Collections On Class Diagrams

Lets say you have an album class which has an ArrayList of tracks. How do you show this in a UML class diagram?

Usually you don't show the collection on the class diagram as a class. All you do is show an association between the album class and the track class. By marking the association with a * multiplicity you imply that a collection is in play.

This raises the question of which collection. To be strict with the UML you should always use a set for your collections since multivalued properties have set semantics in UML. To show an ordered collection, like a list, you need to mark the association as {ordered}. In practice most people use lists for collections (if nothing else it can help testing). That's okay too, although strictly you should be aware of introducing duplicates to the collection.

If you want to show some special collection is being used, then you can use a stereotype such as <<doubly linked list>>. However you should find you only do this very rarely.