Data Structures and Algorithms
with Object-Oriented Design Patterns in Java 
Program 
introduces the AbstractTree class.
The AbstractTree class extends the AbstractContainer class
introduced in Program
and it implements the Tree interface
defined in Program .
The AbstractTree class provides default implementations
for both the depthFirstTraversal
and breadthFirstTraversal methods.
Both of these implementations call abstract methods
such as getKey and getSubtree.
In effect, they are abstract algorithms .
An abstract algorithm describes behavior in the absence of implementation!
Copyright © 1998 by Bruno R. Preiss, P.Eng. All rights reserved.