I think it is very important to remember the interfaces, abstract classes and concrete classes of the Collection utilities. You should also know which type of collection you will require depending on whether you need to store data uniquely, duplicated-doesn't-matter etc...
Therefore, you need to understand about Lists, Sets, Maps, SortedSets, SortedMaps. Their abstract classes (AbstractMap, AbstractSet etc....) and some concrete implementations (LinkedList, ArrayList, TreeMap, TreeSet....etc...)
Most of this classes share the same method names (add, remove, etc..)
Take a look at the Java Tutorial online at this URL:- http://java.sun.com/docs/books/tutorial/collections/intro/index.html