### Question:
Util { public static String toDebugString(ByteString bytes) { int size = bytes.size(); StringBuilder result = new StringBuilder(2 * size); for (int i = 0; i < size; i++) { int value = bytes.byteAt(i) & 0xFF; result.append(Character.forDigit(value >>> 4, 16)); result.append(Character.forDigit(value & 0xF, 16)); } return result.toString(); }  static String autoId(); static int compareBooleans(boolean b1, boolean b2); static int compareIntegers(int i1, int i2); static int compareLongs(long i1, long i2); static int compareDoubles(double i1, double i2); static int compareMixed(double doubleValue, long longValue); @SuppressWarnings("unchecked") static Comparator<T> comparator(); static FirebaseFirestoreException exceptionFromStatus(Status error); static Exception convertThrowableToException(Throwable t); static Continuation<Void, Void> voidErrorTransformer(); static List<Object> collectUpdateArguments(
      int fieldPathOffset, Object field, Object val, Object... fieldsAndValues); static String toDebugString(ByteString bytes); static String typeName(@Nullable Object obj); static void crashMainThread(RuntimeException exception); static int compareByteStrings(ByteString left, ByteString right);  }### Answer:
@Test public void testToDebugString() { assertEquals("", Util.toDebugString(ByteString.EMPTY)); assertEquals("00ff", Util.toDebugString(TestUtil.byteString(0, 0xFF))); assertEquals("1f3b", Util.toDebugString(TestUtil.byteString(0x1F, 0x3B))); assertEquals( "000102030405060708090a0b0c0d0e0f", Util.toDebugString( TestUtil.byteString(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0xA, 0xB, 0xC, 0xD, 0xE, 0xF))); }