### Question:
SpanId { public static boolean isValid(String spanId) { return (spanId.length() == HEX_SIZE) && !INVALID.equals(spanId) && BigendianEncoding.isValidBase16String(spanId); } private  SpanId(); static int getSize(); static int getHexLength(); static String getInvalid(); static String fromLong(long id); static byte[] bytesFromHex(String src, int srcOffset); static boolean isValid(String spanId); static String bytesToHex(byte[] spanId);  }### Answer:
@Test void isValid() { assertThat(SpanId.isValid(SpanId.getInvalid())).isFalse(); assertThat(SpanId.isValid(SpanId.bytesToHex(firstBytes))).isTrue(); assertThat(SpanId.isValid(SpanId.bytesToHex(secondBytes))).isTrue(); assertThat(SpanId.isValid("000000000000z000")).isFalse(); }