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

遇到这个问题的解决方法:

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"

 

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