Data Structures and Algorithms
with Object-Oriented Design Patterns in Java 
Program 
defines an abstract class
called AbstractVisitor that implements the Visitor interface.
This class is provided simply as a convenience.
It provides a default implementation for the visit method
which does nothing and
a default implementation for the isDone method
which always returns false.
Program: AbstractVisitor class.
Copyright © 1998 by Bruno R. Preiss, P.Eng. All rights reserved.