【转】gen already exists but is not a source folder.

【ERROR】gen already exists but is not a source folder. Convert to a source folder or rename it
This is a common error and you can solve it following these steps:

Right click on the project and go to “Properties”
Select “Java Build Path” on the left
Open “Source” tab
Click “Add Folder…”
Check “gen” folder and click Ok and Ok again
Again right click on the project and in the “Andriod Tools” click on “Fix Project Properties”