A default method is a method define in an interface with a default implementation .it uses the default keyword and allows interfaces to have behavior without forcing all implementing classes to override it .

Syntax:

interface Vehicle
{
  void start();
  default void fuelType(){
   System.out.println("petrol"); 
  }
}

The main purpose of default methods is to enable backward combability when evolving interfaces before java8 , adding a new method to an interface would break all implementing classes with default methods, you can add new functionality without affecting existing code .

Features of Default Methods in Java

1. Method with Implementation in Interface
   Default methods allow you to add method implementations directly inside an interface using the default keyword.
2. Backward Compatibility
   Existing classes that implement the interface do not need to override the new default method, preventing breaking changes.
3. Supports Multiple Inheritance of Behavior
   A class can inherit default methods from multiple interfaces, enabling flexible design.
4. Can Be Overridden
   Implementing classes can override default methods to provide custom behavior.
5. Part of Java 8 Functional Programming
   Default methods support functional programming by working alongside lambda expressions and functional interfaces.

Interface with default method

• Inside Interface every variable is always public, static and final . we can not declare instance variable .
• Interface never talks about state of object.
• Inside interface we can not declare constructor .
• Inside Interface we can not declare instance and static blocks .
• Functional interface with default method can refer lambda expressions .
• Inside interface we can not override object class methods .

Abstract class

• Inside abstract class we can declare instance variables , which are required to the child class .
• Abstract class can talk about state of object .
• Inside Abstract class we can declare constructor .
• we can declare instance and static block .
• Abstract class Can not refer Lambda expressions .
• Inside Abstract class we can override object class Method .

Use Cases of Default Methods in Java

1. Adding New Features to Existing Interfaces
   When you need to extend an interface without breaking older implementations.
2. Providing Common Utility Methods
   Useful for logging, validation, or helper methods shared by multiple classes.
3. Framework & Library Development
   Widely used in Java collections, streams, and APIs for smooth version upgrades.
4. Supporting Multiple Inheritance of Behavior
   When a class needs to inherit functionality from multiple interfaces.

What is Static Method in an Interface

A static method in an interface is a method that belongs to the interface itself , not to the instances of the classes that implement it . it is declared using the static keyword and can be called using the interface name .

Syntax :

Interface Utility {
 static void printMessage(String msg){
   System.out.println("Message :"+msg);  
 }
}

Interface static methods always you can call by using interface Name only .

Utility.printMessage("Hello from static method !")

Purpose of static methods in interfaces

1.Utitility or Helper Methods

Interfaces can now include related utility methods without needing a separate utility class .

2.Encpsulation of logic

Static methods allow encapsulating logic that is specific to the interface and not meant to be overridden by implementing classes .

Interface Static methods with respect to overriding

Interface static method by default not available to the implementation class . that’s why overriding concept not applicable for static method .

interface parentinterf
{
 public static void m1()
 {
 
  }
}

public class childInterf implements parentInterf
{
 public static void m1()
 {
 
 }
}

above code compile successful because static method not participating in overriding .