Can you implement an equivalent of extension…

Yes, directly — no, because Java does not support them at the syntax level like Kotlin. But you can imitate their behavior using static helper methods or the Decorator pattern.

1. Static helper methods:

Create a class with static methods that accept the object you want to "extend" as the first argument.

public class StringExtensions {
    // Method that mimics extension for String
    public static String capitalizeFirstLetter(String input) {
        if (input == null || input.isEmpty()) {
            return input;
        }
        return input.substring(0, 1).toUpperCase() + input.substring(1);
    }
}

Usage:

String myString = "hello";
String capitalizedString = StringExtensions.capitalizeFirstLetter(myString);

2. Decorator pattern:

Create a wrapper class that implements the same interface as the original class (or extends it) and adds new functionality.

public interface MyInterface {
    void originalMethod();
}

public class MyClass implements MyInterface {
    @Override
    public void originalMethod() {
        System.out.println("Original method");
    }
}

// Decorator
public class MyInterfaceDecorator implements MyInterface {
    private MyInterface decorated;

    public MyInterfaceDecorator(MyInterface decorated) {
        this.decorated = decorated;
    }

    @Override
    public void originalMethod() {
        decorated.originalMethod();
    }

    // "Extended" functionality
    public void newMethod() {
        System.out.println("New extended method");
    }
}

Usage:

MyClass original = new MyClass();
MyInterfaceDecorator decorated = new MyInterfaceDecorator(original);
decorated.originalMethod();
decorated.newMethod();

Although these approaches are not as convenient as native extension functions in Kotlin, they allow adding functionality to existing classes without modifying their source code.

Can you implement an equivalent of extension functions in Java?