break;
      case '!':
        addToken(match('=') ? BANG_EQUAL : BANG);
        break;
      case '=':
        addToken(match('=') ? EQUAL_EQUAL : EQUAL);
        break;
      case '<':
        addToken(match('=') ? LESS_EQUAL : LESS);
        break;
      case '>':
        addToken(match('=') ? GREATER_EQUAL : GREATER);

  private boolean match(char expected) {
    if (isAtEnd())
      return false;
    if (source.charAt(current) != expected)
      return false;
    current++;
    return true;
  }