This document discusses the concept of dual interfaces in object-oriented design, where a class has two types of clients - external clients and subclasses. It notes that making fields private prevents subclasses from accessing or modifying them directly, so protected accessors should be provided instead to empower subclasses while preventing direct field access by external clients. This dual interface approach benefits classes by controlling field access for clients but empowering subclasses.

1. Stéphane Ducasse 1
Stéphane Ducasse
stephane.ducasse@inria.fr
http://stephane.ducasse.free.fr/
Design Points - Dual
Interfaces
Stéphane Ducasse --- 2005

2. S.Ducasse 2
Clients?
• Clients
• Subclasses

3. S.Ducasse 3
What are the consequences?
A
{ private x ;
void foo(){ ... x ...}
}

4. S.Ducasse 4
What can I do as a subclass?
I cannot access x
I cannot replace x
I cannot extend!

5. S.Ducasse 5
The idiom is
Fields are private AND you provide protected
accessors
OR
Fields are protected

6. S.Ducasse 6
Benefits
Your clients cannot access your fields
But your subclasses are empowered

7. S.Ducasse 7
Two users

8. S.Ducasse 8
Two Users

9. S.Ducasse 9
Clients
A class has always two clients:
its clients and its extenders

10. S.Ducasse 9
Clients
A class has always two clients:
its clients and its extenders