How to Use Kotlin in an Existing Android App?

Question

I have an Android App developed using Java. I now want to start using Kotlin for the same app. Is it possible to use Kotlin and Java side-by-side in an existing app?

Answer 1

In Android Studio 4, there is no need to manually configure Kotlin for a Java project.

When adding Kotlin files to a Java project, Android Studio will automatically setup Kotlin.

Here is the full procedure:

1. Add any new Android component (Activity for example)

2. Choose Kotlin as the language

3. Android Studio downloads Kotlin

Note: When it finished, it could not resolve the R file in the Kotlin Activity. To fix this, I just hit hovered over the red R in the editor and hit alt-enter.

Finally, in my MainActivity.java:

Intent launchNewIntent = new Intent(this, KotlinActivity.class);
startActivityForResult(launchNewIntent, 0);

This code opens the KotlinActivity from the MainActivity in Java.