Top 50 Oops Interview Questions and Answers to Prepare

If you are preparing for an Object-Oriented Programming (OOP) interview, it is essential to be well-versed in OOP concepts. OOP is a fundamental paradigm in modern programming and is widely used in languages like Java, C++, Python, and many others which you can learn through programming courses. This article explores the important Java OOPs interview questions with answers. These OOPs interview questions and answers are specifically focused on freshers as well as experienced individuals to help them prepare for their next interviews by improving their reasoning skills to explore lucrative careers in programming.

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OOPs Interview Questions and Answers

Q1. What is Object-Oriented Programming (OOP)?

Ans: This is one of the basic OOPs interview questions and answers to be prepared for. Object-Oriented Programming (OOP) is a programming paradigm that organises code into objects, which are instances of classes. It focuses on encapsulating data and behaviour together, allowing for modularity and reusability in code.

Q2. What are the four pillars of OOP?

Ans: This is another one of the basic OOPs interview questions for freshers. The four pillars of OOP are:

• Encapsulation: Bundling data and methods that operate on that data together within a class.

• Abstraction: Hiding unnecessary implementation details and exposing only relevant features to the outside world.

• Inheritance: Allowing a class to inherit properties and behaviours from another class.

• Polymorphism: The ability of objects to take on multiple forms and behave differently based on their context.

Q3. What is a class in Java?

Ans: This is one of the top OOPs interview questions and answers for freshers. A class in Java is a blueprint or template that defines the structure and behaviour of objects. It encapsulates data members (attributes) and member functions (methods).

Q4. What is an object?

Ans: An object is an instance of a class. It represents a real-world entity or concept and has its own unique state and behaviour. This one of the frequently asked Java OOPs interview questions and answers must be included in your preparation list.

Q5. What is the difference between a class and an object?

Ans: A class is a blueprint or template that defines the structure and behaviour, whereas an object is an instance of that class, representing a specific entity with its own unique state and behaviour.

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Q6. How is encapsulation achieved in OOP?

Ans: This is amongst the top OOPs interview questions and answers for freshers. Encapsulation is achieved in OOP by making data members private and providing public methods (getters and setters) to access and modify the data.

Q7. What is abstraction in OOP?

Ans: Abstraction is the process of hiding the implementation details of an object and exposing only the relevant features to the outside world. Abstract classes and interfaces are used to achieve abstraction in Java.

Q8. Explain inheritance in Java.

Ans: Inheritance is a mechanism in Java where a class (subclass/derived class) can acquire the properties and behaviours of another class (superclass/base class). It promotes code reuse and supports the "is-a" relationship.

Q9. What is the difference between interface and abstract class?

Ans: To ace your OOPs interview, you must prepare for this best Java OOPs interview question. An abstract class can have both abstract and concrete methods and have instance variables, while an interface can only have abstract methods and constants. A class can implement multiple interfaces, but it can extend only one abstract class.

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Q10. How is polymorphism achieved in Java?

Ans: Polymorphism in Java is achieved through method overloading and method overriding. Method overloading is having multiple methods with the same name but different parameters, and method overriding is providing a specific implementation for a method in a subclass that is already defined in its superclass.

Q11. What is method overloading?

Ans: Method overloading is a feature in Java where multiple methods in the same class can have the same name but different parameter lists (number or types of parameters). This is one of the basic OOPs interview questions for experienced individuals as well as freshers to prepare for.

Q12. What is method overriding?

Ans: Method overriding is a feature in Java where a subclass provides a specific implementation for a method that is already defined in its superclass, using the same method signature. This Java OOPs interview question must be included in your preparation list.

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Q13. What are access modifiers in Java?

Ans: Java access modifiers are keywords that control a programme's classes, methods, and variables' visibility and accessibility. They are essential in regulating the degree of access and isolation of programme elements. Access modifiers (e.g., public, private, protected, and default) control the visibility of classes, methods, and variables in Java. This kind of OOPs interview questions for freshers must be in your preparation list.

Q14. What is the default access modifier in Java?

Ans: The default access modifier, also known as package-private, allows access to classes, methods, and variables within the package but prohibits access from outside of that package. By limiting visibility to only the code components included in its package, it offers a certain amount of encapsulation, improving code organisation and security.

Q15. Can you create an instance of an abstract class in Java?

Ans: No, it cannot be created as an instance of an abstract class directly because abstract classes are incomplete and need to be extended by concrete subclasses. This is amongst the most asked OOPs interview questions for experienced professionals.

Q16. What are constructor and destructor in Java?

Ans: In Java, constructors are special methods used to initialise objects. Java does not have destructors like some other programming languages; instead, it relies on garbage collection to manage memory.

Q17. What is the purpose of the “this” keyword in Java?

Ans: The “this” keyword in Java is used to refer to the current instance of a class. It is primarily used to differentiate between instance variables and method parameters that have the same name.

Q18. Is there a private constructor in Java?

Ans: Private constructors are often used in the Singleton design pattern to restrict object creation to only within the class. These types of java OOPs interview questions and answers should be prepared by the candidates thoroughly.

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Q19. What is the Singleton design pattern?

Ans: The Singleton design pattern is a software design approach that guarantees a class has just one instance throughout the program's execution. This is accomplished by managing the procedure and making sure there is only one single global point of access to that particular instance. This pattern is useful when you wish to limit the production of several objects in order to save resources or ensure consistency in data sharing throughout the application. This is one of the frequently asked OOPs interview questions for experienced professionals that can be asked in the interviews.

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Q20. What is the difference between composition and inheritance?

Ans: Composition is a design principle that allows a class to be composed of one or more objects of other classes, promoting code reuse and flexibility. Inheritance, on the other hand, is a relationship between classes where a subclass inherits properties and behaviors from a superclass.

Q21. What is the final keyword in Java?

Ans: The final keyword in Java acts as a restrictive marker. It prevents inheritance when added to a class, making it impossible to extend. It prevents method overriding when a method is used with it, guaranteeing that the method's behaviour is maintained in subclasses. It denotes a constant value for variables that cannot be changed once it has been assigned. The final keyword essentially protects the integrity and immutability of Java classes, methods, and variables, guaranteeing that they maintain their initial state and functioning.

Q22. What is the method hiding in Java?

Ans: Method hiding occurs when a subclass defines a static method with the same name and method signature as a static method in its superclass. This results in the subclass method hiding the superclass method, and the method called depends on the reference type.

Q23. What is method chaining in Java?

Ans: Method chaining is a technique where multiple methods are called on the same object in a single line of code, often by returning the object itself from each method.

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Q24. What is the super keyword used for in Java?

Ans: The super keyword in Java is used to refer to the superclass of the current subclass. It is used to access and invoke the superclass's methods and constructors.

Q25. What is a static method in Java?

Ans: A static method belongs to the class and not to any specific instance of the class. It can be accessed using the class name and does not require an object to be created to invoke the method.

Q26. Can static methods be overridden in Java?

Ans: Static methods cannot be overridden in Java because method overriding is based on runtime polymorphism, and static methods are resolved at compile time.

Q27. Explain the concept of method reference in Java.

Ans: Method reference is a shorthand notation for referring to a method as a lambda expression. It is used to make code concise when passing a method as an argument to a functional interface.

Q28. What are anonymous classes in Java?

Ans: An anonymous class is a class without a name that is used to create an instance of an interface or an abstract class. It is often used for one-time implementations of an interface or abstract class. This is one of the best OOPs interview questions and answers to be prepared for.

Q29. How does Java support multiple inheritance?

Ans: Java supports multiple inheritance through interfaces. A class can implement multiple interfaces, which allows it to inherit abstract behaviours from multiple sources while avoiding the ambiguity that multiple inheritance of classes can introduce.

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Q30. What are the main characteristics of OOPs?

Ans: This is one of the top OOPs interview questions and answers for freshers. Main characteristics of OOPs include inheritance, Encapsulation, Polymorphism, and Data Abstraction.

Q31. Why adopt OOPs?

Ans: OOPs ensure clarity in programming, enabling straightforward solutions to intricate problems. It facilitates code reusability through inheritance, minimising redundancy. The encapsulation feature binds data and code together, promoting more secure and confidential handling of private data. Moreover, OOPs allow problems to be divided into manageable parts, simplifying the overall solution process. The concept of polymorphism adds flexibility to programs, allowing entities to take multiple forms. This is amongst the types of Java OOPs interview questions and answers that have a high probability of being asked in interview discussions.

Q32. What is a constructor in Java?

Ans: A constructor in Java is a unique method used to initialise objects of a class. It has the same name as the class and is called automatically when an object is created. This is another one of the basic OOPs interview questions and answers for freshers that are considered very important for a thorough understanding of this topic.

Q33. How you can define Encapsulation?

Ans: Encapsulation refers to the binding of data and methods that manipulate them into a single unit such that the sensitive data is hidden from the users. It is applied as the processes which are as follows:

• Data hiding: It is a language feature that restricts access to members of an object. For example, private and protected members in C++.

• Bundling of data and methods together: Data and methods that operate on that data are bundled together.

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Q34. What is the purpose of the equals method in Java?

Ans: The equals method in Java is used to compare the contents of objects for equality. It is often overridden to provide a customised equality comparison for user-defined classes. This type of java OOPs interview question must be in your preparation list.

Q35. What is the hashCode method used for in Java?

Ans: The hashCode method returns a hash code value for an object. It is used in hash-based data structures like HashMap to efficiently locate objects.

Q36. What is the toString method in Java?

Ans: The toString method is used to represent an object as a string. It is often overridden to provide a human-readable string representation of an object's state.

Q37. What is the purpose of the finalise method in Java?

Ans: The finalise method is called by the garbage collector before an object is reclaimed by memory. It can be overridden to release resources or perform cleanup operations. Do learn this Java OOPs interview question as it is very important for thorough preparation.

Q38. What is the difference between method overloading and method overriding?

Ans: This is one of the top most important Java OOPs interview questions and answers. Method overloading involves defining multiple methods in the same class with the same name but different parameters. Method overriding involves providing a specific implementation for a method in a subclass that is already defined in its superclass.

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Q39. What is a package in Java?

Ans: This is a must-know Java OOPs interview question. A package in Java is a way to organise related classes and interfaces. It helps avoid naming conflicts and provides a hierarchical structure for a class organisation.

Q40. What is the instanceof operator used for?

Ans: The instanceof operator is used to check whether an object is an instance of a particular class or interface. It returns a boolean value indicating the result of the check.

Q41. What is the role of the transient keyword in Java?

Ans: The transient keyword is used to indicate that a field should not be serialised when an object is being converted to a byte stream for storage or transmission. You should prepare for these types of OOPs interview questions and answers for your next interview.

Q42. What is the purpose of the volatile keyword in Java?

Ans: The volatile keyword is used to declare a variable as volatile, which ensures that its value is always read from and written to the main memory, avoiding thread-caching issues. This is one of the most important Java OOPs interview questions for freshers as well as experienced.

Q43. What are abstract methods in Java?

Ans: Abstract methods are methods that are declared without implementation in an abstract class or an interface. Subclasses or implementing classes are required to provide concrete implementations for these methods.

Q44. What is the difference between checked and unchecked exceptions?

Ans: Checked exceptions are exceptions that are checked at compile time, requiring the programmer to handle or declare them. Unchecked exceptions (runtime exceptions) are not checked at compile time and do not require explicit handling. This is one of the frequently asked OOPs interview questions and answers.

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Q45. What is method reference in Java 8?

Ans: Method reference is a shorthand notation for referring to a method as a lambda expression. It simplifies lambda expressions by directly referencing a method by name. This type of Java OOPs interview question can be asked by the interviewer.

Q46. What is a lambda expression in Java?

Ans: A lambda expression is an anonymous function that allows you to pass functionality as an argument to a method. It is used to provide a more concise and readable way to write functional interfaces.

Q47. What is the role of the default keyword in interface methods?

Ans: In Java 8 and later, the default keyword is used to define a default implementation for methods in an interface. It allows adding new methods to existing interfaces without breaking implementations.

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Q48. What is the static block in Java?

Ans: A static block is a block of code within a class that is executed only once when the class is loaded into memory. It is often used to initialise static variables or perform setup operations.

Q49. What is the purpose of the try-with-resources statement?

Ans: The try-with-resources statement is used to manage resources that need to be closed explicitly, such as streams or database connections. It ensures that the resources are properly closed when the block exits, even in the presence of exceptions.

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Q50. What is the difference between deep copy and shallow copy?

Ans: Deep copy involves creating a new copy of an object along with copies of all the objects referenced within it. Shallow copy involves creating a new object and copying the references of the objects within, rather than duplicating the referenced objects themselves.

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Conclusion

These are some common OOPs interview questions and answers for freshers as well as experienced individuals that can be asked. Familiarising yourself with these OOPs interview questions and answers, especially the Java OOPs interview question can help you prepare better for your upcoming interviews. By understanding these concepts, you can confidently tackle potential questions and showcase your knowledge during the interview process.

Taking the time to study OOPs interview questions for freshers as well as the OOPs interview questions for experienced developers will undoubtedly improve your chances of succeeding in upcoming interviews and landing your desired role in the field of programming and software development.