Java Programming — OOP in Java Questions & Answers

Q.21 Medium OOP in Java

What is the output of this code?
class Test {
static int x = 10;
public static void main(String[] args) {
System.out.println(x++);
}
}

A 10

B 11

C Compilation error

D Runtime exception

Correct Answer: A. 10

EXPLANATION

The post-increment operator (x++) returns the value before incrementing. So it prints 10, and then x becomes 11.

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Q.22 Medium OOP in Java

Consider a class hierarchy: Animal -> Dog -> Puppy. A Puppy reference can access which of the following?

A Only Puppy methods

B Puppy and Dog methods, but not Animal methods

C Puppy, Dog, and Animal methods

D Only methods declared as public

Correct Answer: C. Puppy, Dog, and Animal methods

EXPLANATION

Through inheritance, a Puppy reference can access all non-private methods from Puppy, Dog, and Animal classes through the inheritance chain.

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Q.23 Medium OOP in Java

Which statement about Java interfaces is INCORRECT according to 2024 specifications?

A Interfaces can have static methods

B Interfaces can have default methods

C Interfaces can have private methods

D Interfaces can be instantiated directly

Correct Answer: D. Interfaces can be instantiated directly

EXPLANATION

Interfaces cannot be instantiated directly. Since Java 8, interfaces can have static and default methods, and since Java 9, they can have private methods.

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Q.24 Medium OOP in Java

What is the correct way to call a parent class constructor from a child class?

A parent();

B super();

C this.parent();

D ParentClass();

Correct Answer: B. super();

EXPLANATION

The super() keyword is used to call the parent class constructor. It must be the first statement in the child class constructor.

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Q.25 Medium OOP in Java

In Java, which of the following is true about abstract classes?

A An abstract class can have no abstract methods

B An abstract class must have all abstract methods

C An abstract class cannot have constructors

D An abstract class cannot have instance variables

Correct Answer: A. An abstract class can have no abstract methods

EXPLANATION

An abstract class can have zero or more abstract methods. It can also have concrete methods, constructors, and instance variables.

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Q.26 Medium OOP in Java

What will happen if you try to override a final method in a subclass?

A It will compile and run successfully

B Compilation error will occur

C Runtime exception will be thrown

D It will override silently

Correct Answer: B. Compilation error will occur

EXPLANATION

A final method cannot be overridden. Attempting to override it results in a compilation error: 'cannot override final method'.

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Q.27 Medium OOP in Java

Which of the following demonstrates proper method overloading in Java?

A public void display() { } and public int display() { }

B public void display(int a) { } and public void display(int b) { }

C public void display(int a) { } and public void display(String a) { }

D public void display() { } and public void display() { return 5; }

Correct Answer: C. public void display(int a) { } and public void display(String a) { }

EXPLANATION

Method overloading requires different parameter types or number of parameters. Options A and D have different return types (not valid for overloading), and option B has the same signature with different parameter names (not overloading).

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Q.28 Easy OOP in Java

Can you instantiate an interface in Java?

A Yes, directly with new keyword

B No, interfaces cannot be instantiated

C Yes, only if it has default methods

D Yes, if it has only abstract methods

Correct Answer: B. No, interfaces cannot be instantiated

EXPLANATION

Interfaces cannot be instantiated directly. You can only create objects of classes that implement interfaces.

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Q.29 Easy OOP in Java

Which keyword is used to prevent a class from being subclassed?

A abstract

B final

C sealed

D static

Correct Answer: B. final

EXPLANATION

The 'final' keyword prevents a class from being extended. For example: final class MyClass {} cannot be subclassed.

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Q.30 Easy OOP in Java

What is the default access modifier for class members in Java if not explicitly specified?

A public

B private

C protected

D package-private (default)

Correct Answer: D. package-private (default)

EXPLANATION

If no access modifier is specified for a class member, it has package-private (default) access, meaning it's accessible only within the same package.

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