Automatic tool that can accurately convert Kotlin code to Java code. Most conversion tools focus on converting Java to Kotlin, because Kotlin was designed to be an improvement over Java in many ways.

That said, it’s certainly possible to manually convert Kotlin code into Java, given that you understand both languages. Here’s a high-level process you might follow:

1. Understand the functionality of the Kotlin code: Before you can translate it to Java, you need to know exactly what it’s doing. Ensure you understand any Kotlin-specific syntax and conventions.

2. Plan the structure of the Java equivalent: Many constructs in Kotlin have direct equivalents in Java, so start by identifying what classes, methods, and variables you’ll need in the Java code.

3. Translate Kotlin-specific constructs: Some things in Kotlin, such as nullable types and extension functions, don’t have direct equivalents in Java, so you’ll need to come up with alternative ways to handle these.

4. Test the Java code: After you’ve done the conversion, make sure to thoroughly test the Java code to ensure it behaves as expected.

You can use this approach to convert even complex Kotlin codebases into Java. However, this could be a time-consuming process, especially for large codebases, and it could introduce bugs if not done carefully.

Another challenge is that Kotlin has features such as coroutines, extension functions, null safety, and more that do not have direct equivalents in Java. If your Kotlin code makes heavy use of these features, the translated Java code could end up looking very different and might not maintain the same functionality.

Remember, though, that both Kotlin and Java compile down to JVM bytecode, which means they’re interoperable. If you have a mixed codebase or are in the process of a transition, you don’t necessarily need to convert all your Kotlin code to Java. You can call Kotlin code from Java and vice versa. This might be a better solution if you’re looking to gradually move from Kotlin to Java