public class StreamMapperTest {

  @Test
  public void test() {

    List<String> addresses = Arrays.asList("test","app","go@com");

    addresses.stream().map(v -> getDomain(v)).forEach(System.out::println);
  }

  private static String getDomain(String address) {
    String[] s = address.split("@");
    if(Arrays.stream(s).count() != 2) {
      return null;
    } else {
      return s[1];
    }
  }