Interface in Java

Source: Dev.to

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.

What is an Interface?

• Declared using the interface keyword.
• Contains abstract methods (by default).
• A class uses the implements keyword to implement an interface.
• Objects cannot be created directly from an interface.

Why Use Interfaces 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();
}

Default Methods (Java 8+)

After Java 8, interfaces can include 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 invoke:

InterfaceName.super.methodName();

Important Rules of Interfaces

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