Description of problem:
If there is an abstract class available to your project (from the project's imports suggestions), Data Modeler will allow you to extend that class. The model will be saved successfully. If you close and reopen the Data Modeler, the extending class is gone.

Trying to build the project will of course fail and the Problems panel informs you that "The type ExtendingClass must implement the inherited abstract method AbstractClass.abstractMethod()".

Looking into the DM, seeing that the new class is not there, you might think everything is okay then, but the DM just doesn't show the class - it's still in the project and you have to remove it or fix it by other means.

There is no way for the user to know that some class from the project's import suggestions is abstract. Since the Data Modeler cannot handle extending such classes, it should probably not even offer them as a possible superclass when creating a new class/data object.
(Though good support for that use case would be much better.)

As a slightly funny side note:
Every new project has java.lang.Number (which is abstract) automatically added to project's import suggestions. New users therefore don't have to go far in search for a class to extend when exploring the Data Modeler.

Version-Release number of selected component (if applicable):
BPMS 6.0 CR2