How to convert a Kotlin source file to a Java source file

you can go to Tools > Kotlin > Show kotlin bytecode

You can compile Kotlin to bytecode, then use a Java disassembler.

The decompiling may be done inside IntelliJ Idea, or using FernFlower https://github.com/fesh0r/fernflower (thanks @Jire)

There was no automated tool as I checked a couple months ago (and no plans for one AFAIK)

As @Vadzim said, in IntelliJ or Android Studio, you just have to do the following to get java code from kotlin:

1. Menu > Tools > Kotlin > Show Kotlin Bytecode
2. Click on the Decompile button
3. Copy the java code

Update:

With a recent version (1.2+) of the Kotlin plugin you also can directly do Menu > Tools > Kotlin -> Decompile Kotlin to Java.