gen already exists but is not a source folder. Convert to a source folder or rename it 的解決辦法

   1. Right click on the project and go to "Properties" //鼠標右鍵點擊項目，然後選中Properties
   2. Select "Java Build Path" on the left //選擇java Build Path
   3. Open "Source" tab // 打開源文件的tab欄
   4. Click "Add Folder..." //點擊添加文件夾
   5. Check "gen" folder and click Ok and Ok again //選中gen目錄 然後點確定  應該就可以了
   6. Again right click on the project and in the "Andriod Tools" click on "Fix Project Properties"  //右鍵點擊項目 選擇     Andriod Tools 點擊 Fix Project Properties