Kotlin Foo::class.java "Unresolved Reference: Java" error - kotlin

Question

I am trying to convert my Java code of HomePage.class to Kotlin. I am following the instructions on Kotlin.org:
getClass()
To retrieve the type information from an object, we use the javaClass
extension property.
val fooClass = foo.javaClass
Instead of Java’s Foo.class use
Foo::class.java.
val fooClass = Foo::class.java
I have a class called HomePage that extends AppCompatActivity (in Android). I am using Android Studio. I tried doing HomePage::class.java and it has an error: Unresolved reference: java
How do I get this to work?

Answer 1

The issue is most likely that you forgot to depend on the reflection libraries which were needed for the reflective functions of Kotlin.
On the Java platform, the runtime component required for using the
reflection features is distributed as a separate JAR file
(kotlin-reflect.jar). This is done to reduce the required size of the
runtime library for applications that do not use reflection features.
If you do use reflection, please make sure that the .jar file is added
to the classpath of your project.
Source

Answer 2

It turns out, I was using an older version of Kotlin, and it wasn't configured correctly. I edited the gradle file to include the latest beta version, and selected the option that configures Kotlin, and it works now.
In gradle:
buildscript {
ext.kotlin_version = '1.0.0-beta-3594'
repositories {
mavenCentral()
}
dependencies {
classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
classpath "org.jetbrains.kotlin:kotlin-android-extensions:$kotlin_version"
}
}
...
dependencies {
compile fileTree(dir: 'libs', include: ['*.jar'])
compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
}

Answer 3

I Put in the beginning of Gradle (Module app)
apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'
apply plugin: 'kotlin-android-extensions'
and
implementation "org.jetbrains.kotlin:kotlin-stdlib-jre7:$kotlin_version"
or
implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlin_version"
in the dependencies section
In the build.gradle (Project)
buildscript {
ext.kotlin_version = '1.2.0'
repositories {
jcenter()
google()
}
dependencies {
classpath 'com.android.tools.build:gradle:3.0.1'
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
}
}

Answer 4

For easy reference, here are the reflection dependencies when using Gradle:
Reflection libraries from the docs for Gradle in your build.gradle
dependencies {
compile "org.jetbrains.kotlin:kotlin-stdlib-jdk8"
compile "org.jetbrains.kotlin:kotlin-reflect"
testCompile "org.jetbrains.kotlin:kotlin-test"
testCompile "org.jetbrains.kotlin:kotlin-test-junit"
}
Reflection libraries syntax for Kotlin Script Gradle DSL in your build.gradle.kts
dependencies {
compile(kotlin("stdlib"))
compile(kotlin("reflect"))
compile(kotlin("test"))
compile(kotlin("test-junit"))
}

Answer 5

In build.gradle (app) I had:
implementation 'androidx.core:core-ktx:1.6.0'
And I replaced it by
implementation 'androidx.core:core-ktx:1.5.0'
And it worked!
The core version also killed some of y functions like: forEach

Answer 6

I used
implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlin_version"
Instead of
implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"

Answer 7

i use this
implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlin_version"
invalided cache & restart android studio!works fine!!

Answer 8

This happened to me after updating Android Studio and the Kotlin plugin, which required migration to a newer version of Kotlin. After configuring everything correctly, the error still persisted. Deleting .idea folder and reloading did not help, but good old File -> Invalidate Caches / Restart did the trick for me.

Answer 9

I copied the class from other project and forgot to change the class package name.
when I changed, it fixed

Answer 10

I saw this in AndroidStudio with Kotlin 1.2.71 and none of the above fixed it for me.
What sadly hilariously worked for me was closing the project, telling AndroidStudio to forget about the project and re-opening from the folder. Presto, no unresolved reference. Weird, I know.

Answer 11

In my case reducing version of kotlin fixed the issue
Change
ext.kotlin_version = '1.3.21'
To
ext.kotlin_version = '1.3.11'

Answer 12

Add this line in your Module build gradle
implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
In Project build gradle:
buildscript {
ext.kotlin_version = '1.3.21'
repositories {
google()
jcenter()
}
dependencies {
classpath 'com.android.tools.build:gradle:3.3.2'
classpath 'com.google.gms:google-services:4.2.0'
classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
// NOTE: Do not place your application dependencies here; they belong
// in the individual module build.gradle files
}
}

Answer 13

I encountered this error when i downloaded and run android studio canary(artic fox). Since then my android studio(normal one) was giving this error i applied all the solutions given above but nothing worked . So i completely uninstalled the android studio and reinstalled the latest one (this was the only option left)