Kotlin - Error: Could not find or load main class _DefaultPackage

For me it worked after I installed the correct JDK. I first had JDK 11 but the tutorial I did was with JDK 8 so after I installed this and set it in the "installed JREs" options it found the main class without having any "mainClassName" or any other option in the build.gradle file.

I have been getting the same issue. And after putting the right compiler output path, it got resolved.

Go to Project -> Project Compiler output : In the text box, fill this: [Absolute Path]/{Project Name}/out

In my case I was having this problem while trying to run the program using the Application Gradle plugin. The problem was in the mainClassName property using single quotes instead of double ones

This didn't work:

mainClassName = 'demo.HelloWorldKt'

With double quotes, it works:

mainClassName = "demo.HelloWorldKt"

This was a severe bug (KT-10221) in automatic generation of Launch Configuration in plugin version 0.4.0. It was fixed in 0.5.0 so the recommendend way to workaround is to update plugin.

The source of the problem was that the plugin used an old pattern for generating name of the class for main function that had been abandoned by Kotlin compiler.

It's possible to workaround it by editing launch configuration (Eclipse Menu -> Run -> Run Configurations...) by hand and changing Main class field in Java Application group. If the file is named hello.kt with no package directive, as it is described in tutorial, than corrected string should be HelloKt.

If file has name other.kt with package my.tutorial than the Main Class should contain my.tutorial.HelloKt. You can read more about it in the section Package-Level Functions of Calling Kotlin From Java page.