10 Abstract Class and Interface Interview Questions Answers in Java

Abstract class and interface is very popular in any object oriented
programming language or Java interview, and there are always one or more
questions from this. Interface is more common,
because of its popularity among designers but questions from abstract class
also pops up now and than. Interview questions from abstract class is more
common on junior level or you say under 2 years experience of  Java programmers, while interface related
questions are mostly asked on senior level Java interview e.g. 4 or 6 years of
experience. They are mostly asked along with other Java design pattern questions,
e.g. Decorator pattern or Factory pattern . Any way, in
this article we will see mix of these interview questions from abstract class
and interface. All questions has been asked in various Java interviews and
difficulty level for these question is easy for most of Java developer. It’s
mostly fact based questions, but some questions like difference between abstract class and interface in Java, and when to prefer abstract class over interface
can be really tricky.

Frequently asked Abstract class and
Interface questions in Java

Here is my list of questions, this not only explains rules related to
abstract class but also shares some tricky questions about using abstract class
and interface. If you have asked any question on this topic, which you don’t
see in this list, than please share with us as comment

1) Can
abstract class have constructors in Java?

Yes, abstract class can declare and define constructor in Java. Since you
can not create instance of abstract class, 
constructor can only be called during constructor chaining, i.e.
when you create instance of concrete implementation class. Now some
interviewer, ask what is the purpose of constructor, if you can not instantiate
abstract class? Well, it can still be used to initialize common variables,
which are declared inside abstract class, and used by various implementation. Also
even if you don’t provide any constructor, compiler will add default no argument constructor
in abstract class, without that your subclass will not compile, since first
statement in any constructor implicitly calls super(), default
super class constructor in Java.

2) Can
abstract class implements interface in Java? does they require to implement all
methods?

Yes, abstract class can implement interface by using implements keyword.
Since they are abstract, they don’t need to implement all methods. It’s good
practice to provide an abstract base class, along with an interface to declare
Type. One example of this is java.util.List interface
and corresponding java.util.AbstractList abstract class. Since AbstractList implements
all common methods,  concrete
implementations like LinkedList and ArrayList are free from burden
of implementing all methods, had they implemented List interface
directly. It’s best of both world, you can get advantage of interface for
declaring type, and flexibility of abstract class to implement common behavior
at one place. Effective Java has a nice
chapter on how to use interface and abstract class in Java, which is worth
reading.

3) Can abstract
class be final in Java?

No, abstract class can not be final in Java. Making them final will stop
abstract class from being extended, which is the only way to use abstract
class. They are also opposite of each other, abstract keyword
enforces to extend a class, for using it, on the other hand, final keyword prevents a class
from being extended. In real world also, abstract signifies incompleteness,
while final is used to demonstrate completeness. Bottom line is, you can not
make your class abstract and final in Java,
at same time, it’s a compile time error.

4) Can
abstract class have static methods in Java?

Yes, abstract class can declare and define static methods, nothing
prevents from doing that. But, you must follow guidelines for making a method
static in Java, as it’s not welcomed in a object oriented design, because static methods can not be overridden in
Java. It’s very rare, you see static methods inside abstract class,
but as I said, if you have very good reason of doing it, then nothing stops
you.

5) Can
you create instance of abstract class?

No, you can not create instance of abstract class in Java, they are
incomplete. Even though, if your abstract class don’t contain any abstract
method, you can not create instance of it. By making a class abstract,  you told compiler that, it’s incomplete and
should not be instantiated. Java compiler will throw error, when a code tries
to instantiate abstract class.

6) Is it
necessary for abstract class to have abstract method?

No, It’s not mandatory for an abstract class to have any abstract method.
You can make a class abstract in Java, by just using abstract keyword in
class declaration. Compiler will enforce all structural restriction, applied to
abstract class, e.g. now allowing to create any instance. By the way, it’s
debatable whether you should have abstract method inside abstract class or
interface. In my opinion, abstract class should have abstract methods, because
that’s the first thing programmer assumes, when he see that class. That would
also go nicely along principle of least surprise.

7)
Difference between abstract class and interface in Java?

This is the most important and one of the classic Java Interview
question. I don’t know, how many times I have seen this question at all most
all levels of Java interviews. One reason, which makes this question
interesting is ability to produce example. It’s easy to answers questions on
core OOPS concepts like Abstraction, Encapsulation, Polymorphism and Inheritance, but when it comes
to subtle points like this, candidate more often fumbled. You can see this post
for all syntactical difference between abstract class and interface, but it
deserve a post on it’s own.

8) When
do you favor abstract class over interface?

This is the follow-up of previous interview questions on abstract class
and interface. If you know syntactical difference, you can answer this question
quite easily, as they are the one, which drives the decision. Since it’s
almost impossible to add a new method on a published interface, it’s better to
use abstract class, when evolution is concern. Abstract class in Java evolves
better than interface. Similarly, if you have too many methods inside
interface, you are creating pain for all it’s implementation, consider
providing an abstract class for default implementation. This is the pattern
followed in Java collection package, you can see AbstractList provides
default implementation for List interface.

9) What
is abstract method in Java?

An abstract method is a method without body. You just declare method,
without defining it and use abstract keyword in method
declaration.  All method declared inside Java Interface are by default
abstract. Here is an example of abstract method in Java

                public
void abstract printVersion();

Now, In order to implement this method, you need to extend abstract class
and override this method.

10) Can
abstract class contains main method in Java ?

Yes, abstract class can contain main method, it just another
static method and you can execute Abstract class with main method, until you
don’t create any instance.

That’s all on this list of interview questions on abstract class,
interface and methods in Java. You can also bookmark this list as FAQ for
abstract class, nice to refresh, before going to interview. On different note,
abstract class and interface are key design decisions on object oriented analysis
and design process, and should be taken by applying due diligence, of course if
you want to write flexible systems.

Source:http://javarevisited.blogspot.com/2013/04/10-abstract-class-and-interface-interview-question-java-answers.html