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.sizes;
  
  import com.puppycrawl.tools.checkstyle.FileStatefulCheck;
  import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
  import com.puppycrawl.tools.checkstyle.api.DetailAST;
  import com.puppycrawl.tools.checkstyle.api.TokenTypes;
  
  /**
   * <p>
   * Checks for the number of types declared at the <i>outer</i> (or <i>root</i>) level in a file.
   * </p>
   * <p>
   * Rationale: It is considered good practice to only define one outer type per file.
   * </p>
   * <ul>
   * <li>
   * Property {@code max} - Specify the maximum number of outer types allowed.
   * Type is {@code int}.
   * Default value is {@code 1}.
   * </li>
   * </ul>
   * <p>
   * To configure the check to accept 1 outer type per file:
   * </p>
   * <pre>
   * &lt;module name="OuterTypeNumber"/&gt;
   * </pre>
   * <p>
   * To configure the check to accept 2 outer types per file:
   * </p>
   * <pre>
   * &lt;module name="OuterTypeNumber"&gt;
   *   &lt;property name="max" value="2"/&gt;
   * &lt;/module&gt;
   * </pre>
   * <p>
   * Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker}
   * </p>
   * <p>
   * Violation Message Keys:
   * </p>
   * <ul>
   * <li>
   * {@code maxOuterTypes}
   * </li>
   * </ul>
   *
   * @since 5.0
   */
  @FileStatefulCheck
  public class OuterTypeNumberCheck extends AbstractCheck {
  
      /**
       * A key is pointing to the warning message text in "messages.properties"
       * file.
       */
      public static final String MSG_KEY = "maxOuterTypes";
  
      /** Specify the maximum number of outer types allowed. */
      private int max = 1;
      /** Tracks the current depth in types. */
      private int currentDepth;
      /** Tracks the number of outer types found. */
      private int outerNum;
  
      @Override
      public int[] getDefaultTokens() {
          return getRequiredTokens();
      }
  
      @Override
      public int[] getAcceptableTokens() {
          return getRequiredTokens();
      }
  
      @Override
      public int[] getRequiredTokens() {
          return new int[] {
              TokenTypes.CLASS_DEF,
              TokenTypes.INTERFACE_DEF,
              TokenTypes.ENUM_DEF,
              TokenTypes.ANNOTATION_DEF,
              TokenTypes.RECORD_DEF,
          };
      }
  
      @Override
      public void beginTree(DetailAST ast) {
          currentDepth = 0;
          outerNum = 0;
      }
  
      @Override
      public void finishTree(DetailAST ast) {
          if (max < outerNum) {
              log(ast, MSG_KEY, outerNum, max);
          }
      }
  
      @Override
      public void visitToken(DetailAST ast) {
          if (currentDepth == 0) {
              outerNum++;
          }
          currentDepth++;
      }
  
      @Override
      public void leaveToken(DetailAST ast) {
          currentDepth--;
      }
  
      /**
       * Setter to specify the maximum number of outer types allowed.
       *
       * @param max the new number.
       */
      public void setMax(int max) {
          this.max = max;
      }
  
  }
  