// FILE: KotlinFile.kt fun Any.foo(): Int { if (this is JavaClass) { something++ return x } return 0 } // FILE: JavaClass.java public abstract class JavaClass extends BaseClass implements JavaInterface { public int getSomething() { return 1; } public void setSomething(int value) { } } // FILE: BaseClass.java public abstract class BaseClass implements JavaInterface { } // FILE: JavaInterface.java public interface JavaInterface { int getX(); }