code: public static String valueOf(String source, int offset16) {
          switch (bounds(source, offset16)) {
          case LEAD_SURROGATE_BOUNDARY:
              return source.substring(offset16, offset16 + 2);
          case TRAIL_SURROGATE_BOUNDARY:
              return source.substring(offset16 - 1, offset16 + 1);
          default:
              return source.substring(offset16, offset16 + 1);
          }
      }
docs: Convenience method corresponding to String.valueOf(codepoint at offset16). Returns a one or
  two char string containing the UTF-32 value in UTF16 format. If offset16 indexes a surrogate
  character, the whole supplementary codepoint will be returned. If a validity check is
  required, use {@link android.icu.lang.UCharacter#isLegal(int)} on the
  codepoint at offset16 before calling. The result returned will be a newly created String
  obtained by calling source.substring(..) with the appropriate indexes.
  
  @param source The input string.
  @param offset16 The UTF16 index to the codepoint in source
  @return string value of char32 in UTF16 format