### Question:
InstantToStringConverter implements Converter<Instant, String> { @Override public String convert(@NonNull Instant source) { ZonedDateTime zonedDateTime = source.atZone(UTC); return FORMATTER.format(zonedDateTime); }  @Override String convert(@NonNull Instant source);  }### Answer:
@Test void should_convert_instant_to_string() { var input = Instant.parse("2019-01-01T12:34:56.789Z"); var expected = "20190101123456789"; InstantToStringConverter converter = new InstantToStringConverter(); var actual = converter.convert(input); assertThat(actual).isEqualTo(expected); }