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

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

a) extends
b) implements
c) uses
d) inherits

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

4. How many interfaces can a single class implement?

a) One
b) Two
c) Four
d) Unlimited

5. Can an interface contain a constructor?

a) Yes
b) No

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

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

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

a) Instance variables
b) Static methods
c) Default methods
d) Abstract methods

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

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

11. Static methods in interfaces...

a) Can be overridden
b) Cannot be overridden
c) Can be instantiated
d) Are always abstract

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

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

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