Allows classes to be built-upon rather than re-invented.
The derived class can use all fields and methods of the base class, and more.
Another viewpoint:
Base class is general,
derived class is specialized.
In a certain sense, object of derived class is also in base class (as well as derived one);
an object can be in >1 class.
Also, derived class can redefine method in base class
Example seen in testGraphics1.java
