Summary of Class Presentation for June 23, 1999
	Topics: Upcasting and Polymorphism

	Upcasting and polymorphism are the two foundations of object
orientation.  They are subtle and often work together, but they are very,
very important.  You should make certain you understand both and keep 
this understanding in your working memory.
	Upcasting means an object of a derived class can serve as if it
were an object of it's base class.  In particular, an object of the 
derived class can be assigned to an object variable of the base class, and
an object of the derived class can be passed as a parameter or returned 
from a method when an object of the base class is specified.  Finally, 
an item of a derived class can be placed in a container ( the only 
container we have covered thus far is the array.  We will cover many other
containers (called collections) on July 1st. ) that specifies objects
of the base class.  Since all classes in Java automatically inherit 
directly ( if they have no explicit base class) or indirectly ( if they
have an explicit base class) from the Object class, a Java container or
parameter or return value or object variable can be specified 
as of the Object 
class and an object of any class can be used.

	Polymorphism comes in many forms.  The form we care about here is
called object-oriented polymorphism.  Object-oriented polymorphism means
when an object of a derived class is serving as an object of a base class,
calling a method of the base class through that object automatically 
calls the corresponding method of the derived class to which that object
actually belongs.  This is often called late binding of methods.  Unlike
in C++, the method in the derived class must have the same name and the
same parameter types in the same order as the method in the base class
for this overriding to occur.  If the method in the derived class has
the same name, but not the same parameter types, the name is overloaded,
but the method used is determined by the types of the parameters actually
passed on the call.

	Upcasting and polymorphism generally are used together.  Several
classes are derived from a common base class.  Then objects of the 
various derived classes are used as if they were objects of the base 
class.  This is upcasting.  Methods of the base class are called through 
base class object variables that actually reference objects of the derived
classes.  Polymorphism ensures that the appropriate methods in the 
derived classes are actually called.

	The combination of upcasting and object-oriented polymorphism 
supports incremental development by allowing a programmer to insert a 
new derived class into an application without disturbing any of the 
application's already present code.  The new derived class's objects 
automatically are used by the existing code just like objects of 
earlier derived classes from the same base class.
	This combination also makes use of commercial libraries for 
which source code is not provided to customers possible.  This 
combination also makes component based development (CBD) possible.