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)

Please show us `MyClassLinkingKotlin.java` particularly line 100 (I think its [line,col]) — ThrowingSpoon, May 15 '17 at 12:56
Possible duplicate of [Error trying to instantiate a Kotlin class from Java](http://stackoverflow.com/questions/43834360/error-trying-to-instantiate-a-kotlin-class-from-java) — Ilya, May 15 '17 at 23:04

score 107 · Accepted Answer · answered May 15 '17 at 14:24

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>

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. — Ferran Maylinch, Jan 22 '20 at 07:21

score 24 · Answer 2 · answered Apr 08 '20 at 21:45

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"
}

Note that 'kotlin-android-extensions' plugin is deprecated. – doctorram Jul 25 '23 at 01:06 — doctorram, Jul 25 '23 at 01:06

score 5 · Answer 3 · answered Jun 09 '17 at 20:48

5

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.

answered Jun 09 '17 at 20:48

Sean

2,315
20
25

1

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? – Michal Vician Apr 06 '21 at 08:30

score 5 · Answer 4 · answered Oct 08 '21 at 09:14

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

Using Kotlin class in Java: Cannot find symbol

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