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.design;
  
  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>
   * Implements Joshua Bloch, Effective Java, Item 17 -
   * Use Interfaces only to define types.
   * </p>
   * <p>
   * According to Bloch, an interface should describe a <em>type</em>. It is therefore
   * inappropriate to define an interface that does not contain any methods
   * but only constants. The Standard interface
   * <a href="https://docs.oracle.com/javase/8/docs/api/javax/swing/SwingConstants.html">
   * javax.swing.SwingConstants</a> is an example of an interface that would be flagged by this check.
   * </p>
   * <p>
   * The check can be configured to also disallow marker interfaces like {@code java.io.Serializable},
   * that do not contain methods or constants at all.
   * </p>
   * <ul>
   * <li>
   * Property {@code allowMarkerInterfaces} - Control whether marker interfaces
   * like Serializable are allowed.
   * Type is {@code boolean}.
   * Default value is {@code true}.
   * </li>
   * </ul>
   * <p>
   * To configure the check:
   * </p>
   * <pre>
   * &lt;module name="InterfaceIsType"/&gt;
   * </pre>
   * <p>Example:</p>
   * <pre>
   * public interface Test1 { // violation
   *     int a = 3;
   *
   * }
   *
   * public interface Test2 { // OK
   *
   * }
   *
   * public interface Test3 { // OK
   *     int a = 3;
   *     void test();
   * }
   * </pre>
   * <p>
   * To configure the check to report violation so that it doesn't allow Marker Interfaces:
   * </p>
   * <pre>
   * &lt;module name=&quot;InterfaceIsType&quot;&gt;
   *   &lt;property name=&quot;allowMarkerInterfaces&quot; value=&quot;false&quot;/&gt;
   * &lt;/module&gt;
   * </pre>
   * <p>Example:</p>
   * <pre>
   * public interface Test1 { // violation
   *     int a = 3;
   * }
   *
   * public interface Test2 { // violation
   *
   * }
   * </pre>
   * <p>
   * Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker}
   * </p>
   * <p>
   * Violation Message Keys:
   * </p>
   * <ul>
   * <li>
   * {@code interface.type}
   * </li>
   * </ul>
   *
   * @since 3.1
   */
  @StatelessCheck
  public final class InterfaceIsTypeCheck
          extends AbstractCheck {
  
      /**
       * A key is pointing to the warning message text in "messages.properties"
       * file.
       */
      public static final String MSG_KEY = "interface.type";
  
      /** Control whether marker interfaces like Serializable are allowed. */
      private boolean allowMarkerInterfaces = true;
  
      @Override
      public int[] getDefaultTokens() {
          return getRequiredTokens();
      }
  
      @Override
      public int[] getRequiredTokens() {
          return new int[] {TokenTypes.INTERFACE_DEF};
      }
  
      @Override
      public int[] getAcceptableTokens() {
          return getRequiredTokens();
      }
  
      @Override
      public void visitToken(DetailAST ast) {
          final DetailAST objBlock =
                  ast.findFirstToken(TokenTypes.OBJBLOCK);
          final DetailAST methodDef =
                  objBlock.findFirstToken(TokenTypes.METHOD_DEF);
          final DetailAST variableDef =
                  objBlock.findFirstToken(TokenTypes.VARIABLE_DEF);
          final boolean methodRequired =
                  !allowMarkerInterfaces || variableDef != null;
  
          if (methodDef == null && methodRequired) {
              log(ast, MSG_KEY);
          }
      }
  
      /**
       * Setter to control whether marker interfaces like Serializable are allowed.
       *
       * @param flag whether to allow marker interfaces or not
       */
      public void setAllowMarkerInterfaces(boolean flag) {
          allowMarkerInterfaces = flag;
      }
  
  }
  