Core Java - Interview Questions and Answers for 'Oops concept' | Search Interview Question

Core Java - Interview Questions and Answers for 'Oops concept' - 57 question(s) found - Order By Rating

Q1. Can a class extend itself in Java ?

Core Java

Ans. No

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Q2. How many VTables are there for each class ?

Core Java

Ans. There is one VTable for each class.

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Q3. How is the virtual method table implemented in Java?

Core Java

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Q4. What is virtual table with respect to method overriding in Java ?

Core Java

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Q5. What could be the reasons for restricting inheritance using final class ?

Core Java

Ans. 1. Enforcing composition over inheritance
2. Restricting overriding of certain methods
3. Final methods are faster than regular instance methods
4. Enforcing Immutability

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Q6. Does use of Final class enforces composition over Inheritance in Java ?

Core Java

Ans. Yes, to a certain extent. But the objective for Final class could be beyond just enforcing composition as certain classes might have been created without inheritance or composition in mind.

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Q7. Can we have multiple inheritance in java using abstract class ?

Core Java

Ans. No

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Q8. Which of the OOP's feature is most important ?

Core Java

Ans. Encapsulation facilitate hiding and restricted access and hence more of a security feature. Encapsulation is definitely a great feature as when applications expand criss cross communication between objects / modules could lead to blunders.

Inheritance facilitates code reuse.

Polymorphism comprise of method overloaded ( which to me is negligible usage ) and method overriding. Method overriding is of great usage as it facilitates concept of interfaces and plugin development.

So it’s Security / Organization vs

Code Reuse / Support for other features like overriding vs

Contracts / Plugin Development facilitating the creation of frameworks / libraries.

Which is more important may vary from application to application , its scale , its use , its sensitivity etc.

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Q9. Which of the polymorphism type - static or runtime - is more important ?

Core Java

Ans. Method overloading / static polymorphism compared to method overriding / runtime polymorphism has very limited usage as it just opens up an alternate way of defining a different method with the same name.

Method Overriding on other hand opens up many other features like contracting , interfacing , pluging development and hence development of libraries and frameworks.

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Q10. Which of the OOP's feature - encapsulation or abstraction - is more important ?

Core Java

Ans. Encapsulation facilitates security by hiding data and logic whereas Abstraction simplifies organization of data and related logic.

As applications scale, both concepts are required for easy management and maintenance. Encapsulation for security and criss cross communication between objects / modules will make it vulnerable. and Abstraction for better organization that enables better understanding of application code and easy maintainability.

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Q11. What are the characteristics of the object ?

Core Java

Ans. 1. State

Instance variable or member elements forms the object state.

2. Behavior

Member functions forms the object behavior

3. Identity

Every object has a unique identity within JVM through a unique Id.

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Q12. What is Overriding ?

Core Java

Ans. Overriding refers to the Methods with same name and parameters, such that the later over rides the former method.

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Q13. What is composition ?

Core Java

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Sample Code for composition

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Q14. Explain OOps concepts.

Core Java

Ans. There are four main OOP concepts in Java. These are:

Abstraction. Abstraction means using simple things to represent complexity. We all know how to turn the TV on, but we don?t need to know how it works in order to enjoy it. In Java, abstraction means simple things like objects, classes, and variables represent more complex underlying code and data. This is important because it lets avoid repeating the same work multiple times.

Encapsulation. This is the practice of keeping fields within a class private, then providing access to them via public methods. It?s a protective barrier that keeps the data and code safe within the class itself. This way, we can re-use objects like code components or variables without allowing open access to the data system-wide.

Inheritance. This is a special feature of Object Oriented Programming in Java. It lets programmers create new classes that share some of the attributes of existing classes. This lets us build on previous work without reinventing the wheel.

Polymorphism. This Java OOP concept lets programmers use the same word to mean different things in different contexts. One form of polymorphism in Java is method overloading. That?s when different meanings are implied by the code itself. The other form is method overriding. That?s when the different meanings are implied by the values of the supplied variables. See more on this below.

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Q15. how does abstraction help your application ?

Core Java

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Q16. What is abstraction ?

Design

Ans. Abstraction is a process of hiding the implementation details and describing only the functionality to the user.

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Q17. Is Runtime Polymorphism possible without Inheritance ?

Core Java

Ans. Yes, Runtime Polymprohism requires either Class inheritance or Interface implementation.

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Q18. What are the uses of Abstraction ?

Core Java

Ans. Loose Coupling
Facilitates designing complex applications by making them modular.

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Q19. Explain Inheritance and Polymorphism.

Core Java

Ans. Getting parent class object properties into child class is called Inheritance.

The process of representing one form into multiple forms is called polymorphism.

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Q20. Give an example of how Object Oriented Programming Concepts can be implemented.

Design

Ans. You can implement encapsulation in Java by keeping the fields (class variables) private and providing public getter and setter methods to each of them. Java Beans are examples of fully encapsulated classes. Encapsulation in Java: Restricts direct access to data members (fields) of a class.

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Very frequently asked. Usually among very first few questions.

Q21. Define encapsulation in Java ?

Core Java

Ans. Encapsulation is a feature of OOP's that binds the data and it's associated methods together as a single unit and facilitate protection and data hiding by providing minimal interface to outside. For example - member variables are declared private and are accessed through public methods. Moreover we have private methods that can only be used internally and hence providing minimal interface to outside class through use of public methods.

Sample Code for encapsulation

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Q22. How can we hide a class in Java ?

Core Java

Ans. By encapsulating it within another class and declaring it private. In such a case, it will only be accessible through parent class or parent class object.

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Q23. What happens to private variables during inheritance ?

Core Java

Ans. Private variables are not inherited. They are simply ignored and not made part of the derived object body.

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Q24. What is dynamic dispatch in Java ?

Core Java

Ans. Dynamic dispatch in java is also known as runtime polymorphism.It is a process in which a call to an overriden method is resolved at runtime.

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Q25. What is "dynamic method dispatch" in java ?

Core Java

Ans. dynamic method dispatch is a process in which a call to an
overridden method is resolved at runtime rather than at compile-time. This is used to achieve runtime polymorphism in java.

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Q26. How does java identifies which method to be called in method overriding or runtime polymorphism, when both methods share the same name and signature ?

Core Java

Ans. Java identifies the method to be called at runtime by the object that is being referenced.

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Q27. Which of the two - compile time and run time polymorphism - requires signature of the method to be different ?

Core Java

Ans. runtime polymorphism or method overriding doesn't require method name and signature to be different whereas compile time polymorphism or method overloading requires method name to be same but the signature to be different.

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Q28. How are the concept of Association related to Composition and Inheritance ?

Core Java

Ans. Composition and Inheritance are the different types of Associations for Classes.

Composition is a has-a association between classes.
Inheritance is a is-a association between classes.

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Q29. What are the core OOPs concepts ?

Core Java

Ans. Abstraction, Encapsulation, Polymorphism , Composition and Inheritance

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Q30. Why java doesn't support multiple Inheritance ?

or

Explain Java Diamond Problem.

Core Java

Ans. class A {
void test() {
System.out.println("test() method");
}
}

class B {
void test() {
System.out.println("test() method");
}
}

Suppose if Java allows multiple inheritance like this,

class C extends A, B {
}

A and B test() methods are inheriting to C class.

So which test() method C class will take? As A & B class test() methods are different , So here we would Facing Ambiguity.

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