How to access fields in a Kotlin object from Java

An interesting thing I ran into today when using mixed Kotlin and Java code on an Android project is that if you want to declare constants in a Kotlin object, you’ll either want to do this:

object Constants {

    @JvmField internal val UPDATE_INTERVAL_IN_MILLISECONDS: Long = 10000
    @JvmField internal val REQUEST_CHECK_SETTINGS = 100
    @JvmField internal val last_known_location = "last_known_location"

}

or this:

object Constants {

    internal const val UPDATE_INTERVAL_IN_MILLISECONDS: Long = 10000
    internal const val REQUEST_CHECK_SETTINGS = 100
    internal const val last_known_location = "last_known_location"

}

Android Studio reports that the second version that uses const is preferred, and I can confirm that both approaches work. Regardless of which Kotlin approach you use, you can now access these fields in your Kotlin object in your Java code like this:

resolveableException.startResolutionForResult(
    MainActivity.this, 
    Constants.REQUEST_CHECK_SETTINGS
);

if (savedInstanceState.containsKey(Constants.last_known_location)) { ...

If you don’t use one of those approaches you’ll have to reference your Kotlin fields as “get” methods from Java — and nobody wants to do that.

A few notes from the Kotlin docs before I go:

• internal means “visible everywhere in the same module”
• Properties annotated with const (in classes as well as at the top level) are turned into static fields in Java
• Annotating a property with @JvmField makes it a static field with the same visibility as the property itself

In summary, if you ever need to access fields in a Kotlin object from Java code, I hope this example is helpful.

Reporting from Broomfield, Colorado,
Alvin Alexander