Must Read 47 OOPS Interview Questions & Answers For Freshers & Experienced [2020]

The programming world and the languages have come a long way since the inception of programming languages. However, one thing that has stood out the test of time is the Object-Oriented programming concepts. Whether you are looking for a starting a career in programming or trying to improve your skills, you should develop a solid understanding of object-oriented programming or object-oriented programming systems (OOPs). 

In this article, we will help you prepare the most crucial OOPs interview questions. Let us review the questions around the key features of OOPs, advantages, and limitations. These OOPs interview questions cover the essential aspects of object-oriented programming. Understanding these concepts and the questions that are asked will help you prepare well for a job interview and tackle the real-world programming assignments.

Top OOPs Interview Questions & Answers

1. What is Object-oriented programming?

This is the most basic OOPs interview question. Your answer should be: 

Object-Oriented Programming refers to the programming paradigm defined using objects instead of only functions and methods. The objects contain data, called fields or attributes, and methods that provide the logic or supporting code. It provides capabilities such as inheritance, polymorphism, encapsulation, abstraction.

2. What are the main features of object-oriented programming?

In this OOPs interview question, make sure you list the four main features:

  • Inheritance
  • Encapsulation
  • Polymorphism
  • Data Abstraction

3. What are the advantages of Object-oriented programming?

  • Problems of any level of complexity can be supported by object-oriented programming.
  • Highly complex problems can be handled by object-oriented programming
  • It provides an efficient mechanism for code reuse using inheritance which reduces redundancy 
  • It provides a mechanism for hiding data
  • It is based on a bottom-up approach
  • It offers flexibility through polymorphism 
  • It improves maintainability of the code

4. What is Structural programming?

Structural programming refers to the traditional method of programming, which is based on functions. The overall program logic is divided into functions to provide a logical structure. It is based on a top-down approach. Structural programming is suitable for easy to moderately complex problems. 

5. What is a class?

A class defines the template or the definition of an object. It is used for creating objects at run time. It provides the data structure, provides initial values for the attributes, and methods that provide the logic for the intended behaviour of the object. The class does not consume memory at runtime. A class refers to a logical entity. E.g., a vehicle may be defined as a class. 

6. What do you mean by an object?

An object refers to the run time instance created from the class during program execution. Objects can refer to real-world entities that have attributes or properties and methods to support the behaviour. Objects consume memory space when they are initialized.

7. Is it always necessary to create objects from class?

No, it is possible to call a base class method if it is defined as a static method.

8. What is a constructor?

A constructor method is used for initializing the objects. They are special types of methods and have the same name as the class. 

Read: Java Interview Questions & Answers

9. List the various types of constructors

Multiple types of constructors that are supported across multiple object-oriented programming languages are:

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

10. What is a destructor?

A destructor is a method used for freeing up the resources allocated to an object. This method is automatically invoked when an object is being destroyed. 

11. What is meant by a copy constructor?

A copy constructor helps in cloning objects by replicating the values from one object into another object which belongs to the same class. 

12. Can you please highlight the difference between a class and a structure?

A class means a user-defined template from which objects are created at runtime. A class is made up of methods that provide the logic for various behaviours supported by the objects.

A structure means a user-defined combination of attributes of various data types.

13. Can you please explain the concept of inheritance with an example?

Inheritance is a powerful feature of object-oriented programming which allows classes to inherit properties and methods from other classes. This helps improve code reuse.  

For example, a base class represents a logical concept, such as a vehicle that may define only the common properties shared by all types of vehicles. However, child classes can inherit from this base class to define more specific types of classes such as a truck, a car, or a bus. In this case, the child classes will inherit the common attributes of the vehicle, and will be able to define attributes, method specific to its own. 

14. What are the limitations of inheritance?

The inheritance requires more processing time for the programs as it has to navigate various classes during execution. Due to inheritance, the parent and child class are tightly coupled. When any changes are needed in the logic, it may require changes in both parent and child classes. 

If the inheritance is not correctly implemented, it can lead to undesired results. 

15. What are the various types of inheritance?

  • Single 
  • Multiple 
  • Multi-level 
  • Hierarchical 
  • Hybrid 

16. What is the meaning of hierarchical inheritance?

When multiple subclasses inherit a base class, it is called hierarchical inheritance. 

17. Distinguish between multiple and multi-level inheritances? 

In the case of the multiple inheritance, a class inherits more than one parent class. In contrast, multi-level inheritance means that class inherits from another class, which is a subclass of some other parent class.

18. How do you define hybrid inheritance?

The hybrid inheritance is defined as the usage of multiple and multilevel inheritance in a single class.

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19. What is a subclass?

The child class which inherits from another class is referred to as the subclass. 

20. Define a superclass? 

A superclass implies a class from which other classes inherit. e.g., the vehicle will be referred to as superclass of classes car, bus, or truck if they all inherited from the same superclass. 

21. What is meant by an interface?

An interface allows a declaration of methods without providing a definition. 

You cannot create objects from the interface. When a class implements an interface, it needs to implement the methods provided by the interface. 

22. What is polymorphism?

Polymorphism is a significant feature of object-oriented programming. It means an ability to exist in multiple forms. A single interface can be implemented in multiple ways by providing various definitions. 

Read more: Top 6 Reasons Why Java Is So Popular With Developers

23. What is meant by static polymorphism?

The static polymorphism or static binding allows us to link a function with objects during compilation. It can be implemented by method overloading of operator overloading. 

24. What is meant by dynamic polymorphism?

A dynamic polymorphism or dynamic binding allows for a call to an overridden method at the run time. 

25. What is method overloading?

The method overloading is a very useful feature of object-oriented programming in which multiple methods can have the same method name; however, they have different arguments. The call to the method is resolved based on the arguments.

26. What is the meaning of method overriding?

Method overriding allows the child class to redefine methods of parent class by applying its implementations. However, the method name, arguments, and return types remain the same. 

27. Can you explain what operator overloading is?

The term operator overloading means that depending on the arguments passed, the operators’ behaviour can be changed. However, it works only for user-defined types. 

28. How do you explain the difference between overloading and overriding?

Overloading a method means that multiple methods share the same method name but have different arguments. However, in the case of the overriding, the child class can redefine the implementation of a method by retaining the same arguments. Another difference is that the overloading is resolved at compile-time while overriding is resolved at run time.

29. What do you know about encapsulation?

Encapsulation is an important feature of object-oriented programming. It allows the binding of the data and the logic together in a single entity. It also allows the hiding of data. 

30. What is meant by data abstraction?

The data abstraction refers to the ability of object-oriented programming that allows hiding the implementation details of logic yet allows for access to only important information.

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31. How can data abstraction be accomplished?

Data abstraction can be accomplished through either an abstract class or an abstract method. 

32. What is meant by abstract class?

An abstract class is made of abstract methods. The abstract methods are only declared, however, not implemented. When a subclass needs to use the methods, it needs to implement those methods. 

33. Can you please elaborate on ‘access specifiers’?

Support this OOPs interview question’s answer with examples:

Access specifiers are special keywords that control the accessibility of methods or classes etc. They are also called access modifiers and are used to achieve the encapsulation. e.g., the keywords public, private, and protected are some examples of access specifiers. 

34. How do you create an instance of an abstract class?

You cannot create an instance of an abstract class since it lacks implementation logic in its methods. You first need to create a subclass that implements all the methods before an object can be initialized. 

35. What is a virtual function?

A virtual function is defined in the parent class and may have definitions implemented. A subclass can override these definitions. 

36. What is a pure virtual function?

A pure virtual function is only declared in the parent class. It is also referred to as an abstract function. Pure virtual functions do not contain any definition in the base class. They must be redefined in the subclass for the implementation needed. 

37. Distinguish between data abstraction and encapsulation.

Data abstraction is the ability to hide unwanted information. 

The encapsulation refers to the ability to hide the data as well as the method together.

38. What are the differences between interfaces and abstract classes?

An abstract class can support both abstract and non-abstract methods. However, the interface allows only abstract methods. 

In the case of an abstract class, both final and non-final variables are supported. However, the interface has variables that are, by default, defined as final. 

The abstract class can have private, and public attributes, but interfaces have attributes as public by default. 

39. What is a final variable?

A final variable means a variable whose value doesn’t change.

40. What is meant by an exception?

An exception is an event raised during a program execution caused by undesirable input or a condition that prevents further processing. An exception causes an interruption in the program’s normal execution and must be handled via exception handling logic to avoid the program’s termination. 

41. Define exception handling

Exception handling refers to the mechanism used for handling the exceptions raised during program execution. It allows for the graceful handling of undesirable results.

42. Is an error basically the same as an exception?

An error means a problem that the program should not catch while the exception implies a condition that should be caught by the program.

43. What is a try-catch block?

A try-catch block is used for exception handling. The set of statements that may cause a potential error are enclosed in a try block. When an exception is raised, it is caught by the catch block. The logic to handle an exception is placed inside the catch block.

44. What is a finally block?

A ‘finally’ block is used for executing essential statements such as to free the memory, close files, or database connections, even if an exception occurs. The finally block always runs. 

45. What is the method ‘finalize’ used for?

The finalize method is called to free the unused resources before the garbage collector gets initiated. 

46. What is a Garbage Collection, and how does it work?

The garbage collection is the ability of the programming language to perform automatic memory management. It automatically frees up the memory by removing the objects that are no longer required.

47. Should you always use Object-oriented programming? Are there any limitations of Object-oriented programming?

Though object-oriented programming offers many advantages, it has some disadvantages too. First of all, it has a steep learning curve compared to procedural programming. It may take a while to get used to thinking and program in terms of objects for many people. Secondly, it may take more experience to design a program in terms of objects. Using OOPs concepts for smaller programming tasks may not be efficient. 

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Conclusion

We hope that you got a good summary of Object-oriented programming concepts. Object-oriented programming takes some time to master. We recommend that you practice a lot to get the fundamentals clear, which will help you answer the questions during an interview. 

We would like to conclude with one trick to help you practice the concept in real life. Whenever you are looking at real-world objects, try to relate them to Object-oriented concepts, and try to identify their attributes, methods. Once you get the hang of this, programming will become much easier and enjoyable.

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