case '/':
        if (match('/')) {
          // A comment goes until the end of the line.
          while (peek() != '\n' && !isAtEnd())
            advance();
        } else {
          addToken(SLASH);
        }
        break;

      case ' ':
      case '\r':
      case '\t':
        // Ignore whitespace.
        break;
      case '\n':
        line++;
        break;

  }
  private char peek() {
    if (isAtEnd())
      return '\0';
    return source.charAt(current);