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 java.util.BitSet;
  import java.util.function.Predicate;
  
  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>
   * Detects double brace initialization.
   * </p>
   * <p>
   * Rationale: Double brace initialization (set of
   * <a href="https://docs.oracle.com/javase/specs/jls/se12/html/jls-8.html#jls-8.6">
   * Instance Initializers</a> in class body) may look cool, but it is considered as anti-pattern
   * and should be avoided.
   * This is also can lead to a hard-to-detect memory leak, if the anonymous class instance is
   * returned outside and other object(s) hold reference to it.
   * Created anonymous class is not static, it holds an implicit reference to the outer class
   * instance.
   * See this
   * <a href="https://blog.jooq.org/dont-be-clever-the-double-curly-braces-anti-pattern/">
   * blog post</a> and
   * <a href="https://www.baeldung.com/java-double-brace-initialization">
   * article</a> for more details.
   * Check ignores any comments and semicolons in class body.
   * </p>
   * <p>
   * To configure the check:
   * </p>
   * <pre>
   * &lt;module name=&quot;AvoidDoubleBraceInitialization&quot;/&gt;
   * </pre>
   * <p>
   * Which results in the following violations:
   * </p>
   * <pre>
   * class MyClass {
   *     List&lt;Integer&gt; list1 = new ArrayList&lt;&gt;() { // violation
   *         {
   *             add(1);
   *         }
   *     };
   *     List&lt;String&gt; list2 = new ArrayList&lt;&gt;() { // violation
   *         ;
   *         // comments and semicolons are ignored
   *         {
   *             add("foo");
   *         }
   *     };
   * }
   * </pre>
   * <p>
   * Check only looks for double brace initialization and it ignores cases
   * where the anonymous class has fields or methods.
   * Though these might create the same memory issues as double brace,
   * the extra class members can produce side effects if changed incorrectly.
   * </p>
   * <pre>
   * class MyClass {
   *     List&lt;Object&gt; list = new ArrayList&lt;&gt;() { // OK, not pure double brace pattern
   *         private int field;
   *         {
   *             add(new Object());
   *         }
   *     };
   * }
   * </pre>
   * <p>
   * Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker}
   * </p>
   * <p>
   * Violation Message Keys:
   * </p>
   * <ul>
   * <li>
   * {@code avoid.double.brace.init}
   * </li>
   * </ul>
   *
   * @since 8.30
   */
  @StatelessCheck
  public class AvoidDoubleBraceInitializationCheck extends AbstractCheck {
  
      /**
       * A key is pointing to the warning message text in "messages.properties"
       * file.
       */
      public static final String MSG_KEY = "avoid.double.brace.init";
  
      /**
       * Set of token types that are used in {@link #HAS_MEMBERS} predicate.
       */
      private static final BitSet IGNORED_TYPES = TokenUtil.asBitSet(
          TokenTypes.INSTANCE_INIT,
          TokenTypes.SEMI,
          TokenTypes.LCURLY,
          TokenTypes.RCURLY
      );
  
      /**
       * Predicate for tokens that is used in {@link #hasOnlyInitialization(DetailAST)}.
       */
      private static final Predicate<DetailAST> HAS_MEMBERS =
          token -> !IGNORED_TYPES.get(token.getType());
  
      @Override
      public int[] getDefaultTokens() {
          return getRequiredTokens();
      }
  
      @Override
      public int[] getAcceptableTokens() {
          return getRequiredTokens();
      }
  
      @Override
      public int[] getRequiredTokens() {
          return new int[] {TokenTypes.OBJBLOCK};
      }
  
      @Override
      public void visitToken(DetailAST ast) {
          if (ast.getParent().getType() == TokenTypes.LITERAL_NEW
              && hasOnlyInitialization(ast)) {
              log(ast, MSG_KEY);
          }
      }
  
      /**
       * Checks that block has at least one instance init block and no other class members.
       *
       * @param objBlock token to check
       * @return true if there is least one instance init block and no other class members,
       *     false otherwise
       */
      private static boolean hasOnlyInitialization(DetailAST objBlock) {
          final boolean hasInitBlock = objBlock.findFirstToken(TokenTypes.INSTANCE_INIT) != null;
          return hasInitBlock
                    && TokenUtil.findFirstTokenByPredicate(objBlock, HAS_MEMBERS).isEmpty();
      }
  }
  