Understanding Java Interfaces: A Comprehensive Guide to Interview Questions

In Java, interfaces play a vital role in achieving abstraction, defining contracts, and enabling polymorphism. A thorough understanding of interfaces is essential for any Java developer. In this article, we will delve into the concept of interfaces in Java and explore some commonly asked interview questions related to interfaces. By the end, you will have a solid understanding of interfaces in Java and be well-prepared to tackle interface-related questions in your next Java interview.

  1. What is an interface in Java?

In Java, an interface is a reference type that defines a contract or a set of method signatures that a class implementing the interface must follow. It serves as a blueprint for classes, providing a way to achieve abstraction and multiple inheritance of behavior. An interface can declare methods, constants, and nested types.

  1. How is an interface different from a class?
  • An interface cannot be instantiated directly, whereas a class can be instantiated to create objects.
  • An interface cannot have instance variables, while a class can have variables.
  • An interface can only declare method signatures, while a class can provide method implementations.
  • A class can extend only one superclass, but it can implement multiple interfaces.
  1. How do you define an interface in Java?

To define an interface in Java, you use the interface keyword followed by the interface name. Here’s an example:

public interface Drawable {
    void draw();
    int calculateArea();

In the above example, we define an interface named Drawable. It declares two methods: draw() and calculateArea(). Any class that implements this interface must provide implementations for these methods.

  1. Can an interface have variables?

Interfaces can declare constants, which are implicitly static and final. However, starting from Java 8, interfaces can also have static and default methods.

Static methods: Starting from Java 8, interfaces can include static methods that provide utility or helper methods. These methods belong to the interface itself and can be called without instantiating any class that implements the interface.

Default methods: Default methods are methods with implementations declared in an interface. They provide a default behavior for methods that may or may not be overridden by implementing classes. Default methods allow adding new methods to existing interfaces without breaking the backward compatibility of implementing classes.

Here’s an example illustrating static and default methods in an interface:

public interface Calculator {
    static double add(double a, double b) {
        return a + b;

    default double subtract(double a, double b) {
        return a - b;

    double multiply(double a, double b);
    double divide(double a, double b);

In the above example, the Calculator interface includes a static method add() and a default method subtract(). Implementing classes can choose to override the multiply() and divide() methods according to their specific requirements.

  1. How are interfaces used for achieving abstraction and polymorphism?

Interfaces in Java are used to achieve abstraction by providing a way to define a contract or behavior without specifying the implementation details. When a class implements an interface, it must provide concrete implementations for all the methods declared in the interface.

Interfaces also enable polymorphism by allowing objects of different classes to be treated as objects of the interface type. This allows for writing code that is more flexible and reusable.


Let’s consider a scenario where you have an interface called Shape that defines methods for calculating the area and perimeter of various shapes. You have classes such as Circle and Rectangle that implement the Shape interface.

public interface Shape {
    double calculateArea();
    double calculatePerimeter();

public class Circle implements Shape {
    private double radius


    public Circle(double radius) {
        this.radius = radius;

    public double calculateArea() {
        return Math.PI * radius * radius;

    public double calculatePerimeter() {
        return 2 * Math.PI * radius;

public class Rectangle implements Shape {
    private double length;
    private double width;

    public Rectangle(double length, double width) {
        this.length = length;
        this.width = width;

    public double calculateArea() {
        return length * width;

    public double calculatePerimeter() {
        return 2 * (length + width);

In the above example, the Shape interface defines the calculateArea() and calculatePerimeter() methods. The Circle and Rectangle classes implement the Shape interface, providing their own implementations for these methods. This allows you to treat both Circle and Rectangle objects as Shape objects, enabling polymorphism.


In this article, we have explored the concept of interfaces in Java and covered some commonly asked interview questions related to interfaces. Understanding interfaces and their role in achieving abstraction and polymorphism is essential for writing flexible and reusable Java code. By familiarizing yourself with interface concepts, syntax, and usage, you will be well-prepared to handle interface-related questions in Java interviews. Remember to practice implementing interfaces and utilizing polymorphism to gain hands-on experience and solidify your understanding of interfaces in Java.

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