Core Java - Interview Questions and Answers for 'Abstract class' | Search Interview Question

Core Java - Interview Questions and Answers for 'Abstract class' - 23 question(s) found - Order By Newest

Very Frequently asked. Have been asked in HCL Technologies very frequently ( based on 3 feedback ). Among first few questions in many interviews.

Q1. Differences between abstract class and interface ?

Core Java

Ans. Abstract classes can have both abstract methods ( method declarations ) as well as concrete methods ( inherited to the derived classes ) whereas Interfaces can only have abstract methods ( method declarations ).

A class can extend single abstract class whereas it can implement multiple interfaces.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java classes abstract class interfaces abstract class vs interface abstract classes vs interfaces Asked in 82 Companies basic frequent}

Q2. Difference between Abstract and Concrete Class ?

Core Java

Ans. Abstract classes are only meant to be sub classed and not meant to be instantiated whereas concrete classes are meant to be instantiated.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java oops abstract classes Abstract vs Concrete Class Asked in 1 Companies basic frequent}

Q3. If an Abstract class has only abstract methods, What's the difference between such a class and an interface ?

Core Java

Ans. Such a class still can have member elements which can be inherited and hence facilitate code reuse. Moreover Abstract class can have non final static elements whereas interfaces are only allowed to have static final elements.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve abstract class interfaces Asked in 2 Companies Basic}

Q4. What are the design considerations while making a choice between using interface and abstract class ?

Core Java

Ans. Keep it as a Abstract Class if its a "Is a" Relationsship and should do subset/all of the functionality. Keep it as Interface if its a "Should Do" relationship.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java oops abstract classes interfaces advanced Asked in 3 Companies}

Q5. How is Abstraction implemented in Java ?

Core Java

Ans. Abstraction is provided in Java by following ways -

Coding to the ( Interfaces / Abstract Classes ) or contracts

By Encapsulating details within classes and exposing the minimal Door ( few public methods )

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java oops concepts abstraction interfaces abstract class encapsulation object oriented programming (oops) oops concepts Asked in 3 Companies basic frequent}

Q6. Why can't we create an instance of abstract class ? Why is it restricted by compiler ?

Core Java

Ans. Because Abstract class is incomplete i.e there are abstract methods that needs to be defined by extending classes.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve abstract class}

Q7. Can we have multiple inheritance in java using abstract class ?

Core Java

Ans. No

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve abstract class multiple inheritance object oriented programming (oops) oops concepts Asked in 2 Companies}

Q8. Difference between Base Class, Derived Class , Abstract Class and Concrete Class ?

Core Java

Ans. Base Class is a relative term used to signify the Parent class in Parent - Child or Base - derived relationship.

Derived Class is a relative term used to signify the Child class in Parent - Child or Base - derived relationship.

Abstract Class is a class that is not allowed to be instantiated and hence can serve only a base class for it's usage.

Concrete Class is the class which is supposed to be instantiated and hence provide definition for all implementing interface methods and extending abstract methods.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve Base Class Derived Class Abstract Class Concrete Class}

Very frequently asked. Usually asked in this format or as difference with interfaces / concrete classes.

Q9. What is an abstract class ?

Core Java

Ans. Abstract class is the class that is not supposed to be instantiated. The purpose of the class to only have extension to the derived class.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve abstract class Asked in 6 Companies basic frequent}

Q10. what will be the output ?

class Animal {
public void eat() throws Exception {
}
}

class Dog2 extends Animal {
public void eat(){}
public static void main(){
Animal an = new Dog2();
an.eat();
}
}

Core Java

Ans. Compile Time Error: Unhandled exception type Exception

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java code coding overridding late binding exception handling abstract class abstract methods}

Q11. Will this code Work ? If not , Why ?

java.util.Calendar c = new java.util.Calendar();

Core Java

Ans. No. It gives the error "Cannot Instantiate the type Calendar". Calendar is an abstract class and hence Calendar object should be instantiated using Calendar.getInstance().

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java date calendar abstract class yes-no}

Q12. Is java.util.Date an abstract Class ? Is java.util.Calendar an abstract Class ?

Core Java

Ans. Date is not a abstract class whereas Calendar is.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java date calendar abstract class}

Q13. Will this code compile ?

public class BuggyBread1{
abstract public void test();
}

Core Java

Ans. No. It will give the compilation error saying "The abstract method test in type BuggyBread1 can only be defined by an abstract class".

We need to declare the class abstract for it to have any abstract method.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java oops abstract class abstract methods java compilation error}

Q14. Will this Code compile ?

abstract public class BuggyBread1{
abstract public void test(){};
}

Core Java

Ans. No. This will give a compilation error saying "Abstract methods do not specify a body".

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java abstract classes abstract methods java compilation error java coding java code coding yes-no}

Q15. In the following code , how many methods needs to be implemented in Class B ?

public interface A{
   public void method1();
   public void method2();
   public void method3();
}

abstract class B implements A{
}

Core Java

Ans. As Class B has been declared abstract , we can either implement any of these methods and just declare rest of them abstract.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve interfaces abstract classes code coding}

Try 2 Question(s) Test

Q16. What should a class do if its extending an abstract class ?

Core Java

Ans. It should either implement the abstract methods or re-declare them abstract.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve abstract class Asked in 2 Companies Basic Frequent}

Q17. When should we use Abstract classes ?

Core Java

Ans. Abstract classes provide a mechanism of interfacing ( using abstract method ) as well as inheritance ( extending abstract class ). So they should be used in place of interfaces in case there is some code ( methods ) or object body ( member elements ) that can be reused with inheritance.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve abstract classes Asked in 1 Companies}

Q18. What was the need of creating an abstract class in Java when we cannot instantiate an object of it ?

Core Java

Ans. The objective of an abstract class is to provide an interface as well as code reuse. Though we cannot instantiate an instance of abstract class but we can reuse the code by extending such a class. Moreover abstract class provides interfacing to outside components just like interfaces.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve abstract class}

Q19. What is the importance of Abstract Class ?

Core Java

Ans. Abstract classes provide a mechanism of interfacing ( using abstract method ) as well as code reuse through inheritance ( extending abstract class )

Comparing to concrete class they have an advantage of providing interface which a concrete class doesn't provide.

Comparing to interfaces they have an advantage of providing code reuse through inheritance which interfaces dont provide.

Sample Code for abstract class

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve abstract class importance of abstract class Basic frequent}

Q20. What are the advantage of Abstract classes over interfaces with respect to Java 7 ? and What changed in Java 8 to help facilitate that in Java 8 ?

Core Java

Ans. Abstract Classes provide default implementations of methods that are inherited by the classes that extend them, which was not the case for Interfaces. This changed in Java 8, where default implementations are provided for methods.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve abstract classes interfaces default method Asked in 1 Companies expert}

Q21. What is the relationship between Base Class and Abstract Class ?

Core Java

Ans. Abstract Class is a class that's only allowed to be a Base class for it's usage. It can never be instantiated on it's own.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve base class abstract class basic}

Q22. Can an abstract class be a derived class ?

Core Java

Ans. Yes, an abstract class can extend another class.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve abstract class}

Q23. Does abstract class have public constructor?

Core Java

Ans. Yes, an abstract class can have a constructor in Java.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve abstract class public constructor Asked in 1 Companies}

Q24. Which of the following is not true for abstract classes ? Reference

Core Java

a. Abstract Class is only meant to be sub classed and not supposed to be instantiated.

b. Abstract class handlers can be used to handle derived class objects.

c. We can't have an abstract class without abstract methods.

d. Abstract class has member elements.

Ans.c. We can't have an abstract class without abstract methods.

Search Interview Questions

Help us and Others Improve. Please let us know the questions asked in any of your previous interview.