### Question:
B3Propagator implements TextMapPropagator { @Override public List<String> fields() { return FIELDS; } private  B3Propagator(
      B3PropagatorInjector b3PropagatorInjector, B3PropagatorExtractor b3PropagatorExtractor); static B3Propagator getSingleHeaderPropagator(); static B3Propagator getMultipleHeaderPropagator(); @Override List<String> fields(); @Override void inject(Context context, C carrier, Setter<C> setter); @Override Context extract(Context context, C carrier, Getter<C> getter);  }### Answer:
@Test void fieldsList() { assertThat(b3Propagator.fields()) .containsExactly( B3Propagator.TRACE_ID_HEADER, B3Propagator.SPAN_ID_HEADER, B3Propagator.SAMPLED_HEADER); }