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.
a TextField is the normal way to handle user input, but you need to include a fair amount of error handling. If the range of 'number of triangles' is limited, it might be easier for you to use a java.awt.Choice, where the user can select the number from a range you have set.