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.whitespace;
  
  import java.io.File;
  
  import com.puppycrawl.tools.checkstyle.StatelessCheck;
  import com.puppycrawl.tools.checkstyle.api.AbstractFileSetCheck;
  import com.puppycrawl.tools.checkstyle.api.FileText;
  
  /**
   * <p>
   * Checks that there are no tab characters ({@code '\t'}) in the source code.
   * </p>
   * <p>
   * Rationale:
   * </p>
   * <ul>
   * <li>
   * Developers should not need to configure the tab width of their text editors in order
   * to be able to read source code.
   * </li>
   * <li>
   * From the Apache jakarta coding standards: In a distributed development environment,
   * when the commit messages get sent to a mailing list, they are almost impossible to
   * read if you use tabs.
   * </li>
   * </ul>
   * <ul>
   * <li>
   * Property {@code eachLine} - Control whether to report on each line containing a tab,
   * or just the first instance.
   * Type is {@code boolean}.
   * Default value is {@code false}.
   * </li>
   * <li>
   * Property {@code fileExtensions} - Specify file type extension of files to process.
   * Type is {@code java.lang.String[]}.
   * Default value is {@code ""}.
   * </li>
   * </ul>
   * <p>
   * To configure the check to report only the first instance in each file:
   * </p>
   * <pre>
   * &lt;module name=&quot;FileTabCharacter&quot;/&gt;
   * </pre>
   * <p>
   * Example - Test.java:
   * </p>
   * <pre>
   * public class Test {
   *   int a;     // violation, indented using tab
   *
   *   public void foo (int arg) { // OK, indented using tab, only first occurrence in file reported
   *     a = arg;                  // OK, indented using spaces
   *   }                           // OK, indented using spaces
   * }
   * </pre>
   * <p>
   * To configure the check to report each instance in each file:
   * </p>
   * <pre>
   * &lt;module name=&quot;FileTabCharacter&quot;&gt;
   *   &lt;property name=&quot;eachLine&quot; value=&quot;true&quot;/&gt;
   * &lt;/module&gt;
   * </pre>
   * <p>
   * Example - Test.java:
   * </p>
   * <pre>
   * public class Test {
   *   int a;     // violation, indented using tab
   *
   *   public void foo (int arg) { // violation, indented using tab
   *     a = arg;                  // OK, indented using spaces
   *   }                           // OK, indented using spaces
   * }
   * </pre>
   * <p>
   * To configure the check to report instances on only certain file types:
   * </p>
   * <pre>
   * &lt;module name=&quot;FileTabCharacter&quot;&gt;
   *   &lt;property name=&quot;fileExtensions&quot; value=&quot;java, xml&quot;/&gt;
   * &lt;/module&gt;
   * </pre>
   * <p>
   * Example - Test.java:
   * </p>
   * <pre>
   * public class Test {
   *   int a;     // violation, indented using tab
   *
   *   public void foo (int arg) { // OK, indented using tab, only first occurrence in file reported
   *     a = arg;                  // OK, indented using spaces
   *   }                           // OK, indented using spaces
   * }
   * </pre>
   * <p>
   * Example - Test.xml:
   * </p>
   * <pre>
   * &lt;?xml version="1.0" encoding="UTF-8" ?&gt;
   * &lt;UserAccount&gt;
   *   &lt;FirstName&gt;John&lt;/FirstName&gt; &lt;!-- violation, indented using tab --&gt;
   *   &lt;LastName&gt;Doe&lt;/LastName&gt;    &lt;!-- only first occurrence in file reported --&gt;
   * &lt;/UserAccount&gt;
   * </pre>
   * <p>
   * Example - Test.html:
   * </p>
   * <pre>
   * &lt;head&gt;
   *   &lt;title&gt;Page Title&lt;/title&gt; &lt;!-- no check performed, html file extension --&gt;
   * &lt;/head&gt;                     &lt;!-- not specified in check config --&gt;
   * &lt;body&gt;
   *   &lt;p&gt;This is a simple html document.&lt;/p&gt;
   * &lt;/body&gt;
   * </pre>
   * <p>
   * Parent is {@code com.puppycrawl.tools.checkstyle.Checker}
   * </p>
   * <p>
   * Violation Message Keys:
   * </p>
   * <ul>
   * <li>
   * {@code containsTab}
   * </li>
   * <li>
   * {@code file.containsTab}
   * </li>
   * </ul>
   *
   * @since 5.0
   */
  @StatelessCheck
  public class FileTabCharacterCheck extends AbstractFileSetCheck {
  
      /**
       * A key is pointing to the warning message text in "messages.properties"
       * file.
       */
      public static final String MSG_CONTAINS_TAB = "containsTab";
  
      /**
       * A key is pointing to the warning message text in "messages.properties"
       * file.
       */
      public static final String MSG_FILE_CONTAINS_TAB = "file.containsTab";
  
      /** Control whether to report on each line containing a tab, or just the first instance. */
      private boolean eachLine;
  
      @Override
      protected void processFiltered(File file, FileText fileText) {
          int lineNum = 0;
          for (int index = 0; index < fileText.size(); index++) {
              final String line = fileText.get(index);
              lineNum++;
              final int tabPosition = line.indexOf('\t');
              if (tabPosition != -1) {
                  if (eachLine) {
                      log(lineNum, tabPosition, MSG_CONTAINS_TAB);
                  }
                  else {
                      log(lineNum, tabPosition, MSG_FILE_CONTAINS_TAB);
                      break;
                  }
              }
          }
      }
  
      /**
       * Setter to control whether to report on each line containing a tab, or just the first
       * instance.
       *
       * @param eachLine Whether report on each line containing a tab.
       */
      public void setEachLine(boolean eachLine) {
          this.eachLine = eachLine;
      }
  
  }
  