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.
It's not possible in the general case, but for certain limited cases you can do it. Get the current ClassLoader, and if it's a URLClassLoader, then get its URLs, turn them into URIs, turn those into Files (if they are file URIs), and then list the files in the File object corresponding to the directory representing your package.
The question though is why? What are you trying to accomplish that you think you need to do this?