Designing a Class Hierarchy

A class hierarchy refers to a set of classes that are related by inheritence.
A relationship that can be describe as X has a Y commonly indicates that X is a class and Y is a data member or member function of the class.
A relationship that can be describe as A is a B commonly indicates that A and B are both classes, and B is a super class of A.
In this case, A is considered a child class and B is considered the parent class.
A good way to think if it is that the child class is a specialized version of the parent class.
When designing a class hierarchy, you should push the more general characterics farther up the hierarchy.
This includes both noun characteristics (data members) and verb characterstics (member functions).
If a class farther down in the hierarchy needs different behavior, you can override the member functions, and either completely replace or augment the corresponding member function in the superclass.
In Java, the syntax for creating a child class is to use the keyword extends public class LabelledLine extends Line
Any public member functions that are not overidden in the derived class will be inherited from the super class.
If you have a method whose implemented is not necessarily needed in the parent, but you want it for using polymorphism, you can make it an abstract method.