This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma 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.