This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
I'm not sure what you mean by a "textarea". However, choosing one of the possibilities, the JTextComponent class has a "read" method which accepts a Reader; you could pass it a FileReader which refers to your C file.