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.TokenUtil;
  
  /**
   * <p>
   * Checks for over-complicated boolean return statements.
   * For example the following code
   * </p>
   * <pre>
   * if (valid())
   *   return false;
   * else
   *   return true;
   * </pre>
   * <p>
   * could be written as
   * </p>
   * <pre>
   * return !valid();
   * </pre>
   * <p>
   * The idea for this Check has been shamelessly stolen from the equivalent
   * <a href="https://pmd.github.io/">PMD</a> rule.
   * </p>
   * <p>
   * To configure the check:
   * </p>
   * <pre>
   * &lt;module name=&quot;SimplifyBooleanReturn&quot;/&gt;
   * </pre>
   * <p>Example:</p>
   * <pre>
   * public class Test {
   *
   *  private boolean cond;
   *  private Foo a;
   *  private Foo b;
   *
   *  public boolean check1() {
   *   if (cond) { // violation, can be simplified
   *     return true;
   *   }
   *   else {
   *     return false;
   *   }
   *  }
   *
   *  // Ok, simplified version of check1()
   *  public boolean check2() {
   *   return cond;
   *  }
   *
   *  // violations, can be simplified
   *  public boolean check3() {
   *   if (cond == true) { // can be simplified to "if (cond)"
   *     return false;
   *   }
   *   else {
   *     return true; // can be simplified to "return !cond"
   *   }
   *  }
   *
   *  // Ok, can be simplified but doesn't return a Boolean
   *  public Foo choose1() {
   *   if (cond) {
   *     return a;
   *   }
   *   else {
   *     return b;
   *   }
   *  }
   *
   *  // Ok, simplified version of choose1()
   *  public Foo choose2() {
   *   return cond ? a: b;
   *  }
   *
   * }
   * </pre>
   * <p>
   * Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker}
   * </p>
   * <p>
   * Violation Message Keys:
   * </p>
   * <ul>
   * <li>
   * {@code simplify.boolReturn}
   * </li>
   * </ul>
   *
   * @since 3.0
   */
  @StatelessCheck
  public class SimplifyBooleanReturnCheck
      extends AbstractCheck {
  
      /**
       * A key is pointing to the warning message text in "messages.properties"
       * file.
       */
      public static final String MSG_KEY = "simplify.boolReturn";
  
      @Override
      public int[] getAcceptableTokens() {
          return getRequiredTokens();
      }
  
      @Override
      public int[] getDefaultTokens() {
          return getRequiredTokens();
      }
  
      @Override
      public int[] getRequiredTokens() {
          return new int[] {TokenTypes.LITERAL_IF};
      }
  
      @Override
      public void visitToken(DetailAST ast) {
          // LITERAL_IF has the following four or five children:
          // '('
          // condition
          // ')'
          // thenStatement
          // [ LITERAL_ELSE (with the elseStatement as a child) ]
  
          // don't bother if this is not if then else
          final DetailAST elseLiteral =
              ast.findFirstToken(TokenTypes.LITERAL_ELSE);
          if (elseLiteral != null) {
              final DetailAST elseStatement = elseLiteral.getFirstChild();
  
              // skip '(' and ')'
              final DetailAST condition = ast.getFirstChild().getNextSibling();
              final DetailAST thenStatement = condition.getNextSibling().getNextSibling();
  
              if (canReturnOnlyBooleanLiteral(thenStatement)
                  && canReturnOnlyBooleanLiteral(elseStatement)) {
                  log(ast, MSG_KEY);
              }
          }
      }
  
      /**
       * Returns if an AST is a return statement with a boolean literal
       * or a compound statement that contains only such a return statement.
       *
       * <p>Returns {@code true} iff ast represents
       * <pre>
       * return true/false;
       * </pre>
       * or
       * <pre>
       * {
       *   return true/false;
       * }
       * </pre>
       *
       * @param ast the syntax tree to check
       * @return if ast is a return statement with a boolean literal.
       */
      private static boolean canReturnOnlyBooleanLiteral(DetailAST ast) {
          boolean result = true;
          if (!isBooleanLiteralReturnStatement(ast)) {
              final DetailAST firstStatement = ast.getFirstChild();
              result = isBooleanLiteralReturnStatement(firstStatement);
          }
          return result;
      }
  
      /**
       * Returns if an AST is a return statement with a boolean literal.
       *
       * <p>Returns {@code true} iff ast represents
       * <pre>
       * return true/false;
       * </pre>
       *
       * @param ast the syntax tree to check
       * @return if ast is a return statement with a boolean literal.
       */
      private static boolean isBooleanLiteralReturnStatement(DetailAST ast) {
          boolean booleanReturnStatement = false;
  
          if (ast != null && ast.getType() == TokenTypes.LITERAL_RETURN) {
              final DetailAST expr = ast.getFirstChild();
  
              if (expr.getType() != TokenTypes.SEMI) {
                  final DetailAST value = expr.getFirstChild();
                  booleanReturnStatement = TokenUtil.isBooleanLiteralType(value.getType());
              }
          }
          return booleanReturnStatement;
      }
  }
  