Can I write Java code in Kotlin file?

One can mix code of Java and Kotlin while designing an Android project .

Kotlin is the preferred language for Android development in 2021 . Both Java and Kotlin can be used to build performant, useful applications, but Google’s libraries, tooling, documentation, and learning resources continue to embrace a Kotlin-first approach; making it the better language for Android today.

Open your Kotlin project in the IntelliJ IDEA / Android Studio. Then navigate to Tools > Kotlin > Show Kotlin Bytecode. You will get the bytecode of your Kotin file. Now click on the Decompile button to get your Java code from the bytecode.