Using Kotlin class in Java: Cannot find symbol

Question

I have found this similar question regarding Android, but I am using plain Java with Maven as build tool. I think it is better to post a new question.

I have created a Kotlin class to which I am trying to refer from Java class as MyKotlinClass.class. The Maven build fails, whereas compilation in IntelliJ Idea works fine. I have already added Kotlin plugin to maven:

        <plugin>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-maven-plugin</artifactId>
            <version>${kotlin.version}</version>
            <executions>
                <execution>
                    <id>compile</id>
                    <phase>compile</phase>
                    <goals>
                        <goal>compile</goal>
                    </goals>
                </execution>
                <execution>
                    <id>test-compile</id>
                    <phase>test-compile</phase>
                    <goals>
                        <goal>test-compile</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>

However that does not help:

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:2.3.2:compile (default-compile) on project app: Compilation failure
[ERROR] MyClassLinkingKotlin.java:[100,40] error: cannot find symbol

The line/column exactly refers to symbol MyKotlinClass.class. It will fail even when using like this:

System.err.println(MyKotlinClass.class)

Giovannagiovanni

Please show us MyClassLinkingKotlin.java particularly line 100 (I think its [line,col]) – Selfconsequence

Added to the question. – Haricot

Possible duplicate of Error trying to instantiate a Kotlin class from Java – Hankering

Answer 1

Your Maven configuration adds the Kotlin compiler plugin, but doesn't adjust the Java compiler plugin execution so that the Java compiler runs after the Kotlin compiler. Therefore, the Java compiler runs before Kotlin, and doesn't see Kotlin-compiled classes.

Here's a snippet showing the correct configuration for a mixed-language project (taken from the documentation):

<build>
    <plugins>
        <plugin>
            <artifactId>kotlin-maven-plugin</artifactId>
            <groupId>org.jetbrains.kotlin</groupId>
            <version>${kotlin.version}</version>
            <executions>
                <execution>
                    <id>compile</id>
                    <goals> <goal>compile</goal> </goals>
                    <configuration>
                        <sourceDirs>
                            <sourceDir>${project.basedir}/src/main/kotlin</sourceDir>
                            <sourceDir>${project.basedir}/src/main/java</sourceDir>
                        </sourceDirs>
                    </configuration>
                </execution>
                <execution>
                    <id>test-compile</id>
                    <goals> <goal>test-compile</goal> </goals>
                    <configuration>
                        <sourceDirs>
                            <sourceDir>${project.basedir}/src/test/kotlin</sourceDir>
                            <sourceDir>${project.basedir}/src/test/java</sourceDir>
                        </sourceDirs>
                    </configuration>
                </execution>
            </executions>
        </plugin>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>3.5.1</version>
            <executions>
                <!-- Replacing default-compile as it is treated specially by maven -->
                <execution>
                    <id>default-compile</id>
                    <phase>none</phase>
                </execution>
                <!-- Replacing default-testCompile as it is treated specially by maven -->
                <execution>
                    <id>default-testCompile</id>
                    <phase>none</phase>
                </execution>
                <execution>
                    <id>java-compile</id>
                    <phase>compile</phase>
                    <goals> <goal>compile</goal> </goals>
                </execution>
                <execution>
                    <id>java-test-compile</id>
                    <phase>test-compile</phase>
                    <goals> <goal>testCompile</goal> </goals>
                </execution>
            </executions>
        </plugin>
    </plugins>
</build>

Neufer

Thank you. So, unfortunately, it seems that intellij doesn't setup the pom file correctly when there is Java and Kotlin together. I had to do it manually by following the documentation. – Chirpy

Answer 2

If you are using Android Studio and have already added the Kotlin plugins via Settings -> Plugins -> Kotlin, then it could be that you haven't setup the rest of Gradle to use Kotlin. Here's a snippet from https://medium.com/@elye.project/setup-kotlin-for-android-studio-1bffdf1362e8:

Step 1: Setup the Kotlin Plugin in Android Studio

Android Studio → Preferences… →Plugins → Browse Repository → type “Kotlin” in search box → install

Step 2: Add Kotlin classpath to project Build.Gradle

buildscript {
    ext.kotlin_version = "1.1.1"
    ext.supportLibVersion = "25.3.0"
    repositories {
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:2.3.0'
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
        classpath "org.jetbrains.kotlin:kotlin-android-extensions:$kotlin_version"

        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

Step 3: Add Kotlin library and apply Kotlin Plugins in your module Build.gradle

apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'
apply plugin: 'kotlin-android-extensions'

android {
    // ... various gradle setup
}

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    testCompile 'junit:junit:4.12'
    compile "com.android.support:appcompat-v7:$supportLibVersion"
    compile "com.android.support:recyclerview-v7:$supportLibVersion"
    compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
}

Pauperize

Note that 'kotlin-android-extensions' plugin is deprecated. – Myocardiograph

Answer 3

I ran into this same error, but had the correct pom setup. My issue was that I had just converted a Java class to a Kotlin class with Intellij, which left that Kotlin file in src/main/java.

The solution for me was to create a src/main/kotlin and move my Kotlin class there, and leave my Java files in src/main/java. But you do definitely need the maven setup that @yole's answer shows.

Certainty

Just curious: why this works is that after Kotlin files are moved to src/main/kotlin they are compiled before Java files? That's how this works? – Ppm

Answer 4

If anyone is using Plain Java project with Gradle instead of Maven then change your module gradle file as follows:

Say you have following plugin in your module:

apply plugin: 'java-library'

If you wanna have kotlin plugin then add kotlin plugin before java plugin:

apply plugin: 'kotlin'
apply plugin: 'java-library'

Reasson is

Your configuration adds the Kotlin compiler plugin, but doesn't adjust the Java compiler plugin execution so that the Java compiler runs after the Kotlin compiler. Therefore, the Java compiler runs before Kotlin, and doesn't see Kotlin-compiled classes.

as explained by @yole in answers

Adp