In Lab2, when I was modifying and saving FirstActivity.java.bak while using NLED, the editor automatically saved a FirstActivity.java.bak in the same folder. When running 'ant debug' to compile, it generated error, explaining that unauthorized file was contained in the source folder. The way to solve this issue is to simply remove the .bak file or any other file extensions that is not the .java extension in the source directory. -- PeterHuang