code: public boolean startsWith(final String str) {
          if (str == null) {
              return false;
          }
          final int len = str.length();
          if (len == 0) {
              return true;
          }
          if (len > size) {
              return false;
          }
          for (int i = 0; i < len; i++) {
              if (buffer[i] != str.charAt(i)) {
                  return false;
              }
          }
          return true;
      }
docs: Checks whether this builder starts with the specified string.
  <p>
  Note that this method handles null input quietly, unlike String.
  
  @param str  the string to search for, null returns false
  @return true if the builder starts with the string