如何让 Android Studio 的 Java 编译器识别 Kotlin class？

Question

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 symbol ActionBar`. 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?

Answer 1

In my case, I forgot to add a dependency to my Java project. The compiler was complaining about Kotlin files.

implementation 'androidx.core:core-ktx:1.9.0'

Answer 2

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.