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;
  
  /**
   * <p>
   * Checks that string literals are not used with <code>==</code> or <code>&#33;=</code>.
   * Since <code>==</code> will compare the object references, not the actual value of the strings,
   * <code>String.equals()</code> should be used.
   * More information can be found
   * <a href="https://stackoverflow.com/questions/513832/how-do-i-compare-strings-in-java/">
   * in this article</a>.
   * </p>
   * <p>
   * Rationale: Novice Java programmers often use code like:
   * </p>
   * <pre>
   * if (x == "something")
   * </pre>
   * <p>
   * when they mean
   * </p>
   * <pre>
   * if ("something".equals(x))
   * </pre>
   * <p>
   * To configure the check:
   * </p>
   * <pre>
   * &lt;module name=&quot;StringLiteralEquality&quot;/&gt;
   * </pre>
   * <p>
   * Examples of violations:
   * </p>
   * <pre>
   * String status = "pending";
   *
   * if (status == "done") {} // violation
   *
   * while (status != "done") {} // violation
   *
   * boolean flag = (status == "done"); // violation
   *
   * boolean flag = (status.equals("done")); // OK
   *
   * String name = "X";
   *
   * if (name == getName()) {}
   * // OK, limitation that check cannot tell runtime type returned from method call
   * </pre>
   * <p>
   * Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker}
   * </p>
   * <p>
   * Violation Message Keys:
   * </p>
   * <ul>
   * <li>
   * {@code string.literal.equality}
   * </li>
   * </ul>
   *
   * @since 3.2
   * @noinspection HtmlTagCanBeJavadocTag
   * @noinspectionreason HtmlTagCanBeJavadocTag - encoded symbols were not decoded
   *      when replaced with Javadoc tag
   */
  @StatelessCheck
  public class StringLiteralEqualityCheck extends AbstractCheck {
  
      /**
       * A key is pointing to the warning message text in "messages.properties"
       * file.
       */
      public static final String MSG_KEY = "string.literal.equality";
  
      @Override
      public int[] getDefaultTokens() {
          return getRequiredTokens();
      }
  
      @Override
      public int[] getAcceptableTokens() {
          return getRequiredTokens();
      }
  
      @Override
      public int[] getRequiredTokens() {
          return new int[] {TokenTypes.EQUAL, TokenTypes.NOT_EQUAL};
      }
  
      @Override
      public void visitToken(DetailAST ast) {
          final boolean hasStringLiteralChild = hasStringLiteralChild(ast);
  
          if (hasStringLiteralChild) {
              log(ast, MSG_KEY, ast.getText());
          }
      }
  
      /**
       * Checks whether string literal or text block literals are concatenated.
       *
       * @param ast ast
       * @return {@code true} if string literal or text block literals are concatenated
       */
      private static boolean hasStringLiteralChild(DetailAST ast) {
          DetailAST currentAst = ast;
          boolean result = false;
          while (currentAst != null) {
              if (currentAst.findFirstToken(TokenTypes.STRING_LITERAL) == null
                      && currentAst.findFirstToken(TokenTypes.TEXT_BLOCK_LITERAL_BEGIN) == null) {
                  currentAst = currentAst.findFirstToken(TokenTypes.PLUS);
              }
              else {
                  result = true;
                  break;
              }
          }
          return result;
      }
  
  }
  