classes:

1. Constructors
2. How to override toString() and equals()
    methods, and what they do by default.
3. The difference between primitive types
   and reference types. 
      - where are the data stored?
      - how to assignment work?
      - the problem of aliasing.
4. the difference between overloading and
   overriding. 
 

How to use Sets and Maps
 what are the basic methods? what do they do?