Introduction to Interfaces in Java

What is an Interface?

In Java, an interface is a programming construct that defines a contract of methods that must be implemented by classes that choose to use the interface. It allows you to define a set of methods that form a common interface, ensuring that implementing classes provide specific behavior.

Defining an Interface

An interface is defined using the interface keyword, followed by the interface name and a list of method signatures (without method bodies). Here's an example of a simple interface:

interface Drawable {
void draw();
void resize(int percent);

Implementing an Interface

To use an interface, a class must implement it using the implements keyword. The class is then required to provide implementations (method bodies) for all the methods declared in the interface.

class Circle implements Drawable {
public void draw() {
System.out.println("Drawing a circle.");
public void resize(int percent) {
System.out.println("Resizing the circle by " + percent + "%.");

Benefits of Interfaces

Interfaces offer several advantages in Java:

  • Define a contract for classes to adhere to.
  • Support multiple inheritance, as a class can implement multiple interfaces.
  • Promote code reusability and flexibility by allowing classes to share common behavior.
  • Facilitate the use of polymorphism, where objects of different classes that implement the same interface can be treated interchangeably.

Using Interfaces

Interfaces are commonly used to create a common interface for a group of related classes. For example, you can define an interface Drawable and have various shapes like circles, rectangles, and triangles implement it.

Drawable circle = new Circle();
Drawable rectangle = new Rectangle();
circle.draw(); // Calls the draw method of the Circle class
rectangle.draw(); // Calls the draw method of the Rectangle class


Interfaces are a vital part of Java that allows you to define common behavior and contracts for classes. You've learned about defining, implementing, and using interfaces in this guide. As you continue your journey in Java programming, interfaces will play a key role in structuring your code and promoting reusability.