code: ///////////////////////////////////////////////////////////////////////////////////////////////
  // checkstyle: Checks Java source code and other text files for adherence to a set of rules.
  // Copyright (C) 2001-2023 the original author or authors.
  //
  // This library is free software; you can redistribute it and/or
  // modify it under the terms of the GNU Lesser General Public
  // License as published by the Free Software Foundation; either
  // version 2.1 of the License, or (at your option) any later version.
  //
  // This library is distributed in the hope that it will be useful,
  // but WITHOUT ANY WARRANTY; without even the implied warranty of
  // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  // Lesser General Public License for more details.
  //
  // You should have received a copy of the GNU Lesser General Public
  // License along with this library; if not, write to the Free Software
  // Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  ///////////////////////////////////////////////////////////////////////////////////////////////
  
  package com.puppycrawl.tools.checkstyle.checks.coding;
  
  import com.puppycrawl.tools.checkstyle.StatelessCheck;
  import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
  import com.puppycrawl.tools.checkstyle.api.DetailAST;
  import com.puppycrawl.tools.checkstyle.api.TokenTypes;
  import com.puppycrawl.tools.checkstyle.utils.CommonUtil;
  import com.puppycrawl.tools.checkstyle.utils.JavadocUtil;
  import com.puppycrawl.tools.checkstyle.utils.TokenUtil;
  
  /**
   * <p>
   * Checks for illegal tokens. By default, labels are prohibited.
   * </p>
   * <p>
   * Rationale: Certain language features can harm readability, lead to
   * confusion or are not obvious to novice developers. Other features
   * may be discouraged in certain frameworks, such as not having
   * native methods in Enterprise JavaBeans components.
   * </p>
   * <ul>
   * <li>
   * Property {@code tokens} - tokens to check
   * Type is {@code anyTokenTypesSet}.
   * Default value is
   * <a href="https://checkstyle.org/apidocs/com/puppycrawl/tools/checkstyle/api/TokenTypes.html#LABELED_STAT">
   * LABELED_STAT</a>.
   * </li>
   * </ul>
   * <p>
   * To configure the check:
   * </p>
   * <pre>
   * &lt;module name=&quot;IllegalToken&quot;/&gt;
   * </pre>
   * <p>Example:</p>
   * <pre>
   * public void myTest() {
   *     outer: // violation
   *     for (int i = 0; i &lt; 5; i++) {
   *         if (i == 1) {
   *             break outer;
   *         }
   *     }
   * }
   * </pre>
   * <p>
   * To configure the check to report violation on token LITERAL_NATIVE:
   * </p>
   * <pre>
   * &lt;module name=&quot;IllegalToken&quot;&gt;
   *   &lt;property name=&quot;tokens&quot; value=&quot;LITERAL_NATIVE&quot;/&gt;
   * &lt;/module&gt;
   * </pre>
   * <p>Example:</p>
   * <pre>
   * public native void myTest(); // violation
   * </pre>
   * <p>
   * Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker}
   * </p>
   * <p>
   * Violation Message Keys:
   * </p>
   * <ul>
   * <li>
   * {@code illegal.token}
   * </li>
   * </ul>
   *
   * @since 3.2
   */
  @StatelessCheck
  public class IllegalTokenCheck
      extends AbstractCheck {
  
      /**
       * A key is pointing to the warning message text in "messages.properties"
       * file.
       */
      public static final String MSG_KEY = "illegal.token";
  
      @Override
      public int[] getDefaultTokens() {
          return new int[] {
              TokenTypes.LABELED_STAT,
          };
      }
  
      @Override
      public int[] getAcceptableTokens() {
          return TokenUtil.getAllTokenIds();
      }
  
      @Override
      public int[] getRequiredTokens() {
          return CommonUtil.EMPTY_INT_ARRAY;
      }
  
      @Override
      public boolean isCommentNodesRequired() {
          return true;
      }
  
      @Override
      public void visitToken(DetailAST ast) {
          log(
              ast,
              MSG_KEY,
              convertToString(ast)
          );
      }
  
      /**
       * Converts given AST node to string representation.
       *
       * @param ast node to be represented as string
       * @return string representation of AST node
       */
      private static String convertToString(DetailAST ast) {
          final String tokenText;
          switch (ast.getType()) {
              case TokenTypes.LABELED_STAT:
                  tokenText = ast.getFirstChild().getText() + ast.getText();
                  break;
              // multiline tokens need to become singlelined
              case TokenTypes.COMMENT_CONTENT:
                  tokenText = JavadocUtil.escapeAllControlChars(ast.getText());
                  break;
              default:
                  tokenText = ast.getText();
                  break;
          }
          return tokenText;
      }
  
  }
  