need to feed its stdout back) without modifying the target program?

in jabba: make a Process with ProcessBuilder, then use getOutputStream() to get an object that pipes data into it and getInputStream() for an object that reads returned data

man expect