55 object-oriented programming questions for developers

If you’re a human resources manager, recruiter, or hiring manager used to hiring developers, you’ll know that object-oriented programming (OOP) is one of the most popular programming paradigms. 

And indeed, hiring a talented OOP programmer can add great value to an organization.

Finding the right talent – i.e. professionals who have experience with different programming languages (e.g., Python, PHP, etc.) – has never been more important. It’s why it’s so important that you ask the right object-oriented programming interview questions and know how to evaluate answers. This will enable you to hire only the best candidates. 

Plus, with a skills-testing platform like TestGorilla, you can make your life easier by using a skills test to measure applicants’ abilities and knowledge even before the interview phase.

Candidates who perform well on our Object-oriented Programming test will allow your business to build programs efficiently while providing quality and speed to end-users.

Or, otherwise said, if you use skills tests and interviews you’ll have a winning combination enabling you to hire the best talent.

In this article, we’ve compiled a list of 55 object-oriented programming interview questions to include in your next assessment. And we’ve provided sample answers to help you pick the best candidates. 

1. Explain what object-oriented programming is.

Object-oriented programming, often abbreviated to OOP, is a programming paradigm that uses objects instead of procedures and functions. Programmers can group objects into classes to implement polymorphism, data hiding, inheritance, and so on.

2. List the core features of object-oriented programming.

The core features of object-oriented programming are:

• Data Abstraction

• Inheritance

• Polymorphism

• Encapsulation

3. Define the term “object” in object-oriented programming.

An object is a basic unit of object-oriented programming, and can be a combination of functions, variables, or data structures. Different objects will have different behaviors, states, or attributes.

4. Define class in object-oriented programming.

A class is made up of objects that have several common methods but have different behaviors and are in different states. 

5. List the differences between a class and an object.

A class is different from an object in the following ways:

• A class is a logical entity, while an object is a real-world, physical entity 

• When you create a class, it will not take up memory space, whereas an object will

• A class will bind data and methods into one unit, while an object will act like a class variable

• A class is declared once, whereas an object can be declared as often as required 

6. Explain the difference between class and structure.

A class is made up of methods or instructions to be performed on objects. A structure is a collection of variables that are user-defined.

7. Define data abstraction.

Data abstraction is a crucial aspect of object-oriented programming and refers to the ability to hide implementation details with only important information visible. 

8. Explain the term “constructor”.

In object-oriented programming, a constructor is a method used to initialize objects in a given class. It’s special because it shares its name with the class. 

9. What are the five types of constructors?

The five types of constructors in OOP are: 

• Default 

• Private 

• Copy 

• Static 

• Parameterized 

10. Define what an exception is.

In object-oriented programming, an exception is a notification that cuts short the program execution and sends the error for a resolution to the exception handler.

11. Explain what virtual functions are.

Virtual functions are used to produce runtime polymorphism. They are overridden by the subclass and present in the parent class. 

12. Explain how you would accomplish data abstraction. 

You can accomplish data abstraction via:

• Abstract class

• Abstract method

13. Define encapsulation.

Encapsulation happens when data and code are bound together into a single unit. An example of this is a class. You can specify data in one class and hide it from other classes, which is known as data hiding.

14. Explain the differences between structured programming and object-oriented programming.

Structured programming and object-oriented programming are different in a number of important ways: 

• With structural programming, programmers divide programs into functions and procedures, while object-oriented programming is based on objects

• Structured programming uses a top-down approach, while object-oriented uses the opposite, bottom-up approach

• Structured programming does not allow for data hiding, while object-oriented programming does

• Structured programming can only solve problems of moderate complexity, while object-oriented programming can solve complex problems 

• Reusing code is not possible with structured programming; object-oriented programming, on the other hand, allows it, reducing redundancy

15. Define static polymorphism. 

Static polymorphism is static binding, or a type of polymorphism that happens at compile time (e.g., method overloading).

16. Define dynamic polymorphism.

Dynamic polymorphism is dynamic binding, also known as runtime polymorphism, and resolves during runtime (e.g., method overriding).

17. Define hybrid inheritance.

Hybrid inheritance is the merger of multilevel inheritance and multiple inheritance.

18. Define hierarchical inheritance. 

In object-oriented programming, hierarchical inheritance happens when a base class has two or more subclasses.

19. Explain why you would use object-oriented programming over structured programming.

There are several reasons why OOP might make more sense than structured programming for a specific purpose. For example: 

• Object-oriented programming can solve complex problems, while structural programming can only solve simple or moderate-complexity problems

• You can hide data with object-oriented programming, which can be crucial if  confidentiality is a concern

• You can divide problems into separate parts, which makes them easier to solve

• You can reuse code (via inheritance) which means you can reduce redundancy

• You can bind code and data together via encapsulation

• You can allow entities to have more than one form (which is known as polymorphism); this makes object-oriented programming more flexible

20. For what purpose would you use a try/catch block?

You would use this for handling exceptions. The try block defines possible errors (via a set of statements), while the catch block catches the exceptions.

21. Explain what a finally block is.

In object-oriented programming, a finally block executes a critical code (e.g., closing a connection) as soon as the try block exits. It will also execute even if there is an unexpected exception.

22. What is finalize used for?

You can utilize the finalize method to clean up and release unmanaged resources before garbage collection (GC). 

23. Explain what a destructor is.

In object-oriented programming, a destructor is an automatically-invoked method that happens if there is object destruction. Additionally, it will close the database connections and files of the object, recover the space allotted, and so on. 

24. Define access specifiers. 

In OOP, access specifiers regulate the accessibility of classes or methods to allow encapsulation implementation. They are also known as access modifiers. 

There are three commonly used access specifiers: private, protected, and public. Depending on which programming language you use, additional access specifiers might exist.

25. Explain what method overloading is. 

Object-oriented programming enables you to allocate the same name to several methods inside a class so long as it passes different arguments. This is known as method overloading.

26. What is method overriding?

In object-oriented programming, method overriding allows the subclass/child class to redefine methods inside the parent/base class. 

27. Define subclass.

Also known as child class, a subclass is a class that inherits attributes from a base or parent class.  

28. Define inheritance. 

In object-oriented programming, inheritance is the process in which a class inherits common attributes from another class. The purpose is to eliminate redundant code, which helps reduce code size.

29. Explain the differences between private, protected, and public access modifiers. 

The three types of access modifiers are defined, as follows: 

• Private access modifiers make a method or property accessible by its own class, but not by the derived class or from anywhere else

• Protected access modifiers mean that the method or property are accessible by their own and derived classes, but not from everywhere else

• Public access modifiers make methods or properties accessible by own and derived classes as well as from everywhere else

30. What is the difference between a method and a class?

In object-oriented programming, a method is a set of callable instructions to be performed on data. A class is a template that connects data and code into one unit. 

31. List the limitations of object-oriented programming

Object-oriented programming has several limitations: 

• It needs appropriate planning

• You can’t use it on small problems

• It needs in-depth testing 

• Problems take longer to solve

32. List the different inheritance types.

The different inheritance types are: 

• Single 

• Multiple 

• Hierarchical 

• Multilevel 

• Hybrid 

33. Define interface in OOP.

This object-oriented programming paradigm lets you announce methods without the need to define them. Unlike classes, which are blueprints, interfaces are not blueprints as they do not have comprehensive actions or instructions to be performed. 

34. Explain what a copy constructor is.

In OOP, copy constructors generate new objects from existing ones. They do this by copying variables of objects in the same class. 

35. Explain what operator overloading is. 

Operator overloading refers to a type of polymorphism, in which different operators have different implementations depending on user-defined types and arguments.

36. What is the difference between overriding and overloading?

Overriding is when the name and parameters (method signature) are the same in both the child class and the superclass. Overloading is when more than one method within the same class has different parameters but has the same names. Also, you resolve overriding during runtime, whereas overloading is resolved during compile time. 

37. Define abstract class. 

In object-oriented programming, an abstract class is a class made up of abstract methods which are declared. However, these methods are not defined. If you wish to use abstract methods in a subclass, you need to define the subclass exclusively. 

38. How would you create an instance of an abstract class?

You can’t. These instances don’t have a complete implementation. Therefore, you can’t create an instance of an abstract class. 

39. Explain the difference between an exception and an error. 

An exception pinpoints conditions that an application should catch. An error indicates a serious problem; an application should not encounter errors.

40. What’s the difference between multilevel inheritance and multiple inheritance?

Multilevel inheritance is when a class (which is a subclass) inherits from another class. Multiple inheritance is when a class inherits two or more base classes.

41. Define polymorphism

Polymorphism is the quality of existing in more than one form. This means you can give numerous definitions to one interface. 

42. What’s the difference between encapsulation and data abstraction?

There are two main differences between encapsulation and data abstraction: 

• Encapsulation solves problems at implementation, while data abstraction solves problems at design

• Encapsulation is when data and code are bound together into one unit and hidden, while data abstraction shows important aspects but hides implementation details

43. Define exception handling.

Exception handling is a crucial aspect of OOP when used for managing errors. The role of an exception handler is to catch errors and resolve them by implementing a centralized mechanism. 

44. List all inheritance limitations.

Inheritance has several limitations, because: 

• It tightly couples the parent and child class

• It needs careful implementation so as not to create incorrect results

• Implementing program modifications means changing both parent and child classes

• It involves jumping from one class to another, so it takes time and effort to execute a program

45. What are pure virtual functions?

Pure virtual functions (also known as abstract functions) don’t contain any definitions and are only declared in the base class. Redefinition is necessary in the subclass. 

46. Define garbage collection (GC).

Garbage collection is the process of freeing up space taken up by objects that no longer exist. It’s a form of automated management of memory. 

47. Define superclass.

A superclass, also known as base class, is the parent class to one or more other classes.

48. What are the differences between an interface and an abstract class?

The differences between an interface and an abstract class is as follows: 

• Regarding data members’ accessibility, an interface is public by default, while an abstract class can be public, private, or protected

• Regarding methods, an interface has abstract methods only, while an abstract class can have abstract and other methods

• Regarding implementation, an interface can’t provide the implementation of an abstract class, while an abstract class can provide the implementation of an interface

• Regarding final variables, an interface declares variables as final by default, while an abstract class can contain non-final and final variables

49. Explain what a final variable is. 

A final variable is a variable where the value doesn’t change. 

50. What is the name of the developer who developed the first OOP language?

Alan Kay. He is considered the godfather of object-oriented programming by many.

51. Name the programming language that doesn’t support polymorphism.

The Ada language does not support polymorphism.

52. How much memory does a class consume?

This is a trick question, because classes do not take up any memory. Classes are just blueprints used to create objects. Only when you create objects is any memory consumed. 

53. What was the first programming language developed as an OOP language?

Smalltalk was the first OOP language created in the 1970s.

54. Which popular OOP language doesn’t support every inheritance type? 

Java does not support multiple or hybrid inheritance. It only supports single, multilevel, and hierarchical inheritance.  

55. Name some popular OOP languages.

Some of the most widely-used OOP languages are: 

• C++

• Python

• PHP

• Java

• Javascript

Recruit the best OOP developers by asking the right questions

Object-oriented programming is growing in demand, because it solves real-world problems better. Code is shorter and therefore easier to write, read, and understand, as well as to modify, debug, and maintain. In addition to that, OOP languages make it simpler to create large, complex programs. 

For all these reasons, finding developers who have the right experience and knowledge of OOP languages and principles is key for any organization that uses object-oriented programming to achieve its software development goals. 

One way of identifying the best talent is to ask the right object-oriented programming interview questions. We’ve provided a comprehensive list above to help you prepare for interviews with your candidates.

But before you even start interviewing applicants, we advise you to use skills testing.

Assess your candidates’ expertise at the start of your recruitment process to eliminate the need to screen resumes and make your hiring process more objective and fair. And, what’s more, thorough skills testing helps you save both time and money.

For the best results, you can use our Object-oriented Programming test and combine it with other programming tests for the programming language(s) required for the role, such as a C# Data Structures test, Java Data Structures test, .NET test, and more. 

With TestGorilla as your testing and assessment partner, you’ll be able to quickly recruit gifted OOP developers to help you achieve your organization’s objectives.

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