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.naming;
  
  import java.util.Objects;
  
  import com.puppycrawl.tools.checkstyle.api.DetailAST;
  import com.puppycrawl.tools.checkstyle.api.TokenTypes;
  import com.puppycrawl.tools.checkstyle.utils.TokenUtil;
  
  /**
   * <p>
   * Checks lambda parameter names.
   * </p>
   * <ul>
   * <li>
   * Property {@code format} - Specifies valid identifiers.
   * Type is {@code java.util.regex.Pattern}.
   * Default value is {@code "^[a-z][a-zA-Z0-9]*$"}.
   * </li>
   * </ul>
   * <p>
   * To configure the check:
   * </p>
   * <pre>
   * &lt;module name="LambdaParameterName"/&gt;
   * </pre>
   * <p>Code Example:</p>
   * <pre>
   * Function&lt;String, String&gt; function1 = s -&gt; s.toLowerCase(); // OK
   * Function&lt;String, String&gt; function2 = S -&gt; S.toLowerCase(); // violation, name 'S'
   *                                                // must match pattern '^[a-z][a-zA-Z0-9]*$'
   * </pre>
   * <p>
   * An example of how to configure the check for names that begin
   * with a lower case letter, followed by letters is:
   * </p>
   * <pre>
   * &lt;module name="LambdaParameterName"&gt;
   *   &lt;property name="format" value="^[a-z]([a-zA-Z]+)*$"/&gt;
   * &lt;/module&gt;
   * </pre>
   * <p>
   * Code Example:
   * </p>
   * <pre>
   * class MyClass {
   *   Function&lt;String, String&gt; function1 = str -&gt; str.toUpperCase().trim(); // OK
   *   Function&lt;String, String&gt; function2 = _s -&gt; _s.trim(); // violation, name '_s'
   *                                              // must match pattern '^[a-z]([a-zA-Z]+)*$'
   *
   *   public boolean myMethod(String sentence) {
   *     return Stream.of(sentence.split(" "))
   *             .map(word -&gt; word.trim()) // OK
   *             .anyMatch(Word -&gt; "in".equals(Word)); // violation, name 'Word'
   *                                                      // must match pattern '^[a-z]([a-zA-Z]+)*$'
   *   }
   * }
   *
   * </pre>
   * <p>
   * Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker}
   * </p>
   * <p>
   * Violation Message Keys:
   * </p>
   * <ul>
   * <li>
   * {@code name.invalidPattern}
   * </li>
   * </ul>
   *
   * @since 8.11
   */
  public class LambdaParameterNameCheck extends AbstractNameCheck {
  
      /** Creates new instance of {@code LambdaParameterNameCheck}. */
      public LambdaParameterNameCheck() {
          super("^[a-z][a-zA-Z0-9]*$");
      }
  
      @Override
      public int[] getDefaultTokens() {
          return getRequiredTokens();
      }
  
      @Override
      public int[] getAcceptableTokens() {
          return getRequiredTokens();
      }
  
      @Override
      public int[] getRequiredTokens() {
          return new int[] {
              TokenTypes.LAMBDA,
          };
      }
  
      @Override
      public void visitToken(DetailAST ast) {
          final boolean isInSwitchRule = ast.getParent().getType() == TokenTypes.SWITCH_RULE;
  
          if (Objects.nonNull(ast.findFirstToken(TokenTypes.PARAMETERS))) {
              final DetailAST parametersNode = ast.findFirstToken(TokenTypes.PARAMETERS);
              TokenUtil.forEachChild(parametersNode, TokenTypes.PARAMETER_DEF, super::visitToken);
          }
          else if (!isInSwitchRule) {
              super.visitToken(ast);
          }
      }
  
      @Override
      protected boolean mustCheckName(DetailAST ast) {
          return true;
      }
  
  }
  