Top 99+ OOPs Interview Questions And Answers in 2024
Here’s a comprehensive list of the top 99 Object-Oriented Programming (OOP) interview questions for 2024, each including a brief explanation. This list covers a wide range of OOP concepts and practices.
1. What is Object-Oriented Programming (OOP)?
Answer: OOP is a programming paradigm based on the concept of objects, which combine data and methods. It aims to improve code reusability, scalability, and maintainability.
2. What are the four main principles of OOP?
Answer: The four main principles are Encapsulation, Abstraction, Inheritance, and Polymorphism.
3. What is a class in OOP?
Answer: A class is a blueprint for creating objects. It defines attributes and methods that the objects created from the class will have.
4. What is an object in OOP?
Answer: An object is an instance of a class. It represents a specific realization of the class with its own values and behaviors.
5. What is encapsulation?
Answer: Encapsulation is the concept of wrapping data and methods into a single unit (a class) and restricting access to some of the object’s components to protect the object’s state.
6. What is abstraction?
Answer: Abstraction is the principle of hiding the complex implementation details and showing only the essential features of an object.
7. What is inheritance?
Answer: Inheritance is a mechanism where a new class (subclass) derives properties and behavior from an existing class (superclass).
8. What is polymorphism?
Answer: Polymorphism allows objects of different classes to be treated as objects of a common superclass, usually through method overriding or method overloading.
9. What is method overloading?
Answer: Method overloading is defining multiple methods in the same class with the same name but different parameters (types or number of parameters).
10. What is method overriding?
Answer: Method overriding occurs when a subclass provides a specific implementation of a method that is already defined in its superclass.
11. What is a constructor in OOP?
Answer: A constructor is a special method called when an object is instantiated. It initializes the object’s state.
12. What is a destructor in OOP?
Answer: A destructor is a special method called when an object is destroyed. It is used to perform cleanup operations and release resources.
13. What is a static method?
Answer: A static method belongs to the class rather than any instance of the class. It can be called without creating an instance of the class.
14. What is a static variable?
Answer: A static variable is shared among all instances of a class and retains its value across instances.
15. What is a class variable?
Answer: A class variable is a variable defined within a class that is shared by all instances of that class.
16. What is a private access modifier?
Answer: The private access modifier restricts access to class members, making them accessible only within the class itself.
17. What is a protected access modifier?
Answer: The protected access modifier allows access to class members within the class itself and its subclasses.
18. What is a public access modifier?
Answer: The public access modifier allows access to class members from any part of the program.
19. What is the difference between a class and an object?
Answer: A class is a blueprint for creating objects, whereas an object is an instance of a class with its own data and behavior.
20. What is an abstract class?
Answer: An abstract class is a class that cannot be instantiated and may contain abstract methods that must be implemented by subclasses.
21. What is an interface?
Answer: An interface is a contract that defines a set of methods without implementing them. Classes can implement interfaces to agree to the contract’s methods.
22. What is the difference between an abstract class and an interface?
Answer: An abstract class can have implemented methods and state, whereas an interface only defines method signatures. A class can implement multiple interfaces but can inherit from only one abstract class.
23. What is a concrete class?
Answer: A concrete class is a class that can be instantiated and provides implementations for all its methods.
24. What is multiple inheritance?
Answer: Multiple inheritance is the ability of a class to inherit features from more than one base class. It is supported differently across programming languages.
25. What is a single inheritance?
Answer: Single inheritance is when a class inherits from only one base class.
26. What is hierarchical inheritance?
Answer: Hierarchical inheritance occurs when multiple subclasses inherit from a single superclass.
27. What is multilevel inheritance?
Answer: Multilevel inheritance is a form of inheritance where a class derives from another class, which itself is derived from another class.
28. What is a dependency injection?
Answer: Dependency injection is a design pattern where an object receives other objects it depends on, rather than creating them internally.
29. What is a factory pattern?
Answer: The factory pattern is a creational design pattern that provides an interface for creating objects without specifying the exact class of the object that will be created.
30. What is a singleton pattern?
Answer: The singleton pattern ensures that a class has only one instance and provides a global point of access to that instance.
31. What is a builder pattern?
Answer: The builder pattern is a creational design pattern that separates the construction of a complex object from its representation, allowing the same construction process to create different representations.
32. What is a prototype pattern?
Answer: The prototype pattern is a creational design pattern that creates new objects by copying an existing object, known as a prototype.
33. What is the observer pattern?
Answer: The observer pattern is a behavioral design pattern where an object, known as the subject, maintains a list of its dependents, known as observers, and notifies them of state changes.
34. What is a strategy pattern?
Answer: The strategy pattern is a behavioral design pattern that defines a family of algorithms, encapsulates each algorithm, and makes them interchangeable.
35. What is a decorator pattern?
Answer: The decorator pattern is a structural design pattern that allows adding new functionality to an object dynamically without altering its structure.
36. What is a command pattern?
Answer: The command pattern is a behavioral design pattern that encapsulates a request as an object, thereby allowing for parameterization of clients with different requests and queuing of requests.
37. What is a mediator pattern?
Answer: The mediator pattern is a behavioral design pattern that defines an object that encapsulates how a set of objects interact, promoting loose coupling by keeping objects from referring to each other explicitly.
38. What is a state pattern?
Answer: The state pattern is a behavioral design pattern that allows an object to alter its behavior when its internal state changes.
39. What is a template method pattern?
Answer: The template method pattern is a behavioral design pattern that defines the skeleton of an algorithm in a base class but lets subclasses redefine certain steps of the algorithm without changing its structure.
40. What is a composite pattern?
Answer: The composite pattern is a structural design pattern that allows you to compose objects into tree-like structures to represent part-whole hierarchies.
41. What is a chain of responsibility pattern?
Answer: The chain of responsibility pattern is a behavioral design pattern where a request is passed along a chain of handlers until one of them handles it.
42. What is an adapter pattern?
Answer: The adapter pattern is a structural design pattern that allows incompatible interfaces to work together by converting the interface of a class into another interface that clients expect.
43. What is a proxy pattern?
Answer: The proxy pattern is a structural design pattern that provides a surrogate or placeholder for another object to control access to it.
44. What is a flyweight pattern?
Answer: The flyweight pattern is a structural design pattern that reduces the cost of creating and manipulating a large number of similar objects by sharing as much data as possible.
45. What is an abstract factory pattern?
Answer: The abstract factory pattern is a creational design pattern that provides an interface for creating families of related or dependent objects without specifying their concrete classes.
46. What is encapsulation used for in OOP?
Answer: Encapsulation is used to protect an object’s internal state from unintended or harmful modifications and to restrict access to certain components.
47. How does inheritance promote code reusability?
Answer: Inheritance allows a new class to inherit attributes and methods from an existing class, promoting code reuse and reducing duplication.
48. What is polymorphism, and how does it enhance flexibility?
Answer: Polymorphism allows objects of different classes to be treated as objects of a common superclass. It enhances flexibility by enabling one interface to be used for a general class of actions.
49. How do abstract classes and interfaces differ in terms of implementation?
Answer: Abstract classes can have method implementations and maintain state, while interfaces only define method signatures without implementation.
50. What are the advantages of using the singleton pattern?
Answer: The singleton pattern ensures that only one instance of a class exists and provides a global point of access to it, reducing memory usage and ensuring consistent behavior.
51. What is the difference between composition and inheritance?
Answer: Composition involves building classes from other classes by including objects of other classes, while inheritance involves creating new classes from existing classes.
52. How do you implement method overloading in Java?
Answer: Method overloading in Java is implemented by defining multiple methods with the same name but different parameter lists within the same class.
53. How do you implement method overriding in Java?
Answer: Method overriding in Java is implemented by providing a new implementation of a method in a subclass that is already defined in its superclass.
54. What is the difference between a shallow copy and a deep copy?
Answer: A shallow copy copies an object’s references to other objects, while a deep copy creates copies of the objects referenced by the original object.
55. What are constructors and destructors used for in C++?
Answer: Constructors are used to initialize objects, while destructors are used to clean up resources and perform necessary cleanup before an object is destroyed.
56. What is the use of the ‘super’ keyword in Java?
Answer: The ‘super’ keyword in Java is used to refer to the superclass of the current object, allowing access to superclass methods and constructors.
57. How does Java handle multiple inheritance?
Answer: Java handles multiple inheritance through interfaces, allowing a class to implement multiple interfaces to achieve similar functionality.
58. What is the use of the ‘this’ keyword in OOP?
Answer: The ‘this’ keyword refers to the current object instance and is used to differentiate between instance variables and parameters with the same name.
59. What is a ‘final’ class in Java?
Answer: A ‘final’ class in Java cannot be subclassed or extended. It is used to prevent inheritance and ensure that the class’s behavior remains unchanged.
60. What is the ‘abstract’ keyword used for?
Answer: The ‘abstract’ keyword is used to define abstract classes and methods that cannot be instantiated or must be implemented by subclasses.
61. What is a ‘sealed’ class in C#?
Answer: A ‘sealed’ class in C# is a class that cannot be inherited. It is used to prevent further extension of the class.
62. How do you define a virtual method in C++?
Answer: In C++, a virtual method is defined using the ‘virtual’ keyword in the base class. It allows derived classes to override the method.
63. What is the role of a ‘virtual destructor’ in C++?
Answer: A ‘virtual destructor’ ensures that the destructor of the derived class is called when deleting an object through a base class pointer, preventing resource leaks.
64. What is a ‘friend’ function in C++?
Answer: A ‘friend’ function in C++ is a function that can access private and protected members of a class, even though it is not a member of that class.
65. What is the purpose of the ‘default’ constructor in C++?
Answer: The ‘default’ constructor is a constructor that takes no arguments and is automatically provided by the compiler if no other constructors are defined.
66. What is an ‘interface’ in the context of multiple inheritance?
Answer: An interface is a contract that classes can implement to achieve multiple inheritance of behavior, allowing a class to adopt multiple interface definitions.
67. How does OOP support code reuse?
Answer: OOP supports code reuse through inheritance, which allows new classes to inherit existing functionality, and through composition, which enables classes to use instances of other classes.
68. What is the difference between ‘composition’ and ‘aggregation’?
Answer: Composition implies a strong lifecycle dependency between the contained and containing objects, whereas aggregation represents a weaker relationship where the contained object can exist independently.
69. What is the ‘diamond problem’ in multiple inheritance, and how is it resolved?
Answer: The diamond problem occurs when a class inherits from two classes that share a common base class, leading to ambiguity. It is resolved in languages like C++ using virtual inheritance and in Java using interfaces.
70. What is the purpose of the ‘new’ operator in C++?
Answer: The ‘new’ operator is used to dynamically allocate memory for objects and initialize them, returning a pointer to the allocated memory.
71. How does C++ handle operator overloading?
Answer: C++ allows operators to be overloaded by defining special member functions or friend functions that specify the new behavior of the operators for user-defined types.
72. What is a ‘namespace’ in C++ and its purpose?
Answer: A namespace in C++ is a container that allows organizing code into logical groups to avoid name conflicts and improve code readability.
73. What is the ‘protected’ access modifier used for?
Answer: The ‘protected’ access modifier allows access to class members within the class itself and its subclasses, but not from outside the class hierarchy.
74. What is the ‘final’ keyword in C++?
Answer: In C++, the ‘final’ keyword is used to prevent a class from being inherited or a method from being overridden, providing a way to finalize class behavior.
75. How do you implement encapsulation in Java?
Answer: Encapsulation in Java is implemented by defining private variables and providing public getter and setter methods to access and modify the variables.
76. What is the ‘instanceof’ operator in Java used for?
Answer: The ‘instanceof’ operator checks if an object is an instance of a specific class or implements a specific interface, helping to ensure type safety.
77. How does Java support multiple inheritance?
Answer: Java supports multiple inheritance through interfaces, allowing a class to implement multiple interfaces, but not multiple classes.
78. What is ‘duck typing’ in dynamic languages like Python?
Answer: Duck typing is a concept in dynamic languages where an object’s suitability is determined by the presence of certain methods and properties, rather than the actual type of the object.
79. What is ‘composition over inheritance’?
Answer: Composition over inheritance is a design principle where a class is composed of objects from other classes, rather than inheriting from them, to achieve greater flexibility and reuse.
80. How does ‘dependency inversion’ principle contribute to OOP?
Answer: The dependency inversion principle suggests that high-level modules should not depend on low-level modules but on abstractions, which helps to reduce coupling and increase flexibility.
81. What is ‘inheritance hierarchy’?
Answer: Inheritance hierarchy refers to the arrangement of classes in a tree-like structure where subclasses inherit from parent classes, forming a hierarchy of relationships.
82. What are ‘design patterns’ and how do they relate to OOP?
Answer: Design patterns are reusable solutions to common problems in software design. They leverage OOP principles to provide standardized approaches for solving specific design issues.
83. What is the difference between ‘deep’ and ‘shallow’ copy in OOP?
Answer: A shallow copy duplicates an object’s top-level structure, while a deep copy duplicates the entire object along with any objects it references, creating a fully independent copy.
84. How does the ‘Liskov Substitution Principle’ apply to OOP?
Answer: The Liskov Substitution Principle states that objects of a superclass should be replaceable with objects of a subclass without affecting the correctness of the program.
85. What is ‘interface segregation’ in OOP?
Answer: Interface segregation is a principle that suggests clients should not be forced to depend on interfaces they do not use. It advocates for creating smaller, more specific interfaces.
86. How does ‘open/closed principle’ benefit software design?
Answer: The open/closed principle states that software entities should be open for extension but closed for modification, allowing new functionality to be added without altering existing code.
87. What is the ‘composition’ relationship in UML diagrams?
Answer: In UML diagrams, composition represents a strong “whole-part” relationship where the lifecycle of the part is dependent on the whole, usually depicted with a filled diamond.
88. What is the ‘aggregation’ relationship in UML diagrams?
Answer: Aggregation represents a weaker “whole-part” relationship where the part can exist independently of the whole, typically depicted with an empty diamond.
89. What is ‘class responsibility collaborator’ (CRC) cards?
Answer: CRC cards are a brainstorming tool used to define and document the responsibilities of classes and their interactions with other classes in object-oriented design.
90. How does the ‘SOLID’ acronym relate to OOP principles?
Answer: SOLID stands for five principles: Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion, which guide the design of maintainable and scalable software.
91. What is ‘code smell’ and how does it relate to OOP?
Answer: Code smell refers to indicators of potential problems in code, such as poor design or implementation issues. In OOP, addressing code smells often involves refactoring to adhere to OOP principles.
92. What is ‘object composition’ and how does it differ from inheritance?
Answer: Object composition involves building classes using instances of other classes to achieve functionality, whereas inheritance involves creating new classes based on existing ones.
93. What is the ‘principle of least astonishment’ in OOP design?
Answer: The principle of least astonishment suggests that software should behave in a way that is least surprising to users, ensuring that it aligns with their expectations and understanding.
94. How does ‘dynamic dispatch’ work in OOP?
Answer: Dynamic dispatch is a mechanism where the method that gets called is determined at runtime based on the actual object’s type, allowing for method overriding and polymorphism.
95. What is a ‘factory method’ design pattern?
Answer: The factory method pattern defines an interface for creating objects but lets subclasses alter the type of objects that will be created, promoting loose coupling and easier maintenance.
96. What is a ‘singleton’ design pattern?
Answer: The singleton pattern ensures that a class has only one instance and provides a global point of access to that instance, often used for managing resources like database connections.
97. What is the ‘strategy’ design pattern?
Answer: The strategy pattern defines a family of algorithms, encapsulates each algorithm, and makes them interchangeable, allowing clients to choose the appropriate algorithm at runtime.
98. What is the ‘observer’ design pattern?
Answer: The observer pattern defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
99. What is the ‘decorator’ design pattern?
Answer: The decorator pattern allows behavior to be added to individual objects, either statically or dynamically, without affecting the behavior of other objects from the same class.
Other Important Q&A List :
- OOPs Interview Questions And Answers
- SQL Interview Questions and Answers
- Content Writing Interview Questions and Answers
- Email Marketing Interview Questions and Answers
- Django Interview Questions and Answers
- C Programming Interview Questions and Answers
- ReactJS Interview Questions and Answers
- Angular Interview Questions and Answers
- Power BI Interview Questions and Answers
- Top DBMS Interview Questions and Answers
- Python Interview Questions and Answers
- MySQL Interview Questions and Answers
- Java Interview Questions and Answers
- Data Structures Interview Questions
- Flutter Interview Questions and Answers
- JavaScript Interview Questions and Answers
- PHP Interview Questions and Answers
- CSS Interview Questions and Answers
- HTML Interview Questions and Answers
