Java Extension Methods

In this tutorial, we will explore Java Extension Methods, a powerful feature introduced in Java 8. Extension methods allow developers to add new functionality to existing classes without modifying their source code. This feature is especially useful when working with library classes or third-party APIs, as it enables us to extend their capabilities without having to subclass or create wrapper classes.


Users can follow the syntax below to create extension methods in Java−

public interface SomeInterface {
   static returnType methodName(arguments) {
      // Method implementation

In the above syntax, we begin by declaring a public interface using the public interface keyword, followed by the desired name for the interface. Inside the interface, we define a static method using the static keyword. We specify the return type of the method and choose a meaningful name for the extension method. Additionally, we can provide any necessary arguments in the arguments section.

Example 1

In this example, we have a List of integers called numbers. We want to square each element of the list using an extension method.

First, we create an extension class called ListExtensions. This class is not related to the built-in List class but provides an extension method for it.

The extension method square takes a List<Integer> as its argument and returns a new List<Integer> with each element squared. Inside the method, we utilize the Stream API to map each number to its square and collect the results into a new list.

In the main method, we create a List of numbers and then call the extension method square directly on the List<Integer> object. The squared numbers are stored in a separate list called squaredNumbers.

Finally, we print the original numbers and the squared numbers to the console.

The extension methods in this example demonstrate how we can extend the functionality of existing classes (in this case, List<Integer>) by adding additional methods to them. This approach enhances code reusability, readability, and modularity by providing a more intuitive syntax and promoting the separation of concerns.

Note − In Java, the concept of extension methods is not natively supported. This example showcases a simulated implementation of extension methods using a separate extension class.

import java.util.List;

public class ExtensionMethodsExample { 
   public static void main(String[] args) {
      List<Integer> numbers = List.of(1, 2, 3, 4, 5);
      // Calling the extension method on the List<Integer> object
      List<Integer> squaredNumbers = square(numbers);
      System.out.println("Original Numbers: " + numbers);
      System.out.println("Squared Numbers: " + squaredNumbers);

   // Extension method to square each element of a List<Integer>
   public static List<Integer> square(List<Integer> list) {
         .map(num -> num * num)


Original Numbers: [1, 2, 3, 4, 5]
Squared Numbers: [1, 4, 9, 16, 25]

Example 2

In this example, we have a String text containing a sentence. We want to count the number of words in this sentence using an extension method.

We create an extension class called StringExtensions that contains the extension method countWords. This method takes a String as its argument and returns the count of words in the string. Inside the method, we use the split method to split the string into an array of words based on whitespace characters, and then we return the length of the array.

We initialize the text variable with a sample sentence in the main method. We then call the extension method countWords directly on the String object text to get the word count.

Finally, we print the original text and the word count to the console

public class ExtensionMethodsExample { 
   public static void main(String[] args) {
      String text = "Hello, World!";
      // Calling the extension method on the String object
      int wordCount = countWords(text);
      System.out.println("Text: " + text);
      System.out.println("Word Count: " + wordCount);

   // Extension method to count the number of words in a String
   public static int countWords(String text) {
      String[] words = text.trim().split("\s+");
      return words.length;


Text: Hello, World!
Word Count: 2


In conclusion, Java Extension Methods offer several powerful features that enhance code modularity, reusability, and flexibility. They provide a non-invasive way to extend existing classes or interfaces, improving the expressiveness and maintainability of code. By leveraging extension methods, developers can write more modular and reusable code, resulting in cleaner and more flexible software systems.

Updated on: 24-Jul-2023

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