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.HashMap;
  import java.util.Map;
  
  import com.puppycrawl.tools.checkstyle.FileStatefulCheck;
  import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
  import com.puppycrawl.tools.checkstyle.api.DetailAST;
  import com.puppycrawl.tools.checkstyle.api.FullIdent;
  import com.puppycrawl.tools.checkstyle.api.TokenTypes;
  import com.puppycrawl.tools.checkstyle.utils.CheckUtil;
  
  /**
   * <p>
   * Checks that classes that either override {@code equals()} or {@code hashCode()} also
   * overrides the other.
   * This check only verifies that the method declarations match {@code Object.equals(Object)} and
   * {@code Object.hashCode()} exactly to be considered an override. This check does not verify
   * invalid method names, parameters other than {@code Object}, or anything else.
   * </p>
   * <p>
   * Rationale: The contract of {@code equals()} and {@code hashCode()} requires that
   * equal objects have the same hashCode. Therefore, whenever you override
   * {@code equals()} you must override {@code hashCode()} to ensure that your class can
   * be used in hash-based collections.
   * </p>
   * <p>
   * To configure the check:
   * </p>
   * <pre>
   * &lt;module name=&quot;EqualsHashCode&quot;/&gt;
   * </pre>
   * <p>Example:</p>
   * <pre>
   * public static class Example1 {
   *     public int hashCode() {
   *         // code
   *     }
   *     public boolean equals(String o) { // violation, overloaded implementation of 'equals'
   *         // code
   *     }
   * }
   * public static class Example2 {
   *     public boolean equals(Object o) { // violation, no 'hashCode'
   *         // code
   *     }
   *     public boolean equals(String o) {
   *         // code
   *     }
   * }
   * public static class Example3 {
   *     public int hashCode() {
   *         // code
   *     }
   *     public boolean equals(Object o) { // OK
   *         // code
   *     }
   *     public boolean equals(String o) {
   *         // code
   *     }
   * }
   * public static class Example4 {
   *     public int hashCode() {
   *         // code
   *     }
   *     public boolean equals(java.lang.Object o) { // OK
   *         // code
   *    }
   * }
   * public static class Example5 {
   *     public static int hashCode(int i) {
   *         // code
   *     }
   *     public boolean equals(Object o) { // violation, overloaded implementation of 'hashCode'
   *         // code
   *     }
   * }
   * public static class Example6 {
   *     public int hashCode() { // violation, overloaded implementation of 'equals'
   *         // code
   *     }
   *     public static boolean equals(Object o, Object o2) {
   *         // code
   *     }
   * }
   * </pre>
   * <p>
   * Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker}
   * </p>
   * <p>
   * Violation Message Keys:
   * </p>
   * <ul>
   * <li>
   * {@code equals.noEquals}
   * </li>
   * <li>
   * {@code equals.noHashCode}
   * </li>
   * </ul>
   *
   * @since 3.0
   */
  @FileStatefulCheck
  public class EqualsHashCodeCheck
          extends AbstractCheck {
  
      // implementation note: we have to use the following members to
      // keep track of definitions in different inner classes
  
      /**
       * A key is pointing to the warning message text in "messages.properties"
       * file.
       */
      public static final String MSG_KEY_HASHCODE = "equals.noHashCode";
  
      /**
       * A key is pointing to the warning message text in "messages.properties"
       * file.
       */
      public static final String MSG_KEY_EQUALS = "equals.noEquals";
  
      /** Maps OBJ_BLOCK to the method definition of equals(). */
      private final Map<DetailAST, DetailAST> objBlockWithEquals = new HashMap<>();
  
      /** Maps OBJ_BLOCKs to the method definition of hashCode(). */
      private final Map<DetailAST, DetailAST> objBlockWithHashCode = new HashMap<>();
  
      @Override
      public int[] getDefaultTokens() {
          return getRequiredTokens();
      }
  
      @Override
      public int[] getAcceptableTokens() {
          return getRequiredTokens();
      }
  
      @Override
      public int[] getRequiredTokens() {
          return new int[] {TokenTypes.METHOD_DEF};
      }
  
      @Override
      public void beginTree(DetailAST rootAST) {
          objBlockWithEquals.clear();
          objBlockWithHashCode.clear();
      }
  
      @Override
      public void visitToken(DetailAST ast) {
          if (isEqualsMethod(ast)) {
              objBlockWithEquals.put(ast.getParent(), ast);
          }
          else if (isHashCodeMethod(ast)) {
              objBlockWithHashCode.put(ast.getParent(), ast);
          }
      }
  
      /**
       * Determines if an AST is a valid Equals method implementation.
       *
       * @param ast the AST to check
       * @return true if the {code ast} is an Equals method.
       */
      private static boolean isEqualsMethod(DetailAST ast) {
          final DetailAST modifiers = ast.getFirstChild();
          final DetailAST parameters = ast.findFirstToken(TokenTypes.PARAMETERS);
  
          return CheckUtil.isEqualsMethod(ast)
                  && isObjectParam(parameters.getFirstChild())
                  && (ast.findFirstToken(TokenTypes.SLIST) != null
                          || modifiers.findFirstToken(TokenTypes.LITERAL_NATIVE) != null);
      }
  
      /**
       * Determines if an AST is a valid HashCode method implementation.
       *
       * @param ast the AST to check
       * @return true if the {code ast} is a HashCode method.
       */
      private static boolean isHashCodeMethod(DetailAST ast) {
          final DetailAST modifiers = ast.getFirstChild();
          final DetailAST methodName = ast.findFirstToken(TokenTypes.IDENT);
          final DetailAST parameters = ast.findFirstToken(TokenTypes.PARAMETERS);
  
          return "hashCode".equals(methodName.getText())
                  && parameters.getFirstChild() == null
                  && (ast.findFirstToken(TokenTypes.SLIST) != null
                          || modifiers.findFirstToken(TokenTypes.LITERAL_NATIVE) != null);
      }
  
      /**
       * Determines if an AST is a formal param of type Object.
       *
       * @param paramNode the AST to check
       * @return true if firstChild is a parameter of an Object type.
       */
      private static boolean isObjectParam(DetailAST paramNode) {
          final DetailAST typeNode = paramNode.findFirstToken(TokenTypes.TYPE);
          final FullIdent fullIdent = FullIdent.createFullIdentBelow(typeNode);
          final String name = fullIdent.getText();
          return "Object".equals(name) || "java.lang.Object".equals(name);
      }
  
      @Override
      public void finishTree(DetailAST rootAST) {
          objBlockWithEquals
              .entrySet().stream().filter(detailASTDetailASTEntry -> {
                  return objBlockWithHashCode.remove(detailASTDetailASTEntry.getKey()) == null;
              }).forEach(detailASTDetailASTEntry -> {
                  final DetailAST equalsAST = detailASTDetailASTEntry.getValue();
                  log(equalsAST, MSG_KEY_HASHCODE);
              });
          objBlockWithHashCode.forEach((key, equalsAST) -> log(equalsAST, MSG_KEY_EQUALS));
      }
  
  }
  