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I think the first step would be to get a clear definition of what is to be done.
Computers have many options in terms of output and those outputs can be interpreted in various ways.
Perhaps you...

I'm not saying an audio signal can't make a motor turn, but there is great risk of damaging the computer.
The audio circuitry was not designed to run a motor. The output is very limited.
What are...

Please explain more about:
"use head phone port to connect to dc motor."
I am not sure I fully understand your intentions yet.

Yes you can use java to control things like small dc motors. Java can be used to control just about anything around you.

I think "how to do it" exceeds the scope of a forum. I think the first...