private static boolean isHexDigit(char c) {
    return (c >= 'A' && c <= 'F') || (c >= '0' && c <= '9');
  }
Write a printable representation of a byte array. Non-printable characters are hex escaped in
the format \\x%02X, eg: \x00 \x05 etc
@param b   array to write out
@param off offset to start at
@param len length to write
@return string output
hbase-common/src/main/java/org/apache/hadoop/hbase/util/Bytes.java