Interface in Java

Introduction

• An interface in Java is used to achieve abstraction and multiple inheritance.
• It defines what a class should do, but not how it should do it.

In simple words:
Interface is a contract that a class must follow.

What is an Interface?

• An interface is declared using the keyword interface
• It contains abstract methods (by default)
• A class uses implements keyword to implement an interface
• Object cannot be created for an interface

Why Use Interface in Java?

• To achieve abstraction
• To support multiple inheritance
• To achieve loose coupling
• To define common behavior for unrelated classes

Syntax:

interface InterfaceName {
    void method1();
    void method2();
}

• After Java 8, interfaces can have default methods. If a class implements two interfaces that contain default methods with the same signature, the class must override the method and explicitly specify which interface method to call using

InterfaceName.super.methodName();

Important Rules of Interface

• All methods are public and abstract by default
• Variables are public static final (constants)
• Interface methods must be implemented by the class
• A class can implement multiple interfaces
• Interface cannot have constructors