MatchLocator.java

public static Map<String, Set<String>> idsAndFields(final Collection<Match> matches) {
  return matches.stream()
    .collect(Collectors.groupingBy(
      match -> match.id,
      Collectors.flatMapping(
        match -> match.positions().stream()
          .map(position -> position.fieldName),
        Collectors.toSet()
      )
    ));
}