Yes, I do understand the title reflects the age old repetitive interview question. It has been asked again, again and again and then just some more times. But why, why is this so important. Why does every software company on the planet has the question sticking out of their heads. We all know the answer to this(duh??), they all know the answer to this(duh??), and nearly every preparation site answers this, then what is the big fuss about?

Well, the fantastic thing about an iceberg is its tip and then comes in the total size. In a very similar fashion, for this question, everybody understands the syntax but fails to understand the semantics. If you have been reading about this, few well know answers are:

  • Interface is a 100% abstract class
  • Java class(es) can implement multiple interfaces but it can extend only one abstract class.
  • Java interface should be implemented using keyword “implements”; A Java abstract class should be extended using keyword “extends”.

All the above arguments talk about the dynamics of implementations of an abstract class and interface(s). Although those are factually correct but lack in answering a fundamental design question, i.e., “When should my software use an abstract class and when should it use an interface”.

To answer this let us begin by asking a few questions, the first one is, Have you ever noticed that when we inherit from interfaces, we use “implements” however when we inherit from an abstract class we use “extends”?, Yes, No? Puzzled?

This little inheritance jargon is the key in understanding the usage of abstract classes vs interfaces. Not only in software engineering but in any engineering field(even in basic english), the keyword “implements” is associated with implementing a functionality and “extends” is associated with extending to enhance the characteristics of a type. Keeping this in mind, Whenever we are implementing we are actually defining “what” a class can do or what it is capable of. For example, a Ball which “is a” Toy is “capable of” bouncing. Got it?, Saw the light?, Not yet?

Maybe Now:

class Ball extends Toy implements Bounceable

So whenever we talk about functionality, think interfaces and whenever we talk about characteristics, think abstract classes(not always, but yes when we analyze in terms of extensions). So, as a general rule of thumb:

Interfaces:

  • Talk about what a class can do.(Functionality)
  • Declare what kind of functionality, a class should implement.
  •  Uses “is capable of” relationship.

Abstract Class(es):

  • Talks about characteristics an object should support.
  • Type of an Object
  • Uses “is a” relationship

Hope this cleared some of the fog.

Advertisements