Object Oriented Programming (OOPs) Interview Questions

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Introduction

Object-Oriented Programming is a programming paradigm based on the concept of “objects”, which can contain data and code. The data is in the form of fields, and the code is in the form of procedures. A common feature of objects is that procedures are attached to them and can access and modify the object’s data fields. Let’s see some of the interview questions of OOPs.

Interview Questions and Answers

1. Q: What is object-oriented programming (OOP)?
A: Object-oriented programming is a programming paradigm that organizes data and behaviors into reusable structures called objects.

2. Q: What are the four fundamental principles of OOP?
A: The four fundamental principles are encapsulation, inheritance, polymorphism, and abstraction.

3. Q: What is encapsulation?
A: Encapsulation is the process of hiding internal implementation details of an object and providing a public interface to interact with it.

4. Q: Explain inheritance.
A: Inheritance allows a class to inherit properties and behaviors from another class, enabling code reuse and creating a hierarchical relationship between classes.

5. Q: What is polymorphism?
A: Polymorphism allows objects of different classes to be treated as objects of a common superclass. It enables methods to be overridden in subclasses and invoked at runtime.

6. Q: What is abstraction?
A: Abstraction is the process of creating simplified models of complex systems by focusing on essential characteristics while hiding unnecessary details.

7. Q: What is a class?
A: A class is a blueprint or template for creating objects. It defines the properties and behaviors that objects of that class will have.

8. Q: What is an object?
A: An object is an instance of a class. It has state (data) and behavior (methods).

9. Q: What is a constructor?
A: A constructor is a special method that is used to initialize an object of a class. It has the same name as the class and is called when an object is created.

10. Q: Explain method overloading.
A: Method overloading is the ability to define multiple methods with the same name but different parameters. The appropriate method is chosen based on the number and types of arguments.

11. Q: What is method overriding?
A: Method overriding is the ability to provide a different implementation of a method in a subclass, which has the same signature as the method in the superclass.

12. Q: What is the difference between an abstract class and an interface?
A: An abstract class can have both defined and undefined methods, while an interface can only have undefined methods. A class can inherit from multiple interfaces, but only one abstract class.

13. Q: What is a static method?
A: A static method belongs to the class rather than an instance of the class. It can be called without creating an object of the class.

14. Q: What is the ‘this’ keyword?
A: The ‘this’ keyword refers to the current object instance within a class. It is used to differentiate between instance variables and local variables with the same name.

15. Q: What is method hiding?
A: Method hiding occurs when a subclass defines a static method with the same name and signature as a static method in the superclass. The method in the superclass is no longer accessible through the subclass.

16. Q: What is a final class?
A: A final class cannot be subclassed. It is often used to create immutable classes or classes with a fixed implementation that should not be modified.

17. Q: What is composition?
A: Composition is a design technique where a class is composed of objects of other classes. It allows for code reuse and promotes a “has-a” relationship between classes.

18. Q: What is an abstract method?
A: An abstract method is a method that is declared in an abstract class or interface but does not have an implementation. It must be overridden in a subclass or implemented in a class that implements the interface.

19. Q: What is the difference between shallow copy and deep copy?
A: Shallow copy creates a new object that references the same memory as the original object, while deep copy creates a new object with its own copy of the data.

20. Q: What is the purpose of the ‘super’ keyword?
A: The ‘super’ keyword is used to refer to the superclass or parent class. It can be used to invoke the superclass constructor or access methods and fields of the superclass.

21. Q: What is method chaining?
A: Method chaining is a technique where multiple methods are called in a single line by returning the object itself after each method call. It allows for concise and fluent code.

22. Q: What is the difference between an instance variable and a class variable?
A: An instance variable belongs to each instance of a class and has a separate copy for each object. A class variable is shared among all instances of the class and is accessed using the class name.

23. Q: Explain the concept of interfaces.
A: Interfaces define a contract that classes must adhere to. They contain abstract methods that must be implemented by any class that implements the interface.

24. Q: What is multiple inheritance?
A: Multiple inheritance is the ability to inherit properties and behaviors from multiple classes. Some programming languages support it, while others provide alternatives like interfaces.

25. Q: What is method visibility or access modifiers?
A: Access modifiers define the visibility of classes, methods, and variables. The common access modifiers are public, private, protected, and default (package-private).

26. Q: What is method overloading and overriding?
A: Method overloading is having multiple methods with the same name but different parameters in the same class. Method overriding is providing a different implementation of a method in a subclass.

27. Q: What is the difference between an instance method and a static method?
A: An instance method operates on an instance of a class and can access instance variables. A static method belongs to the class itself and cannot access instance variables.

28. Q: What is a destructor?
A: A destructor is a special method that is automatically called when an object is no longer needed or when it goes out of scope. It is used for cleanup tasks.

29. Q: What is a design pattern?
A: A design pattern is a reusable solution to a commonly occurring problem in software design. It provides a template for solving specific design problems.

30. Q: What is the purpose of the ‘final’ keyword?
A: The ‘final’ keyword is used to make a class, method, or variable unchangeable or non-extendable.

31. Q: What is the difference between a stack and a queue?
A: A stack is a Last-In-First-Out (LIFO) data structure, while a queue is a First-In-First-Out (FIFO) data structure.

32. Q: What is the difference between composition and inheritance?
A: Composition is a “has-a” relationship where a class contains objects of other classes. Inheritance is an “is-a” relationship where a class inherits properties and behaviors from another class.

33. Q: What is method hiding and method overriding?
A: Method hiding occurs when a subclass defines a static method with the same name as a static method in the superclass. Method overriding occurs when a subclass provides a different implementation of a method defined in the superclass.

34. Q: What is the purpose of the ‘protected’ access modifier?
A: The ‘protected’ access modifier allows access to the member within the same package or subclass of the defining class.

35. Q: What is the difference between early binding and late binding?
A: Early binding (static binding) is the process of resolving method calls at compile-time. Late binding (dynamic binding) is the process of resolving method calls at runtime.

36. Q: What is the ‘instanceof’ operator used for?
A: The ‘instanceof’ operator is used to check if an object is an instance of a particular class or implements a specific interface.

37. Q: What is the difference between an abstract class and an interface?
A: An abstract class can have both defined and undefined methods, while an interface can only have undefined methods. A class can implement multiple interfaces but can only inherit from a single abstract class.

38. Q: What is the purpose of the ‘volatile’ keyword?
A: The ‘volatile’ keyword is used to indicate that a variable may be modified by multiple threads and should not be cached, ensuring that each thread sees the updated value.

39. Q: What is a factory method?
A: A factory method is a method that creates and returns instances of a class. It provides an interface for creating objects while hiding the object creation logic.

40. Q: What is a Singleton pattern?
A: The Singleton pattern ensures that a class has only one instance and provides a global point of access to it.

41. Q: What is a class diagram?
A: A class diagram is a visual representation of the classes, their relationships, and the structure of a system. It depicts the attributes, methods, and associations between classes.

42. Q: What is the difference between shallow copy and deep copy?
A: Shallow copy creates a new object with references to the same data as the original object, while deep copy creates a new object with its own copy of the data.

43. Q: What is the purpose of the ‘default’ keyword in Java interfaces?
A: The ‘default’ keyword is used to define a default implementation for a method in an interface. It allows adding new methods to an existing interface without breaking the implementing classes.

44. Q: What is the Liskov Substitution Principle?
A: The Liskov Substitution Principle states that objects of a superclass should be replaceable with objects of its subclasses without affecting the correctness of the program.

45. Q: What is the Open-Closed Principle?
A: The Open-Closed Principle states that software entities (classes, modules, etc.) should be open for extension but closed for modification. It promotes the use of interfaces and abstraction.

46. Q: What is the Single Responsibility Principle?
A: The Single Responsibility Principle states that a class should have only one reason to change. It promotes separation of concerns and modular design.

47. Q: What is the Dependency Inversion Principle?
A: The Dependency Inversion Principle states that high-level modules should not depend on low-level modules. Both should depend on abstractions. It promotes loose coupling and easier maintainability.

48. Q: What is the difference between composition and aggregation?
A: Composition implies a strong relationship where the lifetime of the contained object is controlled by the container. Aggregation implies a weaker relationship where the contained object can exist independently.

49. Q: What is the SOLID principle?
A: SOLID is an acronym for five principles of software design: Single Responsibility Principle, Open-Closed Principle, Liskov Substitution Principle, Interface Segregation Principle, and Dependency Inversion Principle.

50. Q: What is the purpose of the ‘interface’ keyword in Java?
A: The ‘interface’ keyword is used to declare an interface in Java. An interface defines a contract that classes must adhere to and can be implemented by multiple classes.

 



  • July 10, 2023