gen already exists but is not a source folder. Convert to a source folder or rename it.

Android 代码中无意删除了 .setting文件夹，重建后提示

gen already exists but is not a source folder. Convert to a source folder or rename it.

查了半天，总算有个明白人

1. Right click on the project and go to "Properties"
2. Select "Java Build Path" on the left
3. Open "Source" tab
4. Click "Add Folder..."
5. Check "gen" folder and click Ok and Ok again
6. Again right click on the project and in the "Andriod Tools" click on "Fix Project Properties"