There is an instruction and Kotlin sample code in the following site.
Extend an Android app to Google Assistant with App Actions:
https://codelabs.developers.google.com/codelabs/appactions/#0
Is it possible to implement this function by Java not Kotlin? I am wondering if Google people don't support the function in Java because it is the latest technology. If they still support it in Java, can you share any Java sample code of the function?