导入新工程时报错：gen already exists but is not a source folder. Convert to a source folder or rename it.

解决办法：
1. 右键点击工程，选择 "Properties"
2. 选择左边的 "Java Build Path"
3. 打开 "Source" 标签面板
4. 点击 "Add Folder..."
5. 勾选 "gen" 文件夹，点击OK，点击YES，再点击OK