ClassNotFoundException is thrown when reading an evolved class with
a new field from a serialized stream when the type of the
new field did not exist in the original classes hierarchy.

When ClassNotFoundException is thrown for the scenario described
in description, just make the missing class available in
the original class hierarchy. 

Be sure that the dummy class definition
implements Serializable and the serialVersionUID is explicitly set
to the serialver of the missing class.

In short, the following java.awt classes are not currently backwards compatible
from JDK1.2 to any JDK 1.1.x release.  Color, Font and Component.
All three of these classes had fields added to them in JDK 1.2
with new classes.  The ClassNotFoundException is thrown when a JDK 1.2
serialized stream containining an instance of one of these objects
is read back into JDK 1.1.x.

I believe java.net.URL class is also not currently backwards compatible, but maybe for other reasons (in JDK 
1.2.2)