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

   1. Right click on the project and go to "Properties" //鼠标右键点击项目，然后选中Properties
   2. Select "Java Build Path" on the left //选择java Build Path
   3. Open "Source" tab // 打开源文件的tab栏
   4. Click "Add Folder..." //点击添加文件夹
   5. Check "gen" folder and click Ok and Ok again //选中gen目录 然后点确定  应该就可以了
   6. Again right click on the project and in the "Andriod Tools" click on "Fix Project Properties"  //右键点击项目 选择     Andriod Tools 点击 Fix Project Properties