This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
Yes. Collection can contains Collection, of course we are talking about classes that implement Collection interface. As you will notice the methods in Collection interface like add, or contain take Object as parameter. So you can pretty much put anything except primitive data type in it.