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
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.
原文：http://www.cnblogs.com/BEETLE-SY/p/3554726.html