Java Programming — OOP in Java Questions & Answers

Q.1 Medium OOP in Java

In Java, when you override a method from a parent class, which of the following is NOT a valid reason to use the @Override annotation?

A To enable compile-time checking for correct method signature

B To improve code readability and maintainability

C To automatically invoke the parent class method implementation

D To catch errors if the parent method is removed or signature changes

Correct Answer: C. To automatically invoke the parent class method implementation

EXPLANATION

The @Override annotation helps verify that you are correctly overriding a parent method and improves code clarity, but it does NOT automatically invoke the parent class method. You must use super.methodName() for that. @Override is purely a compile-time marker for verification purposes.

Take Test

Q.2 Hard OOP in Java

In a banking system, Account is a parent class with method withdraw(). SavingsAccount and CurrentAccount both override it. A programmer writes: List accounts = new ArrayList(); accounts.add(new SavingsAccount()); accounts.add(new CurrentAccount()); for(Account a : accounts) a.withdraw(1000); Which concept is primarily demonstrated here?

A Encapsulation

B Abstraction only

C Polymorphism and Runtime Binding

D Inheritance only

Correct Answer: C. Polymorphism and Runtime Binding

EXPLANATION

The same withdraw() call behaves differently for SavingsAccount and CurrentAccount based on the actual object type at runtime. This is polymorphism with runtime (dynamic) binding.

Take Test

Q.3 Hard OOP in Java

Which of the following correctly implements the Factory Design Pattern principle in OOP?

A Creating objects directly using 'new' keyword everywhere

B Using a separate class with static methods to create and return objects of different types

C Using inheritance to create a base factory class

D Making all constructors private

Correct Answer: B. Using a separate class with static methods to create and return objects of different types

EXPLANATION

The Factory Pattern uses a separate class (factory) with methods to create and return objects. This decouples object creation from usage.

Take Test

Q.4 Hard OOP in Java

A system has classes: Vehicle -> Car -> ElectricCar. If Vehicle.start() is overridden in Car and again in ElectricCar, what is the output of: Vehicle v = new ElectricCar(); v.start();

A Vehicle's start() executes

B Car's start() executes

C ElectricCar's start() executes

D Compilation error

Correct Answer: C. ElectricCar's start() executes

EXPLANATION

This demonstrates dynamic (runtime) polymorphism. The actual object type (ElectricCar) determines which method is called, not the reference type (Vehicle).

Take Test

Q.5 Hard OOP in Java

In Java 16+, which sealed class feature allows you to restrict which classes can extend a class?

A Using 'final' keyword on all methods

B Using 'sealed' keyword with 'permits' clause

C Using 'protected' access modifier on constructor

D Using package-private access on the class

Correct Answer: B. Using 'sealed' keyword with 'permits' clause

EXPLANATION

Java 17 introduced 'sealed' classes with the 'permits' clause to explicitly specify which classes can extend a sealed class, providing better control over inheritance.

Take Test

Q.6 Hard OOP in Java

You need to create a class that provides common functionality for all database operations (insert, update, delete). What is the best approach?

A Create an interface DatabaseOperations

B Create an abstract class with abstract methods

C Create a concrete class with final methods

D Create an abstract class with some concrete methods for common logic

Correct Answer: D. Create an abstract class with some concrete methods for common logic

EXPLANATION

An abstract class allows you to define both abstract methods (to be overridden) and concrete methods (shared implementation), which is ideal for this scenario.

Take Test

Q.7 Medium OOP in Java

In Java, a class can implement multiple interfaces but can extend only one class. This design choice primarily supports which principle?

A Single Responsibility Principle

B Liskov Substitution Principle

C Avoiding diamond problem in single inheritance

D Interface Segregation Principle

Correct Answer: C. Avoiding diamond problem in single inheritance

EXPLANATION

Multiple inheritance of implementation can cause the diamond problem. Java avoids this by allowing single class inheritance but multiple interface implementation.

Take Test

Q.8 Medium OOP in Java

Which of the following statements about 'this' keyword in Java is INCORRECT?

A 'this' can be used to refer to the current object

B 'this' can be used to invoke another constructor of the same class

C 'this' can be used in static methods

D 'this' can be used to return the current object

Correct Answer: C. 'this' can be used in static methods

EXPLANATION

'this' cannot be used in static methods because static methods are class-level and don't have an instance context. 'this' always refers to an instance.

Take Test

Q.9 Medium OOP in Java

What will be printed when this code executes?
class A { int x = 5; }
class B extends A { int x = 10; }
public class Test {
public static void main(String[] args) {
A a = new B();
System.out.println(a.x);
}
}

A 5

B 10

C Compilation error

D Runtime exception

Correct Answer: A. 5

EXPLANATION

Variable shadowing occurs here. 'a' is of type A, so a.x refers to A's variable x which is 5. Variables are not overridden, only methods are.

Take Test

Q.10 Medium OOP in Java

Consider a scenario where you have an interface Shape with a method calculateArea(). You implement this in classes Circle and Rectangle. Which OOP concept is being demonstrated?

A Encapsulation

B Abstraction

C Polymorphism

D Both Abstraction and Polymorphism

Correct Answer: D. Both Abstraction and Polymorphism

EXPLANATION

The interface provides abstraction by hiding implementation details. Multiple classes implementing the same interface method demonstrates polymorphism (many forms).

Take Test

All Subjects & Topics