关于android中gen目录的一个小问题

Eclipse+ADT for Android Little problem "Project 'xxx' is missing required source folder: 'gen'

I have a problem with Eclipse+ADT to develop Android App there Eclipse display a error like:
Project 'xxxx' is missing required source folder: 'gen' xxx Build path Build Path Problem

I have fixed it very simple:
Right click project 'xxx' ->Properties ->Java Build Path ->Tab 'Order and Export'->Select 'xxx/gen' ->Click 'Up'
then 'OK', the problem is OK now.