flatMap.java

  // instead of
  public java.util.List<String> legacyFlatMap1(java.util.List<Optional<String>> args) {
    return args
      .stream()
      .filter(Optional::isPresent)
      .map(Optional::get)
      .map(String::toLowerCase)
      .collect(Collectors.toList());
  }

  // or
  public java.util.List<String> legacyFlatMap2(java.util.List<Optional<String>> args) {
    return args
      .stream()
      .flatMap(arg -> arg.map(Stream::of).orElseGet(Stream::empty))
      .map(String::toLowerCase)
      .collect(Collectors.toList());
  }

  // or
  public java.util.List<String> legacyFlatMap3(java.util.List<Optional<String>> args) {
    // since Java 9 introducing Optional.stream()
    return args
      .stream()
      .flatMap(Optional::stream)
      .map(String::toLowerCase)
      .collect(Collectors.toList());
  }

  // do it the functional way
  public io.vavr.collection.Seq vavrFlatMap(io.vavr.collection.Seq<Option<String>> args) {
    return args
      .flatMap(Function.identity()) // sames as .flatMap(arg -> arg)
      .map(String::toLowerCase);
  }