Gerardo,

To execute an external application you should check the java.lang.Runtime and java.lang.Process.

What I do not understand it what do you mean for "given a certain file type". If you mean something like the basic concept 'specific extension => specific software", I don't know how that it's handled. I'm not sure that the operating system will do that for you, but, I think, that you can maintain your own 'association list'.