private static final Map<String, TokenType> keywords;
  static {
    keywords = new HashMap<>();
    keywords.put("and", AND);
    keywords.put("class", CLASS);
    keywords.put("else", ELSE);
    keywords.put("false", FALSE);
    keywords.put("for", FOR);
    keywords.put("fun", FUN);
    keywords.put("if", IF);
    keywords.put("nil", NIL);
    keywords.put("or", OR);
    keywords.put("print", PRINT);
    keywords.put("return", RETURN);
    keywords.put("super", SUPER);
    keywords.put("this", THIS);
    keywords.put("true", TRUE);
    keywords.put("var", VAR);
    keywords.put("while", WHILE);
  }

        } else if (isAlpha(c)) {
          identifier();

  }
  private void identifier() {
    while (isAlphaNumeric(peek()))
      advance();
    String text = source.substring(start, current);
    TokenType type = keywords.get(text);
    if (type == null)
      type = IDENTIFIER;
    addToken(type);

  private boolean isAlpha(char c) {
    return (c >= 'a' && c <= 'z') ||
        (c >= 'A' && c <= 'Z') ||
        c == '_';
  }
  private boolean isAlphaNumeric(char c) {
    return isAlpha(c) || isDigit(c);
  }