解决方法:
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
6. 最后右键点击工程，选择 "Andriod Tools" 里面的 "Fix Project Properties"
gen already exists but is not a source folder ZT,布布扣,bubuko.com
gen already exists but is not a source folder ZT
原文地址：http://www.cnblogs.com/weiryang/p/3793043.html
