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What Is An Interface

In object-oriented programming (OOP), an interface is a fundamental concept that defines a contract or a set of abstract methods that a class must implement. An interface, in essence, specifies a list of methods that a class that implements the interface must provide. Interfaces do not contain any actual method implementations; they only declare the method signatures (names, parameters, and return types).


  • What is an interface?
  • Creating and using an interface.
  • Extending interfaces
  • Implementing multiple interfaces.
  • What are abstract classes?
  • Creating and Using abstract classes.
  • Differences between abstract classes and interface

What is an interface?

  • It is a mechanism by which Java somewhat achieves multiple inheritance.
  • It is a system using which unrelated objects interact with each other.
  • An interface is a class where all the fields are public,static & final and all the methods are public & abstract.
  • But it is different from multiple inheritance

Creating and using an interface

					<Modifier> interface 
<InterfaceName> extends <InterfaceNames>
//statement block
class <ClassName> implements < 
InterfaceName >{
//statement block


Extending interfaces

  • An interface, unlike other classes can extend multiple interfaces.
  • A class implementing an extended interface has to implement the methods of both the interface, as well as its ancestors.

Implementing multiple interfaces

  • A class can choose to implement multiple interfaces.
  • It then has to implement all the methods defined in all the chosen interfaces.
  • Helpful in spreading concept of multiple inheritance, where a class chooses to pick multiple behavioral patterns
					// Define an interface
interface Shape {
    double calculateArea(); 
    // Abstract method
    double calculatePerimeter(); 
    // Abstract method

// Implement the interface in a class
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;


In this example, the Shape interface defines two abstract methods: calculateArea() and calculatePerimeter(). The Circle class implements the Shape interface by providing concrete implementations for these methods.

Interfaces are a fundamental concept in OOP that promotes code reusability, maintainability, and polymorphism, making them a powerful tool in software development.