Programming Interviews Exposed-Secrets to Landing Your Next Job, Second Edition

Author:byJohn Mongan, Noah SuojanenandEric Gigure
Language: eng
Format: mobi
ISBN: 9780470121672
Published: 2012-08-23T02:21:05+00:00

Important Explain the difference between an interface and an abstract class in object-oriented programming.

The specific answer to this depends, of course, on which language you’re developing with, but start the answer by giving some general definitions:

An interface declares a set of related methods, outside of any class.

An abstract class is an incomplete class definition that declares but does not define all of its methods.

Conceptually, then, an interface defines an application programming interface (API) that is independent of any class hierarchy. In fact, interfaces can be used in non-OO programming models, such as componentbased models like COM and CORBA. However, you’re focusing on the use of interfaces in an object-oriented context, where they are useful in their own right. Interfaces are the ultimate encapsulators, because they hide all the details of the classes that implement their methods from the user of the interface. They’re particularly important - almost necessary, in fact - in languages that only support single inheritance (classes can only inherit from one base class). A class that exposes its members via an interface is said to implement the interface.

Unlike an interface, an abstract class is a proper class: It can have data members and can be a subclass of other classes. Unlike a concrete (nonabstract) class, however, some of its behaviors are deliberately left to be defined by its own subclasses. Abstract classes cannot be instantiated because of this - only instances of concrete subclasses can be created.

An interface is almost identical to an abstract class with no data members and no method definitions. In C++ this is exactly how you’d define an interface: by declaring a class with no data members and only pure virtual functions - something like this, for example:

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