How should a boundary object be implemented in the Java language?
My boundary classes are ordinary classes that have an association with what I call a use case controller class. It is the use case controller class that coordinates all the messaging between the entity classes. In a sense the use case controller implements the sequence diagram messaging.
Now, your boundary classes may indeed implement some other well defined interfaces, particularly if these boundary classes are all very similar (i.e., GUI, Reporting, etc.).