Accessing Kotlin extension functions from Java

Question

Is it possible to access extension functions from Java code?

I defined the extension function in a Kotlin file.

package com.test.extensions

import com.test.model.MyModel

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

Where MyModel is a (generated) java class. Now, I wanted to access it in my normal java code:

MyModel model = new MyModel();
model.bar();

However, that doesn't work. The IDE won't recognize the bar() method and compilation fails.

What does work is using with a static function from kotlin:

public fun bar(): Int {
   return 2*2
}

by using import com.test.extensions.ExtensionsPackage so my IDE seems to be configured correctly.

I searched through the whole Java-interop file from the kotlin docs and also googled a lot, but I couldn't find it.

What am I doing wrong? Is this even possible?

Answer 1

All Kotlin functions declared in a file will be compiled by default to static methods in a class within the same package and with a name derived from the Kotlin source file (First letter capitalized and ".kt" extension replaced with the "Kt" suffix). Methods generated for extension functions will have an additional first parameter with the extension function receiver type.

Applying it to the original question, Java compiler will see Kotlin source file with the name example.kt

package com.test.extensions

public fun MyModel.bar(): Int { /* actual code */ }

as if the following Java class was declared

package com.test.extensions

class ExampleKt {
    public static int bar(MyModel receiver) { /* actual code */ }
}

As nothing happens with the extended class from the Java point of view, you can't just use dot-syntax to access such methods. But they are still callable as normal Java static methods:

import com.test.extensions.ExampleKt;

MyModel model = new MyModel();
ExampleKt.bar(model);

Static import can be used for ExampleKt class:

import static com.test.extensions.ExampleKt.*;

MyModel model = new MyModel();
bar(model);