This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma 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.