Java Programming — OOP in Java Questions & Answers

Q.31 Easy OOP in Java

What is the access modifier of members declared in an interface?

A private

B protected

C public

D package-private

Correct Answer: C. public

EXPLANATION

All members in an interface are implicitly public. Variables are public static final, and methods are public abstract (unless default or static).

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Q.32 Hard OOP in Java

In a legacy system, you find code with very deep inheritance hierarchy (5+ levels). What design problem does this indicate?

A Good object-oriented design

B Violation of the Liskov Substitution Principle

C Possible violation of 'favor composition over inheritance' principle

D The code is efficient and should be kept as is

Correct Answer: C. Possible violation of 'favor composition over inheritance' principle

EXPLANATION

Deep inheritance hierarchies are considered a code smell. They suggest that composition might have been a better approach, leading to tighter coupling and reduced flexibility.

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Q.33 Hard OOP in Java

What is the output of the following code?
interface I1 { default void show() { System.out.println("I1"); } }
interface I2 { default void show() { System.out.println("I2"); } }
class C implements I1, I2 { public void show() { I1.super.show(); } public static void main(String[] args) { new C().show(); } }

A I1

B I2

C Compile-time error due to ambiguity

D I1 I2

Correct Answer: A. I1

EXPLANATION

When a class implements multiple interfaces with the same default method, it must explicitly override the method. Using I1.super.show() calls I1's default implementation.

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Q.34 Medium OOP in Java

Which of the following correctly demonstrates composition over inheritance?

A class Engine { } class Car extends Engine { }

B class Engine { } class Car { Engine e = new Engine(); }

C Both approaches are equally good

D Composition is never preferred in Java

Correct Answer: B. class Engine { } class Car { Engine e = new Engine(); }

EXPLANATION

Composition (HAS-A relationship) is often preferred over inheritance (IS-A relationship) because it's more flexible and avoids tight coupling.

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Q.35 Hard OOP in Java

What happens with exception handling in method overriding?

A Child class method can throw any exception

B Child class method can only throw checked exceptions, not unchecked

C Child class method can throw the same or narrower (subclass) exceptions only

D Exception handling rules don't apply to overridden methods

Correct Answer: C. Child class method can throw the same or narrower (subclass) exceptions only

EXPLANATION

Liskov Substitution Principle: A child class method can throw the same exception or a narrower exception than the parent method, not a broader one.

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Q.36 Hard OOP in Java

A real-world scenario: You're designing a banking system. Should you use an abstract class or interface for 'Account'?

A Use interface because it's more flexible

B Use abstract class because Account has state (balance, accountNumber) and shared behavior (deposit, withdraw)

C Use both abstract class and interface together

D Neither, use a concrete class

Correct Answer: B. Use abstract class because Account has state (balance, accountNumber) and shared behavior (deposit, withdraw)

EXPLANATION

Abstract classes are suitable when you have shared state and constructors needed. An Account has properties like balance and account number, making abstract class the better choice.

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Q.37 Medium OOP in Java

Which of the following is true about interface default methods?

A They must be implemented by all implementing classes

B They provide a default implementation that can be overridden or inherited

C They are the same as abstract methods

D Only one interface can have default methods

Correct Answer: B. They provide a default implementation that can be overridden or inherited

EXPLANATION

Default methods in interfaces (Java 8+) provide a default implementation. Implementing classes can use this default implementation or override it as needed.

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Q.38 Hard OOP in Java

What does the following code output?
class A { static void display() { System.out.println("A"); } }
class B extends A { static void display() { System.out.println("B"); } }
public class Test { public static void main(String[] args) { A ref = new B(); ref.display(); } }

A A

B B

C Compile-time error

D Runtime error

Correct Answer: A. A

EXPLANATION

Static methods are resolved at compile-time based on the reference type, not the object type. Hence, A.display() is called, not B.display(). This is method hiding, not overriding.

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Q.39 Medium OOP in Java

What is the relationship between an abstract class and an interface in Java 8+?

A Abstract classes and interfaces are identical

B Interfaces can have default and static methods, while abstract classes can have instance variables and constructors

C Only abstract classes can be extended

D Interfaces are always preferred over abstract classes

Correct Answer: B. Interfaces can have default and static methods, while abstract classes can have instance variables and constructors

EXPLANATION

From Java 8, interfaces can have default and static methods, but abstract classes can have instance variables, constructors, and private methods which interfaces cannot have.

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Q.40 Medium OOP in Java

Consider a scenario where class B extends class A. If class A has a constructor with parameters, what must class B do?

A Ignore the parent constructor completely

B Must explicitly call parent constructor using super()

C Parent constructor is automatically called without any action

D Class B becomes invalid and cannot be compiled

Correct Answer: B. Must explicitly call parent constructor using super()

EXPLANATION

If a parent class has a parameterized constructor and no default constructor, the child class must explicitly call it using super() to initialize parent class members.

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