private void scanToken() {
    char c = advance();
    switch (c) {
      case '(':
        addToken(LEFT_PAREN);
        break;
      case ')':
        addToken(RIGHT_PAREN);
        break;
      case '{':
        addToken(LEFT_BRACE);
        break;
      case '}':
        addToken(RIGHT_BRACE);
        break;
      case ',':
        addToken(COMMA);
        break;
      case '.':
        addToken(DOT);
        break;
      case '-':
        addToken(MINUS);
        break;
      case '+':
        addToken(PLUS);
        break;
      case ';':
        addToken(SEMICOLON);
        break;
      case '*':
        addToken(STAR);
        break;
    }
  }

  }
  private char advance() {
    return source.charAt(current++);
  }
  private void addToken(TokenType type) {
    addToken(type, null);
  }
  private void addToken(TokenType type, Object literal) {
    String text = source.substring(start, current);
    tokens.add(new Token(type, text, literal, line));