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 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>
   * Checks that the clone method is not overridden from the
   * Object class.
   * </p>
   * <p>
   * This check is almost exactly the same as the {@code NoFinalizerCheck}.
   * </p>
   * <p>
   * See
   * <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html#clone()">
   * Object.clone()</a>
   * </p>
   * <p>
   * Rationale: The clone method relies on strange, hard to follow rules that
   * are difficult to get right and do not work in all situations. In some cases,
   * either a copy constructor or a static factory method can be used instead of
   * the clone method to return copies of an object. For more information on rules
   * for the clone method and its issues, see Effective Java:
   * Programming Language Guide First Edition by Joshua Bloch pages 45-52.
   * </p>
   * <p>
   * Below are some rules/reasons why the clone method should be avoided.
   * </p>
   * <ul>
   * <li>
   * Classes supporting the clone method should implement the Cloneable
   * interface but the Cloneable interface does not include the clone method.
   * As a result, it doesn't enforce the method override.
   * </li>
   * <li>
   * The Cloneable interface forces the Object's clone method to work
   * correctly. Without implementing it, the Object's clone method will
   * throw a CloneNotSupportedException.
   * </li>
   * <li>
   * Non-final classes must return the object returned from a call to
   * super.clone().
   * </li>
   * <li>
   * Final classes can use a constructor to create a clone which is different
   * from non-final classes.
   * </li>
   * <li>
   * If a super class implements the clone method incorrectly all subclasses
   * calling super.clone() are doomed to failure.
   * </li>
   * <li>
   * If a class has references to mutable objects then those object
   * references must be replaced with copies in the clone method
   * after calling super.clone().
   * </li>
   * <li>
   * The clone method does not work correctly with final mutable object
   * references because final references cannot be reassigned.
   * </li>
   * <li>
   * If a super class overrides the clone method then all subclasses must
   * provide a correct clone implementation.
   * </li>
   * </ul>
   *
   * <p>Two alternatives to the clone method, in some cases, is a copy constructor
   * or a static factory method to return copies of an object. Both of these
   * approaches are simpler and do not conflict with final fields. They do not
   * force the calling client to handle a CloneNotSupportedException.  They also
   * are typed therefore no casting is necessary. Finally, they are more
   * flexible since they can take interface types rather than concrete classes.
   * </p>
   * <p>Sometimes a copy constructor or static factory is not an acceptable
   * alternative to the clone method.  The example below highlights the
   * limitation of a copy constructor (or static factory). Assume
   * Square is a subclass for Shape.
   * </p>
   * <pre>
   * Shape s1 = new Square();
   * System.out.println(s1 instanceof Square); //true
   * </pre>
   * <p>
   * ...assume at this point the code knows nothing of s1 being a Square
   *    that's the beauty of polymorphism but the code wants to copy
   *    the Square which is declared as a Shape, its super type...
   * </p>
   * <pre>
   * Shape s2 = new Shape(s1); //using the copy constructor
   * System.out.println(s2 instanceof Square); //false
   * </pre>
   * <p>
   * The working solution (without knowing about all subclasses and doing many
   * casts) is to do the following (assuming correct clone implementation).
   * </p>
   * <pre>
   * Shape s2 = s1.clone();
   * System.out.println(s2 instanceof Square); //true
   * </pre>
   * <p>
   * Just keep in mind if this type of polymorphic cloning is required
   * then a properly implemented clone method may be the best choice.
   * </p>
   * <p>Much of this information was taken from Effective Java:
   * Programming Language Guide First Edition by Joshua Bloch
   * pages 45-52.  Give Bloch credit for writing an excellent book.
   * </p>
   * <p>
   * To configure the check:
   * </p>
   * <pre>
   * &lt;module name=&quot;NoClone&quot;/&gt;
   * </pre>
   * <p>Example: </p>
   * <pre>
   * public class Foo {
   *
   *  public Object clone() {return null;} // violation, overrides the clone method
   *
   *  public Foo clone() {return null;} // violation, overrides the clone method
   *
   *  public static Object clone(Object o) {return null;} // OK
   *
   *  public static Foo clone(Foo o) {return null;} // OK
   *
   * }
   * </pre>
   * <p>
   * Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker}
   * </p>
   * <p>
   * Violation Message Keys:
   * </p>
   * <ul>
   * <li>
   * {@code avoid.clone.method}
   * </li>
   * </ul>
   *
   * @since 5.0
   */
  @StatelessCheck
  public class NoCloneCheck extends AbstractCheck {
  
      /**
       * A key is pointing to the warning message text in "messages.properties"
       * file.
       */
      public static final String MSG_KEY = "avoid.clone.method";
  
      @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 visitToken(DetailAST ast) {
          final DetailAST mid = ast.findFirstToken(TokenTypes.IDENT);
          final String name = mid.getText();
  
          if ("clone".equals(name)) {
              final DetailAST params = ast.findFirstToken(TokenTypes.PARAMETERS);
              final boolean hasEmptyParamList =
                  params.findFirstToken(TokenTypes.PARAMETER_DEF) == null;
  
              if (hasEmptyParamList) {
                  log(ast, MSG_KEY);
              }
          }
      }
  
  }
  