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Top 50 Important OOPs Interview Questions and Answers

Published on Aug 19,2019 5K Views
1 / 3 Blog from Interview Questions

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OOPs is one of the main concepts in the programming world, therefore, every interview that you attend requires knowledge of OOPs. This article compiles the most frequently asked OOPs Interview Questions for freshers. So go ahead and get prepared.

Let’s take a quick look at all the topics of discussion:

Basic OOPs Interview Questions:

1. What is the difference between OOP and SOP?

Object-Oriented ProgrammingStructural Programming
Object-Oriented Programming is a type of programming which is based on objects rather than just functions and proceduresProvides logical structure to a program where programs are divided functions
Bottom-up approachTop-down approach
Provides data hidingDoes not provide data hiding
Can solve problems of any complexityCan solve moderate problems
Code can be reused thereby reducing redundancyDoes not support code reusability

2. What is OOPs?

OOPs (Object-Oriented Programming) is a type of programming which is based on objects rather than just functions and procedures. Individual objects are grouped into classes. OOPs implements real-world entities like inheritance, polymorphism, hiding, etc into programming. It also allows binding data and code together.

3. Why use OOPs?

  • OOPs allows clarity in programming thereby allowing simplicity in solving complex problems
  • Code can be reused through inheritance thereby reducing redundancy
  • Data and code are bound together by encapsulation
  • OOPs allows data hiding, therefore, private data is kept confidential
  • Problems can be divided into different parts making it simple to solve
  • The concept of polymorphism gives flexibility to the program by allowing the entities to have multiple forms

4. What are the main features of OOPs?

  • Inheritance
  • Encapsulation
  • Polymorphism
  • Data Abstraction

To know more about OOPs in JAVA, Python, and C++ you can go through the following blogs:

Classes and Objects OOPs Interview Questions:

5. What is an object?

An object is a real-world entity which is the basic unit of OOPs for example chair, cat, dog, etc. Different objects have different states or attributes, and behaviors.

6. What is a class?

A class is a prototype that consists of objects in different states and with different behaviors. It has a number of methods that are common the objects present within that class.

7. What is the difference between a class and a structure?

Class: User-defined blueprint from which objects are created. It consists of methods or set of instructions that are to be performed on the objects.

Structure: A structure is basically a user-defined collection of variables which are of different data types.

8. Can you call the base class method without creating an instance?

Yes, you can call the base class without instantiating it if:

  • It is a static method
  • The base class is inherited by some other subclass

9. What is the difference between a class and an object?

ObjectClass
A real-world entity which is an instance of a classA class is basically a template or a blueprint within which objects can be created
An object acts like a variable of the classBinds methods and data together into a single unit
An object is a physical entityA class is a logical entity
Objects take memory space when they are createdA class does not take memory space when created
Objects can be declared as and when requiredClasses are declared just once

To know more about objects and classes in JAVA, Python, and C++ you can go through the following blogs:

Interview Questions on Features of OOPs

Inheritance:

10. What is inheritance?

Inheritance is a feature of OOPs which allows classes inherit common properties from other classes. For example, if there is a class such as ‘vehicle’, other classes like ‘car’, ‘bike’, etc can inherit common properties from the vehicle class. This property helps you get rid of redundant code thereby reducing the overall size of the code.

11. What are the different types of inheritance?

  • Single inheritance
  • Multiple inheritance
  • Multilevel inheritance
  • Hierarchical inheritance
  • Hybrid inheritance

12. What is the difference between multiple and multilevel inheritance?

Multiple InheritanceMultilevel Inheritance
Multiple inheritance comes into picture when a class inherits more than one base classMultilevel inheritance means a class inherits from another class which itself is a subclass of some other base class
Example: A class defining a child inherits from two base classes Mother and FatherExample: A class describing a sports car will inherit from a base class Car which inturn inherits another class Vehicle

13. What is hybrid inheritance?

Hybrid inheritance is a combination of multiple and multi-level inheritance.

14. What is hierarchical inheritance?

Hierarchical inheritance refers to inheritance where one base class has more than one subclasses. For example, the vehicle class can have ‘car’, ‘bike’, etc as its subclasses.

15. What are the limitations of inheritance?

  • Increases the time and effort required to execute a program as it requires jumping back and forth between different classes
  • The parent class and the child class get tightly coupled
  • Any modifications to the program would require changes both in the parent as well as the child class
  • Needs careful implementation else would lead to incorrect results

To know more about inheritance in Java and Python, read the below articles:

16. What is a superclass?


A superclass or base class is a class that acts as a parent to some other class or classes. For example, the Vehicle class is a superclass of class Car.

17. What is a subclass?

A class that inherits from another class is called the subclass. For example, the class Car is a subclass or a derived of Vehicle class.

Polymorphism

18. What is polymorphism?

Polymorphism refers to the ability to exist in multiple forms. Multiple definitions can be given to a single interface. For example, if you have a class named Vehicle, it can have a method named speed but you cannot define it because different vehicles have different speed. This method will be defined in the subclasses with different definitions for different vehicles.

19. What is static polymorphism?

Static polymorphism (static binding) is a kind of polymorphism that occurs at compile time. An example of compile-time polymorphism is method overloading.

20. What is dynamic polymorphism?

Runtime polymorphism or dynamic polymorphism (dynamic binding) is a type of polymorphism which is resolved during runtime. An example of runtime polymorphism is method overriding.

21. What is method overloading?

Method overloading is a feature of OOPs which makes it possible to give the same name to more than one methods within a class if the arguments passed differ.

22. What is method overriding?

Method overriding is a feature of OOPs by which the child class or the subclass can redefine methods present in the base class or parent class. Here, the method that is overridden has the same name as well as the signature meaning the arguments passed and the return type.

23. What is operator overloading?

Operator overloading refers to implementing operators using user-defined types based on the arguments passed along with it.

24. Differentiate between overloading and overriding.

OverloadingOverriding
Two or more methods having the same name but different parameters or signatureChild class redefining methods present in the base class with the same parameters/ signature
Resolved during compile-timeResolved during runtime

To know more about polymorphism in Java and Python, read the below articles:

Encapsulation

25. What is encapsulation?

Encapsulation refers to binding the data and the code that works on that together in a single unit. For example, a class. Encapsulation also allows data-hiding as the data specified in one class is hidden from other classes.

26. What are ‘access specifiers’?

Access specifiers or access modifiers are keywords that determine the accessibility of methods, classes, etc in OOPs. These access specifiers allow the implementation of encapsulation. The most common access specifiers are public, private and protected. However, there are a few more which are specific to the programming languages.

27. What is the difference between public, private and protected access modifiers?

NameAccessibility from own classAccessibility from derived classAccessibility from world

Public

Yes

Yes

Yes

Private

Yes

No

No

Protected

Yes

Yes

No

To know more about encapsulation read along:

Data abstraction

28. What is data abstraction?

Data abstraction is a very important feature of OOPs that allows displaying only the important information and hiding the implementation details. For example, while riding a bike, you know that if you raise the accelerator, the speed will increase, but you don’t know how it actually happens. This is data abstraction as the implementation details are hidden from the rider.

29. How to achieve data abstraction?

Data abstraction can be achieved through:

  • Abstract class
  • Abstract method

30. What is an abstract class?

An abstract class is a class that consists of abstract methods. These methods are basically declared but not defined. If these methods are to be used in some subclass, they need to be exclusively defined in the subclass.

31. Can you create an instance of an abstract class?

No. Instances of an abstract class cannot be created because it does not have a complete implementation. However, instances of subclass inheriting the abstract class can be created.

32. What is an interface?

It is a concept of OOPs that allows you to declare methods without defining them. Interfaces, unlike classes, are not blueprints because they do not contain detailed instructions or actions to be performed. Any class that implements an interface defines the methods of the interface.

33. Differentiate between data abstraction and encapsulation.

Data abstractionEncapsulation
Solves the problem at the design levelSolves the problem at the implementation level
Allows showing important aspects while hiding implementation detailsBinds code and data together into a single unit and hides it from the world

To know more about data abstraction, below articles might help you:

Methods and Functions OOPs interview questions

34. What are virtual functions?

Virtual functions are functions that are present in the parent class and are overridden by the subclass. These functions are used to achieve runtime polymorphism.

35. What are pure virtual functions?

Pure virtual functions or abstract functions are functions that are only declared in the base class. This means that they do not contain any definition in the base class and need to be redefined in the subclass.

36. What is a constructor?

A constructor is a special type of method that has the same name as the class and is used to initialize objects of that class.

37. What is a destructor?

A destructor is a method that is automatically invoked when an object is destroyed. The destructor also recovers the heap space that was allocated to the destroyed object, closes the files and database connections of the object, etc.

38. Types of constructors

Types of constructors differ from language to language. However, all the possible constructors are:

  • Default constructor
  • Parameterized constructor
  • Copy constructor
  • Static constructor
  • Private constructor

39. What is a copy constructor?

A copy constructor creates objects by copying variables from another object of the same class. The main aim of a copy constructor is to create a new object from an existing one.

40. What is the use of ‘finalize’?

Finalize as an object method used to free up unmanaged resources and cleanup before Garbage Collection(GC). It performs memory management tasks.

41. What is Garbage Collection(GC)?

GC is an implementation of automatic memory management. The Garbage collector frees up space occupied by objects that are no longer in existence.

42. Differentiate between a class and a method.

ClassMethod
A class is basically a template that binds the code and data together into a single unit. Classes consist of methods, variables, etcCallable set of instructions also called a procedure or function that are to be performed on the given data

43. Differentiate between an abstract class and an interface?

Basis for comparisonAbstract ClassInterface

Methods

Can have abstract as well as other methodsOnly abstract methods

Final Variables

May contain final and non-final variablesVariables declared are final by default

Accessibility of Data Members

Can be private, public, etcPublic by default

Implementation

Can provide the implementation of an interfaceCannot provide the implementation of an abstract class

44. What is a final variable?

A variable whose value does not change. It always refers to the same object by the property of non-transversity.

Exception Handling

45. What is an exception?

An exception is a kind of notification that interrupts the normal execution of a program. Exceptions provide a pattern to the error and transfer the error to the exception handler to resolve it. The state of the program is saved as soon as an exception is raised.

46. What is exception handling?

Exception handling in Object-Oriented Programming is a very important concept that is used to manage errors. An exception handler allows errors to be thrown and caught and implements a centralized mechanism to resolve them.

47. What is the difference between an error and an exception?

ErrorException
Errors are problems that should not be encountered by applicationsConditions that an application might try to catch

48. What is a try/ catch block?

A try/ catch block is used to handle exceptions. The try block defines a set of statements that may lead to an error. The catch block basically catches the exception.

49. What is a finally block?

A finally block consists of code that is used to execute important code such as closing a connection, etc. This block executes when the try block exits. It also makes sure that finally block executes even in case some unexpected exception is encountered.

Limitations of OOPs

50. What are the limitations of OOPs?

  • Usually not suitable for small problems
  • Requires intensive testing
  • Takes more time to solve the problem
  • Requires proper planning
  • The programmer should think of solving a problem in terms of objects

Hope you are clear with all that has been shared with you in this tutorial. This brings us to the end of our article on OOPs Interview Questions. Make sure you practice as much as possible and revert your experience.  

To get in-depth knowledge on this concept, you can checkout the live Python online training and  Java Certification Training with 24/7 support and lifetime access. 

Got a question for us? Please mention it in the comments section of this “OOPS Interview Questions” blog and we will get back to you as soon as possible.

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