### Question:
ZipkinKeys { @Nullable public static String sampledToString(@Nullable Boolean sampled) { return sampled == null ? null : (sampled ? DO_SAMPLE_VALUE : DO_NOT_SAMPLE_VALUE); } private  ZipkinKeys(); @Nullable static String sampledToString(@Nullable Boolean sampled); @Nullable static Boolean sampledToBoolean(@Nullable String sampled); static final String TRACE_ID; static final String SPAN_ID; static final String PARENT_SPAN_ID; static final String SAMPLED; static final String FLAGS; static final String DO_SAMPLE_VALUE; static final String DO_NOT_SAMPLE_VALUE; }### Answer:
@Test public void sampledToString_WhenSampledIsNull_ShouldReturnNull() { assertEquals(null, ZipkinKeys.sampledToString(null)); }
@Test public void sampledToString_WhenSampledIsTrue_ShouldReturnQuoted1() { assertEquals("1", ZipkinKeys.sampledToString(Boolean.TRUE)); }
@Test public void sampledToString_WhenSampledIsFalse_ShouldReturnQuoted0() { assertEquals("0", ZipkinKeys.sampledToString(Boolean.FALSE)); }