code: public static boolean isWellFormed(byte[] bytes, int off, int len) {
      int end = off + len;
      checkPositionIndexes(off, end, bytes.length);
      // Look for the first non-ASCII character.
      for (int i = off; i < end; i++) {
        if (bytes[i] < 0) {
          return isWellFormedSlowPath(bytes, i, end);
        }
      }
      return true;
    }
docs: Returns whether the given byte array slice is a well-formed UTF-8 byte sequence, as defined by
  {@link #isWellFormed(byte[])}. Note that this can be false even when {@code
  isWellFormed(bytes)} is true.
  
  @param bytes the input buffer
  @param off the offset in the buffer of the first byte to read
  @param len the number of bytes to read from the buffer