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.
Do you mean you want to replace the .class file? If your application is already deployed and you want to replace the .class file, you can do that but the changes will not be reflected unless you redeploy the application.