WhyCauseIllegalAccessError.java

// Package X
class Foo {
  interface FooInterface {
    void fnFoo();
  }
}

public class Bar {
  public void prepareFoo(VisibleFooInterface foo) {
    foo.fnFoo();
  }
  
  public interface VisibleFooInterface extends Foo.FooInterface {
    // below code is waste whether or not
    // void fnFoo();
  }
}

// Package Y (Kotlin)
class BuzKt {
  fun main(args : Array<String>) { 
    // belows are crash `java.lang.IllegalAccessError: 
    // Illegal class access: 'BuzKt' 
    // attempting to access 'Foo$FooInterface'`
    Bar().prepareFoo(object: VisibleFooInterface { println("Hoge") })
    Bar().prepareFoo { println("Fuga") }
  }
}