What is a Nested Interface in Java?

This blog will help you understand what nested interfaces are, why they’re useful, and how they differ from nested classes.

Nested Interface

• A nested interface is an interface that is declared inside another class or interface.
• It is a static interface by default.
• Nested interfaces can be accessed using the outer class or interface name.
• They help logically group interfaces that are only used by their outer class or interface.
• They can be declared public, private, or protected depending on their enclosing class.

Example of Nested Interface

class OuterClass {
 interface NestedInterface {
 void display();
 }
}

class InnerClass implements OuterClass.NestedInterface {
 public void display() {
 System.out.println("This is a nested interface example.");
 }
}

public class Main {
 public static void main(String[] args) {
 OuterClass.NestedInterface obj = new InnerClass();
 obj.display();
 }
}

Uses of Nested Interface

• Encapsulation: Groups related interfaces with the classes they belong to.
• Modularity: Keeps the code organized and readable.
• Access Control: Provides the ability to restrict access to the interface.
• Namespace Management: Prevents name clashes by placing the interface in a specific scope.

Difference between Nested Interface and Nested Class

	Nested Interface	Nested Class
Definition	An interface declared inside a class or interface.	A class declared inside another class.
Type	Always static by nature.	Can be static or non-static (inner class).
Implementation	Implemented by a class using OuterClass.InterfaceName syntax.	Instantiated using the outer class object (if non-static).
Use Case	To define a contract specific to the outer class or interface.	To group helper classes or for logic encapsulation.
Inheritance	Only methods — no concrete behavior.	Can have fields and concrete methods.