HELP PLEASE WITH THIS QUESTION....INTERVIEW TODAY

Question

In an existing application, the code to implement a particular feature is encapsulated in a single class. However the requirements have just changed and this feature must exhibit an alternative behaviour in some instances, while continuing to exhibit the existing behaviour in other instances. Discuss support in Java for implementing the new requirement.

Answer 1

Thanks for the replies, guys.
It is not as if I can not answer it but this is a
forum and this is a chance for me to get a job. I am
trying to be sure of the right explanation. OK, fair enough.
What I
feel initially is that an abstract class or an
interface will be needed if you want to change the
implementation later i.e. overriding the methods.That's true, if you want to be sure that client code using your class doesn't change they'll want be making the call through an interface or abstract class.
You can inherit and override even if you don't have an interface or abstract class, but in that case polymorphism can't help you and you force clients to change their code. Worse, they have to start checking for instance type.
So yes, you'll have three steps:
(1) Write the common interface that has the signature of the method you'd like to override.
(2) Have the original class implement the interface.
(3) Write the new class such that it implements the interface and uses its new functionality.
If the original class implementation can be considered "default", you can go for an abstract class that implements the interface and provides the default implementation. I'd have both the original and new classes extend the abstract class in that case.
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