Wednesday, August 1, 2007

IS-A principle => interface methods are ALL public

An earlier post asked Q: Must concrete methods be public if born in an interface?
%% A: I think so. why?

short answer: a subtype must appear a good member of the "family", supporting all expected services. A non-public method would not support the expected service.

We call each method a "service" because the client will use it and depend on it.

long answer:

Suppose a caller method M1 calling a method M2, perhaps a factory method.
Suppose M2 returns a Company which is an interface.
Suppose M1 puts the object into a variable "company" of type Company, and calls company.payTax().

Suppose the object is actually a Shop, a subtype of Company.

Since Company.java is an Interface, java syntax dictates Company (or its ancestors) declaring payTax() as public.

Q: can Shop.java define payTax() as private?
A: no. M1, the client, would fail when calling payTax(). In fact, Shop.java would probably fail to compile if payTax() is protected, default or anything but public.
A: You break the IS-A principle -- Shop no longer IS-A Company.
A: a public callable payTax() method is part of the IS-A.
A: a public callable payTax() method is part of being a Company
A: a public callable payTax() method is part of the expected services of the Company family.
A: Every subtype of Company, be it a sub-interface, abstract class or concrete class, must expose a public callable payTax(), otherwise clients will die.


A more real example: Runnable.run() is, thanks to the interface, public ==> concrete run() method is always public. Suppose you instantiate an object R implementing Runnable but with a private run(). Suppose a 3rd-party client receives this object as a Runnable type and faithfully invokes its run(). Bang! The client has to disqualify this object as a genuine Runable.

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