How to get Android Studio's Java compiler to recognize a Kotlin class?

Issue

I added a Kotlin file that contains three classes to an Android Studio project originally written in Java. The file compiles fine within Android Studio, and the project builds until I try to get a Java class to interact with the Kotlin class. Android Studio gives me the following error:

Java compiler:
   [path]/[project]
      app/src/main/java
         [project].jav
            error: cannot find symbol class Quiz_abTime

When I added the file, Android Studio complained about gradle configuration, but I ran the configuration and it says that all modules that use Kotlin are configured.

Note that I have already tried invalidating the cache and restarting Android Studio, so that isn't the problem.

Here are my build.gradle files. The module first:

// Top-level build file where you can add configuration options common to all sub-projects/modules.

buildscript {
    ext.kotlin_version = '1.2.61'
    repositories {
        jcenter()
        google()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.1.4'
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"

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

allprojects {
    repositories {
        jcenter()
        maven {
            url "https://maven.google.com"
        }
    }
}

Now the app's build.gradle:

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

android {
    compileSdkVersion 26

    defaultConfig {
        applicationId [application name deleted for SO]
        minSdkVersion 16
        targetSdkVersion 26
        versionCode 6
        versionName "2.0.5"
        vectorDrawables.useSupportLibrary = true
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'com.android.support:support-v4:26.1.0'
    implementation 'com.android.support:support-vector-drawable:26.1.0'
    implementation 'com.android.support:appcompat-v7:26.1.0' // where X.X.X version
    implementation 'com.github.yukuku:ambilwarna:2.0.1'
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
    implementation "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
}
repositories {
    mavenCentral()
}

Here's another issue that I include only because it may be related. In my main application, I have the following:

import android.support.v7.app.ActionBar;

Curiously, ActionBar is highlighted in red, and it says it Cannot resolve symbolActionBar`. There are several others highligted like this; this started after Android Studio asked for an upgrade. However, at least this builds; building fails only when I try to access the Kotlin class from a Java class.

I've done Kotlin with Java before in Android Studio and this wasn't a problem, but it Can anyone tell me where to look for issues that are preventing the Java compiler from seeing the Kotlin class?

Solution

In this case it turns out that Android Studio didn't prepend the Kotlin file with an important line:

package [packagename]

The moment I added that, it compiled and ran beautifully. I thought this was something AS would do automatically, but apparently not -- or at least not in this case, for whatever reason. The issues with other symbols it can't find are apparently unrelated.

Answered By - John Perry