Calling Kotlin from Java -- error: package demo does not exist

Question

I don't understand the documentation:

Package-Level Functions

All the functions and properties declared in a file example.kt inside a package org.foo.bar, including extension functions, are compiled into static methods of a Java class named org.foo.bar.ExampleKt.

// example.kt
package demo

class Foo

fun bar() {
}

// Java
new demo.Foo();
demo.ExampleKt.bar();

my code below.

compile error; build failed:

thufir@dur:~/NetBeansProjects/kotlin$ 
thufir@dur:~/NetBeansProjects/kotlin$ gradle compileJava

> Task :compileJava
/home/thufir/NetBeansProjects/kotlin/src/main/java/net/bounceme/dur/kotlin/App.java:12: error: package demo does not exist
        new demo.Foo();
                ^
/home/thufir/NetBeansProjects/kotlin/src/main/java/net/bounceme/dur/kotlin/App.java:13: error: package demo does not exist
        demo.ExampleKt.bar();
            ^
2 errors


FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':compileJava'.
> Compilation failed; see the compiler error output for details.

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

* Get more help at https://help.gradle.org

BUILD FAILED in 0s
1 actionable task: 1 executed
thufir@dur:~/NetBeansProjects/kotlin$ 

java source:

package net.bounceme.dur.kotlin;

import java.util.logging.Logger;

public class App {

    private static final Logger LOG = Logger.getLogger(App.class.getName());

    private void run() {
        LOG.info("running");

        new demo.Foo();
        demo.ExampleKt.bar();

    }

    public static void main(String[] args) {
        new App().run();
    }
}

kotlin source:

package demo;

class Foo

fun bar() {
}

project:

thufir@dur:~/NetBeansProjects/kotlin$ 
thufir@dur:~/NetBeansProjects/kotlin$ tree
.
├── build
│   ├── classes
│   │   └── java
│   │       └── main
│   └── tmp
│       └── compileJava
├── build.gradle
├── gradle
│   └── wrapper
│       ├── gradle-wrapper.jar
│       └── gradle-wrapper.properties
├── gradlew
├── gradlew.bat
├── settings.gradle
└── src
    ├── main
    │   ├── java
    │   │   └── net
    │   │       └── bounceme
    │   │           └── dur
    │   │               └── kotlin
    │   │                   └── App.java
    │   └── kotlin
    │       └── example.kt
    └── test
        └── java
            └── AppTest.java

18 directories, 9 files
thufir@dur:~/NetBeansProjects/kotlin$ 

Technics

Apply the kotlin-android plugin to your library module build.gradle (see the docs): apply plugin: 'kotlin-android' – Stickweed

Answer 1

use this in your build.gradle (module):

apply plugin: 'kotlin-android'

Unaware

Answer 2

1. Make sure you have the Kotlin plugin installed in Android Studio
2. add Kotlin to the class path in your projects gradle.build file:

         dependencies {
            classpath "com.android.tools.build:gradle:4.1.2"
            classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:1.4.21"
            classpath "org.jetbrains.kotlin:kotlin-android-extensions:1.4.21"
            // NOTE: Do not place your application dependencies here; they belong
            // in the individual module build.gradle files
        }

3. apply the kotlin-android plugin on your modlues (app) gradle.build like this:

   plugins {
       id 'com.android.application'
       id 'kotlin-android'
   }
   

Smutchy

Answer 3

The demo folder should be under the src/main/kotlin and the example.kt under it.

So it should be src/main/kotlin/demo/example.kt

[edit] It is not required to match directories and packages: source files can be placed arbitrarily in the file system.

[edit2] I made an example worked :

Java code

package test;

import demo.HelloWorldKt;

 public class Test {
    public static void main(String[] args) {
       ExampleKt.bar();
    }
}

Kotlin code :

package demo

fun bar() {
    println("yeah")
}

Where the kotlin file is helloWrold.kt directly under src/main/kotlin

The import of the kotlin file seems missing in your java file.

Golgotha

Absolutley, thx.yes, I made that change already; will update whole question to reflect that. – Technics

no. wait. you say ../kotlin/demo/example.kt but that's NOT what the demo (link at bottom of q) seems to do. They just seem to do ../kotlin/example.kt I think... – Technics

Yeah but the error tell you that it’s try to find demo.example.kt and you don’t have the demo package under your kotlin source files. – Golgotha

new demo.Foo(); is not useful but is just an example of how you can create an kotlin object from a java class. It's confusing though – Golgotha

It's confusing but I read in the documentation that It is not required to match directories and packages: source files can be placed arbitrarily in the file system. – Golgotha

apply plugin: 'kotlin-android' to your library module build.gradle. – Stickweed

Answer 4

//app level build.gradle

    id("com.android.application")
    id("org.jetbrains.kotlin.android") //Added for kotlin
}


//project level build.gradle
plugins {
    id("com.android.application")
    id("org.jetbrains.kotlin.android")
}

Bourbon

Your answer could be improved with additional supporting information. Please edit to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center. – Epizootic