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.regex.Pattern;
  
  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>
   * Ensures that the names of abstract classes conforming to some pattern
   * and check that {@code abstract} modifier exists.
   * </p>
   * <p>
   * Rationale: Abstract classes are convenience base class implementations of
   * interfaces. For this reason, it should be made obvious that a given class
   * is abstract by prefacing the class name with 'Abstract'.
   * </p>
   * <ul>
   * <li>
   * Property {@code format} - Specify valid identifiers.
   * Type is {@code java.util.regex.Pattern}.
   * Default value is {@code "^Abstract.+$"}.</li>
   * <li>
   * Property {@code ignoreModifier} - Control whether to ignore checking for the
   * {@code abstract} modifier on classes that match the name.
   * Type is {@code boolean}.
   * Default value is {@code false}.</li>
   * <li>
   * Property {@code ignoreName} - Control whether to ignore checking the name.
   * Realistically only useful if using the check to identify that match name and
   * do not have the {@code abstract} modifier.
   * Type is {@code boolean}.
   * Default value is {@code false}.
   * </li>
   * </ul>
   * <p>
   * To configure the check:
   * </p>
   * <pre>
   * &lt;module name="AbstractClassName"/&gt;
   * </pre>
   * <p>Example:</p>
   * <pre>
   * class Example1 {
   *   abstract class AbstractFirst {}
   *   abstract class Second {} // violation 'must match pattern '\^Abstract\.\+\$''
   *   class AbstractThird {} // violation 'must be declared as 'abstract''
   *   class Fourth {}
   *   abstract class GeneratorFifth {} // violation 'must match pattern '\^Abstract\.\+\$''
   *   class GeneratorSixth {}
   * }
   * </pre>
   * <p>
   * To configure the check so that it check name
   * but ignore {@code abstract} modifier:
   * </p>
   * <pre>
   * &lt;module name="AbstractClassName"&gt;
   *   &lt;property name="ignoreModifier" value="true"/&gt;
   * &lt;/module&gt;
   * </pre>
   * <p>Example:</p>
   * <pre>
   * class Example2 {
   *   abstract class AbstractFirst {}
   *   abstract class Second {} // violation 'must match pattern '\^Abstract\.\+\$''
   *   class AbstractThird {}
   *   class Fourth {}
   *   abstract class GeneratorFifth {} // violation 'must match pattern '\^Abstract\.\+\$''
   *   class GeneratorSixth {}
   * }
   * </pre>
   * <p>
   * To configure the check to ignore name
   * validation when class declared as 'abstract'
   * </p>
   * <pre>
   * &lt;module name="AbstractClassName"&gt;
   *   &lt;property name="ignoreName" value="true"/&gt;
   * &lt;/module&gt;
   * </pre>
   * <p>Example:</p>
   * <pre>
   * class Example3 {
   *   abstract class AbstractFirst {}
   *   abstract class Second {}
   *   class AbstractThird {} // violation 'must be declared as 'abstract''
   *   class Fourth {}
   *   abstract class GeneratorFifth {}
   *   class GeneratorSixth {}
   * }
   * </pre>
   * <p>
   * To configure the check
   * with {@code format}:
   * </p>
   * <pre>
   * &lt;module name="AbstractClassName"&gt;
   *   &lt;property name="format" value="^Generator.+$"/&gt;
   * &lt;/module&gt;
   * </pre>
   * <p>Example:</p>
   * <pre>
   * class Example4 {
   *   abstract class AbstractFirst {} // violation 'must match pattern '\^Generator\.\+\$''
   *   abstract class Second {} // violation 'must match pattern '\^Generator\.\+\$''
   *   class AbstractThird {}
   *   class Fourth {}
   *   abstract class GeneratorFifth {}
   *   class GeneratorSixth {} // violation 'must be declared as 'abstract''
   * }
   * </pre>
   * <p>
   * Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker}
   * </p>
   * <p>
   * Violation Message Keys:
   * </p>
   * <ul>
   * <li>
   * {@code illegal.abstract.class.name}
   * </li>
   * <li>
   * {@code no.abstract.class.modifier}
   * </li>
   * </ul>
   *
   * @since 3.2
   */
  @StatelessCheck
  public final class AbstractClassNameCheck extends AbstractCheck {
  
      /**
       * A key is pointing to the warning message text in "messages.properties"
       * file.
       */
      public static final String MSG_ILLEGAL_ABSTRACT_CLASS_NAME = "illegal.abstract.class.name";
  
      /**
       * A key is pointing to the warning message text in "messages.properties"
       * file.
       */
      public static final String MSG_NO_ABSTRACT_CLASS_MODIFIER = "no.abstract.class.modifier";
  
      /**
       * Control whether to ignore checking for the {@code abstract} modifier on
       * classes that match the name.
       */
      private boolean ignoreModifier;
  
      /**
       * Control whether to ignore checking the name. Realistically only useful
       * if using the check to identify that match name and do not have the
       * {@code abstract} modifier.
       */
      private boolean ignoreName;
  
      /** Specify valid identifiers. */
      private Pattern format = Pattern.compile("^Abstract.+$");
  
      /**
       * Setter to control whether to ignore checking for the {@code abstract} modifier on
       * classes that match the name.
       *
       * @param value new value
       */
      public void setIgnoreModifier(boolean value) {
          ignoreModifier = value;
      }
  
      /**
       * Setter to control whether to ignore checking the name. Realistically only useful if
       * using the check to identify that match name and do not have the {@code abstract} modifier.
       *
       * @param value new value.
       */
      public void setIgnoreName(boolean value) {
          ignoreName = value;
      }
  
      /**
       * Setter to specify valid identifiers.
       *
       * @param pattern the new pattern
       */
      public void setFormat(Pattern pattern) {
          format = pattern;
      }
  
      @Override
      public int[] getDefaultTokens() {
          return getRequiredTokens();
      }
  
      @Override
      public int[] getRequiredTokens() {
          return new int[] {TokenTypes.CLASS_DEF};
      }
  
      @Override
      public int[] getAcceptableTokens() {
          return getRequiredTokens();
      }
  
      @Override
      public void visitToken(DetailAST ast) {
          visitClassDef(ast);
      }
  
      /**
       * Checks class definition.
       *
       * @param ast class definition for check.
       */
      private void visitClassDef(DetailAST ast) {
          final String className =
              ast.findFirstToken(TokenTypes.IDENT).getText();
          if (isAbstract(ast)) {
              // if class has abstract modifier
              if (!ignoreName && !isMatchingClassName(className)) {
                  log(ast, MSG_ILLEGAL_ABSTRACT_CLASS_NAME, className, format.pattern());
              }
          }
          else if (!ignoreModifier && isMatchingClassName(className)) {
              log(ast, MSG_NO_ABSTRACT_CLASS_MODIFIER, className);
          }
      }
  
      /**
       * Checks if declared class is abstract or not.
       *
       * @param ast class definition for check.
       * @return true if a given class declared as abstract.
       */
      private static boolean isAbstract(DetailAST ast) {
          final DetailAST abstractAST = ast.findFirstToken(TokenTypes.MODIFIERS)
              .findFirstToken(TokenTypes.ABSTRACT);
  
          return abstractAST != null;
      }
  
      /**
       * Returns true if class name matches format of abstract class names.
       *
       * @param className class name for check.
       * @return true if class name matches format of abstract class names.
       */
      private boolean isMatchingClassName(String className) {
          return format.matcher(className).find();
      }
  
  }
  