code: public void append(final String str) {
          final String s = str != null ? str : "null";
          final int strlen = s.length();
          final int newlen = this.len + strlen;
          if (newlen > this.array.length) {
              expand(newlen);
          }
          s.getChars(0, strlen, this.array, this.len);
          this.len = newlen;
      }
docs: Appends chars of the given string to this buffer. The capacity of the
  buffer is increased, if necessary, to accommodate all chars.
  
  @param str    the string.