Know . What an abstract class is and what you can and cannot do with it . What the extends keyword is for in Java .What the implemts keyword is for in java . What is and what is not inherited from the super class in java The difference between public, protected, and private access What inherited members are visible in the subclass . How to call a super class constructor from a subclass . When you will have a default constructor and when you will not . How to implement an 'is a' relationship between classes . How to implement a 'has a' relationship between classes The difference between composition and inheritance . What we are referring to when we talk about the public interface of a class