code: public static String encodeWithColons(byte[] in) {
          if (in == null) {
              return null;
          }
          return encodeWithColons(in, 0, in.length);
      }
docs: Encodes the given bytes into a base-16 string, inserting colons after
  every 2 characters of output.
  
  @param in
  the bytes to encode.
  
  @return The formatted base-16 string, or null if {@code in} is null.