KotlinClass.kt

class KotlinClass {
    // here foo is declared as not nullable (nullable declaration would have been fun makeItCrash(foo: String?) 
    fun makeItCrash(foo: String) {
    }
}

Main.java

public class Main {

    public static void main(String[] args) {
        KotlinClass kotlinClass = new KotlinClass();
        kotlinClass.makeItCrash(null); 
        // AndroidStudio only warns you, but you will get an IllegalArgumentException during execution 
        // if you pass a null value from Java code to a non-nullable Kotlin method parameter.
    }
}