In mock tests i observed that, questions from collections are about the functionality of each class and interfaces.
like 1) To identify the collection which is thread safe. 2) To identify the classes/interfaces which implements/extends collection interface. like that.
In JavaRanch forum you can search with keywords related to collections. Once if u read all the questions posted by the ranchers (most of the questions are from different mock tests) you will get an idea. [ December 23, 2005: Message edited by: Naresh Kumar ]