Abstract class / Interface

[Sandeep Awasthi]

How to choose between Abstract class or interface ? (Interview question)

I answered if you have few methods which have concrete implementation and few methods without implementation then go for abstract class, if you all the methods without implementation, go for interface.

But interviewer was not satisfied with this answer. Is there any other factor to choose between abstract class and interface.

[saikrishna cinux]

and one more point you need to tell

if we want to achieve multiple inheritence then go for interface

[Ernest Friedman-Hill]

In general, prefer interfaces to represent type hierarchies. Use abstract classes mainly as convenient, default implementations of those interfaces.

[Sandeep Awasthi]

Thanks all for your replies. I dont think we can add anything more to this. I dont know what exactly the interviewer was expecting. Once again thanks for replies.

[Atul Shukla]

hey !! read this, might interviewer expect this from you...

if you want to have your class methods to must have implementation by all subclasses then go for abstract classes...but wait...it seems same for interface...no ..
actually we use interface to filter out methods that should be implemented by subclasses on the discretion of the subclass ,confused?
consider example...

you have a class shape that have public method animate(). OK . now consider subclasses rectangle , square , circle subclases of class shape and all subclases have method animate(). hold on now you want to have only specific subclasses to have method ROTATE(), now what, if you put that rotate method() in superclass then all subclasses have that method but you want to filter out....so you declare an interface and declare a method in that and let subclasses implement that interface along with extending shape class... so square and rectangle implements the interface but circle may not.... may be it is cleared to you....
come up with queries if you not understand what i mean to tell you.....

Regards
Atul

[Bauke Scholtz]

Summarized:

Abstract elements describes required elements of a type hierachy. When subclassed, you must implement them all. You can extend from only one superclass.

Interface elements describes optional elements of a type hierachy. It's your choice whether to implement them. You can implement more than one interface.

[Sandeep Awasthi]

Thank you very much Atual and Bauke. I got it now.