Top 50 OOP (Object Oriented Programming) Interview Questions for Java Developers

These are the frequently asked OOP Interview questions for Java developers

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Hello guys, if you are preparing for Java interviews and need some questions to revise Object Oriented concepts then you have come to the right place. In the past, I have shared the best OOP books and courses as well multiple Java interview questions and in this article, I am going to share 50+ object-oriented programming concept questions for you.

OOP Interview question or Object-oriented programming interview question is an integral part of any Java interview. Since Java is an Object-oriented programming language, it’s expected from Java developers that he is good in Object-oriented analysis and design and familiar with essential OOP concepts like Abstraction, Encapsulation, and Polymorphism.

OOP, Interview question in Java is mainly based around fundamental OOPS concepts, and How those are implemented in Java, like Abstraction OOP concept is implemented using an interface and abstract class, Encapsulation is using private keyword, etc.

Question from OOP is also asked as part of the Java design pattern question on the Senior level Java interview. On Freshers and Beginners level interview, OOP Interview Questions are mostly based on fundamentals only with some tricky Java questions like Why Java doesn’t support multiple inheritances, etc.

In this Java article, we will see some frequently asked OOP interview questions and answers.

Some question is very fundamental, and some Object-oriented programming question is difficult to answer, but this mix helps to learn more. If you love to learn more about the OOPS design principle, read 10 OOP design principles for Java programmers.

10 OOP Design Principles You Can Learn in depth

66 Java OOP Concepts Interview Question and Answers

without wasting any more of your time, here is my list of frequently asked Object-Oriented Programming questions for Java programmers. These questions have been collected from many Java interviews, both telephonic and face-bot-face from different levels of Java programmers.

1. What is Class in Object-oriented programming? (answer)
   A class is a blueprint to create objects.
2. What is Object in OOP? (answer)
   Created from classes, represent a particular state of the class.
3. What is the abstraction in Java? (answer)
   An OOP technique to hide complexities from clients.
4. What is Inheritance in Java? (answer)
   An object-oriented technique to reuse code and functionalities.
5. What is Encapsulation or data hiding in Java? (answer)
   An oop way to hide data so that you can change it later without impacting others.
6. What is Polymorphism in Java or OOP? (answer)
   provides flexibility to choose different codes to run at runtime.
7. What is the difference between Polymorphism, Overloading, and Overriding? (answer)
8. Why Java doesn’t support Multiple Inheritance in Java? (answer)
9. When do you use interface and abstract class in Java?(Answer)
10. What is the difference between static and dynamic binding in Java? (answer)
11. What is the difference between abstraction and polymorphism in Java?** (answer)
12. What is the difference between the IS-A relationship and HAS-A? (answer)
    IS-A represents Inheritance and HAS-A represents composition.
13. Why Java doesn’t support operator overloading? (answer)
14. What is an abstract class in Java? (answer)
15. What is an interface in Java? (answer)
16. What is the difference between Inheritance and Composition? (answer)
17. What is the difference between coupling and cohesion? (answer)
    coupling is the dependency between different parts of code while cohesion is about the same part of code.
18. What is a constructor in Java? (answer)
    A special construct that creates objects.
19. What is the difference between Class and Object in Java or OOP? (answer)
20. What is the difference between Inheritance and Polymorphism in Java? (answer)
21. What are SOLID Design Principles? Explain any three of them (answer)
22. What is the difference between Factory and Abstract Factory design patterns? (answer)
23. What is the difference between Overloading, hiding, shadowing and Overriding in OOP? (answer)
24. What is the difference between instance and object in Java? (answer)
25. What is the difference between static and dynamic binding in Java? (answer)
26. What are SOLID Object-oriented principles?** (answer)
27. Difference between Abstract Class and Interface in Java? (answer)
28. Difference between private, protected, and public modifiers in Java? (answer)
29. What is constructor chaining? (answer)
30. Difference between pass by value and pass by reference? (answer)
31. Difference between abstraction and encapsulation? (answer)
32. Difference between association, composition, and aggregation? (answer)
33. Can you explain Open Closed Design Principle? (answer)
34. What is an Observer design pattern? When should you use it? (answer)
35. What is the difference between hiding and shadowing in OOP?** (answer)
36. Can you override a static method in Java? (answer)
37. What is the difference between state and strategy design patterns? (answer)
38. What is the difference between a class and an instance? (answer)
39. What is the difference between Factory and Abstract Factory design patterns? (answer)
40. What is the difference between Dependency injection and Factory Pattern? (answer)
41. What is method overloading in OOP or Java? (answer)
42. What is the method overriding in OOP or Java?** (answer)
43. Is Java a pure object-oriented language? if not why? (answer)
44. What are the rules of method overloading and overriding in Java?** (answer)
45. The difference between method overloading and overriding? (answer)
46. Can we overload a static method in Java? (answer)
47. Can we override the static method in Java?** (answer)
48. Can we override a private method in Java?** (answer)
49. What is the covariant method overriding in Java?** (answer)
50. Can we change the argument list of an overriding method?** (answer)
51. Can we override a method that throws runtime exception without throws clause? (answer)
52. Can we override the final method in Java? (answer)
53. What is the default method of Java 8?** (answer)
54. What is an abstract class in Java? (answer)**
55. What is an interface in Java? What is the real use of an interface?** (answer)
56. What are the differences between Abstract class and interface?** (answer)
57. Can we make a class abstract without an abstract method?** (answer)
58. Can we make a class both final and abstract at the same time?** (answer)
59. Can we overload or override the main method in Java?** (answer)
60. What problem is solved by the Strategy pattern in Java?** (answer)
61. Which OOP concept Decorator design Pattern is based upon?** (answer)
62. When to use the Singleton design pattern in Java?** (answer)
63. What is the difference between State and Strategy Patterns?** (answer)
64. What is the difference between Association, Aggregation, and Composition in OOP? (answer)
65. What is the difference between Decorator, Proxy, and Adapter patterns in Java? (answer)
66. What is the difference between Composition and Inheritance in OOP?(answer)

That’s all about some of the frequently asked OOP Interview Questions for Java Programmers. Most likely you already know answers to these fundamental Object-oriented programming, design, and pattern-related questions.

If you don’t see the links and revise those concepts. You can use this list to revise all key object oriented programming concepts before your interview. If you think a popular OOP question is missing from the list, feel free to share in the comments and I will include it in this list.

Most of these questions are also useful for JavaScript and Python developers because both JavaScript and Python are also object-oriented programming languages.

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Thanks for reading this article so far. If you find these Object-Oriented Programming interview questions useful then please share them with your friends and colleagues. If you have any questions or feedback then please drop a note.

P. S. — If you are new to object oriented programming and need some resources to learn OOP then you can also check-out these free object oriented programming courses to start with.

Top 5 Free Object-Oriented Programming Courses for Java Programmers in depth [UPDATED]