Java Interfaces MCQ (Multiple Choice Questions)

In this blog post, we present multiple-choice questions to test your knowledge of Java interfaces (including Java 8 static and default methods in an interface). Interfaces in Java provide a powerful mechanism for achieving abstraction and multiple inheritance. As of Java 8, they've become even more powerful, allowing for both static and default methods. 

What are Interfaces? 

In Java, an interface is a reference type, similar to a class, which can contain only constants, method signatures, default methods, static methods, and nested types. It cannot contain constructors (as it can’t be instantiated on its own). 

Key Points: 

Abstraction: Interfaces are used to achieve full abstraction, ensuring that all the methods in an interface are abstract (without implementation). 

Multiple Inheritance: Java doesn't support multiple inheritances in classes due to the "diamond problem". However, a single class can implement multiple interfaces, offering a solution to this limitation. 

Default Methods: Introduced in Java 8, default methods have a method body and provide a default implementation. They can be overridden by the implementing class.

Static Methods: Introduced in Java 8. The static methods belong to the interface itself (we should call static methods using the interface name as they belong to the interface), rather than to instances of implementing classes.

Java Interfaces MCQ (Multiple Choice Questions)

1. What is the primary purpose of interfaces in Java?

a) Achieve Polymorphism

b) Achieve Multiple Inheritance

c) Achieve Encapsulation

d) Achieve Abstraction

Answer:

d) Achieve Abstraction

Explanation:

Interfaces are used to achieve full abstraction in Java, ensuring that a class adheres to a certain contract without enforcing how that contract is fulfilled.

2. Which keyword is used to implement an interface in a class?

a) extends

b) implements

c) uses

d) inherits

Answer:

b) implements

Explanation:

The implements keyword is used when a class wants to adhere to the contract defined by an interface.

3. What is the default access modifier of a method in an interface in Java?

a) private

b) protected

c) public

d) None of the above

Answer:

c) public

Explanation:

Interface methods are public by default since the idea is for them to be implemented by other classes.

4. How many interfaces can a single class implement?

a) One

b) Two

c) Four

d) Unlimited

Answer:

d) Unlimited

Explanation:

A class can implement any number of interfaces in Java.

5. Can an interface contain a constructor?

a) Yes

b) No

Answer:

b) No

Explanation:

Interfaces cannot be instantiated directly, so they cannot have constructors.

6. What was the significant change to interfaces in Java 8?

a) Ability to have constructors

b) Introduction of private methods

c) Introduction of static and default methods

d) All methods became final

Answer:

c) Introduction of static and default methods

Explanation:

Java 8 allowed interfaces to have method bodies in the form of static and default methods.

7. Why were default methods introduced in Java 8 interfaces?

a) To provide multiple inheritance

b) To add utility functions

c) To provide backward compatibility with older interface versions

d) For better performance

Answer:

c) To provide backward compatibility with older interface versions

Explanation:

Default methods allow developers to add new methods to interfaces with an implementation without affecting classes that already use this interface.

8. Which of these is not allowed in an interface?

a) Instance variables

b) Static methods

c) Default methods

d) Abstract methods

Answer:

a) Instance variables

Explanation:

Interfaces can have static final variables (constants) but not instance variables.

9. How do you access a static method of an interface?

a) Using the interface name

b) Using the method name directly

c) Through an object of the interface

d) Through an implementation class

Answer:

a) Using the interface name

Explanation:

Static methods in an interface are accessed using the interface name, similar to static methods in classes.

10. What happens if a class implements two interfaces with default methods of the same name?

a) It takes the first interface's method

b) It takes the second interface's method

c) It must override the conflicting method

d) Throws a runtime error

Answer:

c) It must override the conflicting method

Explanation:

If a class implements two interfaces with default methods of the same name, it has to override the method to resolve the conflict.

11. Static methods in interfaces...

a) Can be overridden

b) Cannot be overridden

c) Can be instantiated

d) Are always abstract

Answer:

b) Cannot be overridden

Explanation:

Static methods belong to the interface itself, not to the implementing class. Hence, they cannot be overridden.

12. The "diamond problem" in programming refers to...

a) Memory issues in OOP

b) Multiple inheritance ambiguity

c) Lack of abstraction

d) Too many static methods

Answer:

b) Multiple inheritance ambiguity

Explanation:

The diamond problem occurs when a class inherits from two classes that have a method with the same name, leading to ambiguity. Java avoids this problem by not supporting multiple inheritance of classes.

13. Which of the following is a benefit of using interfaces?

a) They provide a clear contract for classes to follow

b) They can have instance variables

c) They can be instantiated

d) They can be used to create objects directly

Answer:

a) They provide a clear contract for classes to follow

Explanation:

Interfaces set a clear contract that classes must follow if they implement the interface, ensuring that certain methods are implemented in the class.