Accessing Kotlin extension functions from Java

Question

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);

Kotlin top-level extension function are compiled as Java static methods.

Given Kotlin file Extensions.kt in package foo.bar containing:

fun String.bar(): Int {
    ...
}

The equivalent Java code would be:

package foo.bar;

class ExtensionsKt {
    public static int bar(String receiver) { 
        ...
    }
}

Unless, that is, Extensions.kt contained the line

@file:JvmName("DemoUtils")

In which case the Java static class would be named DemoUtils

In Kotlin, extension methods can be declared in other ways. (For example, as a member function or as an extension of a companion object.)

I have a Kotlin file called NumberFormatting.kt that has the following function

fun Double.formattedFuelAmountString(): String? {
    val format = NumberFormat.getNumberInstance()
    format.minimumFractionDigits = 2
    format.maximumFractionDigits = 2
    val string = format.format(this)
    return string
}

In java I simple access it over the file NumberFormattingKt in the following way after the required import import ....extensions.NumberFormattingKt;

String literString = NumberFormattingKt.formattedFuelAmountString(item.getAmount());

You can always see the actual Java code which is getting generated from your Kotlin code by going to Tools > Kotlin > Show Kotlin Bytecode, then clicking Decompile. This can help you tremendously. In your case the Java code will look like this if you have MyModelExtensions.kt

public final class MyModelExtensionsKt {
   public static final int bar(@NotNull MyModel $receiver) {
      Intrinsics.checkParameterIsNotNull($receiver, "$receiver");
      return $receiver.getName().length();
   }
}

you can improve over this by using @JvmName on the file containing bar:

@file:JvmName("MyModels")
package io.sspinc.datahub.transformation

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

and it will result in this code:

public final class MyModels {
   public static final int bar(@NotNull MyModel $receiver) {
      Intrinsics.checkParameterIsNotNull($receiver, "$receiver");
      return $receiver.getName().length();
   }
}

Using MyModels is in line with what Effective Java suggests for utility classes. You can also rename your method like this:

public fun MyModel.extractBar(): Int {
    return this.name.length
}

then from the Java side it will look idiomatic:

MyModels.extractBar(model);

With newer KotlinEx, you can directly call extension in java

ExtensionFileName.foo(field1...)

Basically, what it does is, it makes the receiver as first arguement and other arguments remain being at the same place

So For eg.

You have extension(in file Extension.kt)

Context.showToast(message:String){ 
...
}

In Java, you call it as

ExtensionKt.showToast(context, message);

It works for me:

Kotlin

Java code

My project is an old android project created with Java; now I created the first kotlin file and added String extensions fun String.isNotNullOrEmpty(): Boolean {... }

and I could call it from java file using: StringUtilsKt.isNotNullOrEmpty(thestring).

My kotlin file name is StringUtils