12. Data structures. ArrayList, LinkedList (algorithm efficiency).
    Stacks, queues, trees

Garbage colleciton isn't implemented yet. (It is coming).

So some of these can't really be used yet.

But, if they do get implemented, they could be used to do some
of the same things as arrays (lab 9)

*) a good task is to actually implement a linked list. There could
   be a problem of running out of memory if used, b/c no garbage
   collection. Practice of try...catch 

*) Re-implement the music playing class as a queue

*) A follow-same-path-home as a stack.