How to Access Kotlin Extension Functions from Java

Is it possible to access Kotlin extension functions from Java code?

package com.test.extensions

import com.test.model.MyModel

/**
 * Extension function for MyModel
 */
public fun MyModel.bar(): Int {
    return this.name.length
}

Kotlin extension functions are a powerful feature that allow you to add new functions to existing classes without modifying their source code. However, accessing these functions from Java requires a different approach due to the way Kotlin compiles and handles these functions.

// In Java, call the extension function like a static method:
import com.test.extensions.ExtensionsPackage;

MyModel model = new MyModel();
int result = ExtensionsPackage.bar(model); // Correct way to access the extension function.

Causes

• Kotlin extension functions do not compile to member functions. Instead, they are compiled to static methods.
• In Java, you cannot call an extension function as if it were a method of the class because it is not part of the class's method set.

Solutions

• To call a Kotlin extension function from Java, you need to invoke it as a static method using the extension function's receiver class and a separate import: `ExtensionsPackage.bar(model);` where `ExtensionsPackage` is the generated class containing the static methods for extensions.
• Ensure that your Kotlin function is marked as `@JvmStatic` if you prefer to call it without needing to import the entire package.

Mistake: Attempting to access the extension function as a member method on the class instance (e.g., `model.bar();`).

Solution: Use the static method call format: `ExtensionsPackage.bar(model);`.

Mistake: Not importing the correct package that contains the compiled extension function.

Solution: Ensure you have the correct import statement for `ExtensionsPackage` where the extension function is available.