fix warnings.
we can use CollectionConverters because scala-collection-compat provide CollectionConverters for Scala 2.12

• https://github.com/scala/scala/blob/da17a60eba0bdefad8c80154875ab9da419f94e0/src/library/scala/collection/JavaConverters.scala#L78
• https://github.com/scala/scala-collection-compat/blob/56d849a612862ef3b91745b76fa93a47a295ed3d/compat/src/main/scala-2.11_2.12/scala/jdk/CollectionConverters.scala