How can one add static methods to Java classes in Kotlin

As of Kotlin 1.3, this is not possible. However, it's being considered for a future release!

To help this feature get implemented, go vote on this issue: https://youtrack.jetbrains.com/issue/KT-11968

This idea is very popular in the Kotlin community, so I bet it'll be in soon enough.

I think this is not possible. Documentation says the following:

If a class has a companion object defined, you can also define extension functions and properties for the companion object.

The Math class is a Java class, not a Kotlin one and does not have a companion object in it. You can add a clamp method to the Double class instead.

As of Kotlin 1.2 it is still not possible.

As a workaround, to statically "extend" Environment class I am currently using:

Class EnvironmentExtensions {
    companion object {
        @JvmStatic
        fun getSomething(): File {
            ...
            return Environment.something()
        }
    }
}

It is not an ideal solution but IntelliJ/Android Studio code completion helps with the usage:

val something = EnvironmentExtensions.getSomething()