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.blocks;
  
  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>
   * Finds nested blocks (blocks that are used freely in the code).
   * </p>
   * <p>
   * Rationale: Nested blocks are often leftovers from the
   * debugging process, they confuse the reader.
   * </p>
   * <p>
   * For example, this check finds the obsolete braces in
   * </p>
   * <pre>
   * public void guessTheOutput()
   * {
   *   int whichIsWhich = 0;
   *   {
   *     whichIsWhich = 2;
   *   }
   *   System.out.println("value = " + whichIsWhich);
   * }
   * </pre>
   * <p>
   * and debugging / refactoring leftovers such as
   * </p>
   * <pre>
   * // if (conditionThatIsNotUsedAnyLonger)
   * {
   *   System.out.println("unconditional");
   * }
   * </pre>
   * <p>
   * A case in a switch statement does not implicitly form a block.
   * Thus, to be able to introduce local variables that have case scope
   * it is necessary to open a nested block. This is supported, set
   * the allowInSwitchCase property to true and include all statements
   * of the case in the block.
   * </p>
   * <ul>
   * <li>
   * Property {@code allowInSwitchCase} - Allow nested blocks if they are the
   * only child of a switch case.
   * Type is {@code boolean}.
   * Default value is {@code false}.
   * </li>
   * </ul>
   * <p>
   * To configure the check:
   * </p>
   * <pre>
   * &lt;module name="AvoidNestedBlocks"/&gt;
   * </pre>
   * <p>
   * Example:
   * </p>
   * <pre>
   * public void foo() {
   *   int myInteger = 0;
   *   {                      // violation
   *     myInteger = 2;
   *   }
   *   System.out.println("myInteger = " + myInteger);
   *
   *   switch (a) {
   *     case 1:
   *       {                    // violation
   *         System.out.println("Case 1");
   *         break;
   *       }
   *     case 2:
   *       System.out.println("Case 2");     // OK
   *       break;
   *   }
   * }
   * </pre>
   * <p>
   * To configure the check to allow nested blocks in switch case:
   * </p>
   * <pre>
   * &lt;module name=&quot;AvoidNestedBlocks&quot;&gt;
   *   &lt;property name=&quot;allowInSwitchCase&quot; value=&quot;true&quot;/&gt;
   * &lt;/module&gt;
   * </pre>
   * <p>
   * Example:
   * </p>
   * <pre>
   * public void foo() {
   *   int myInteger = 0;
   *   {                      // violation
   *     myInteger = 2;
   *   }
   *   System.out.println("myInteger = " + myInteger);
   *
   *   switch (a)
   *   {
   *     case 1:
   *       {                    // OK
   *         System.out.println("Case 1");
   *         break;
   *       }
   *     case 2:
   *       System.out.println("Case 2");     // OK
   *       break;
   *   }
   * }
   * </pre>
   * <p>
   * Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker}
   * </p>
   * <p>
   * Violation Message Keys:
   * </p>
   * <ul>
   * <li>
   * {@code block.nested}
   * </li>
   * </ul>
   *
   * @since 3.1
   */
  @StatelessCheck
  public class AvoidNestedBlocksCheck extends AbstractCheck {
  
      /**
       * A key is pointing to the warning message text in "messages.properties"
       * file.
       */
      public static final String MSG_KEY_BLOCK_NESTED = "block.nested";
  
      /**
       * Allow nested blocks if they are the only child of a switch case.
       */
      private boolean allowInSwitchCase;
  
      @Override
      public int[] getDefaultTokens() {
          return getRequiredTokens();
      }
  
      @Override
      public int[] getAcceptableTokens() {
          return getRequiredTokens();
      }
  
      @Override
      public int[] getRequiredTokens() {
          return new int[] {TokenTypes.SLIST};
      }
  
      @Override
      public void visitToken(DetailAST ast) {
          final DetailAST parent = ast.getParent();
          if (parent.getType() == TokenTypes.SLIST
                  && (!allowInSwitchCase || hasSiblings(ast))) {
              log(ast, MSG_KEY_BLOCK_NESTED);
          }
      }
  
      /**
       * Checks whether the AST node has any siblings or not.
       *
       * @param ast node to examine
       * @return {@code true} if the node has one or more siblings
       */
      private static boolean hasSiblings(DetailAST ast) {
          return ast.getPreviousSibling() != null || ast.getNextSibling() != null;
      }
  
      /**
       * Setter to allow nested blocks if they are the only child of a switch case.
       *
       * @param allowInSwitchCase whether nested blocks are allowed
       *                 if they are the only child of a switch case.
       */
      public void setAllowInSwitchCase(boolean allowInSwitchCase) {
          this.allowInSwitchCase = allowInSwitchCase;
      }
  
  }
  