001////////////////////////////////////////////////////////////////////////////////
002// checkstyle: Checks Java source code for adherence to a set of rules.
003// Copyright (C) 2001-2015 the original author or authors.
004//
005// This library is free software; you can redistribute it and/or
006// modify it under the terms of the GNU Lesser General Public
007// License as published by the Free Software Foundation; either
008// version 2.1 of the License, or (at your option) any later version.
009//
010// This library is distributed in the hope that it will be useful,
011// but WITHOUT ANY WARRANTY; without even the implied warranty of
012// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
013// Lesser General Public License for more details.
014//
015// You should have received a copy of the GNU Lesser General Public
016// License along with this library; if not, write to the Free Software
017// Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
018////////////////////////////////////////////////////////////////////////////////
019
020package com.puppycrawl.tools.checkstyle.checks.coding;
021
022/**
023 * <p>
024 * Checks that the clone method is not overridden from the
025 * Object class.
026 * </p>
027 *
028 * <p>Rationale: The clone method relies on strange/hard to follow rules that
029 * do not work it all situations.  Consequently, it is difficult to
030 * override correctly.  Below are some of the rules/reasons why the clone
031 * method should be avoided.
032 *
033 * <ul>
034 * <li>
035 * Classes supporting the clone method should implement the Cloneable
036 * interface but the Cloneable interface does not include the clone method.
037 * As a result, it doesn't enforce the method override.
038 * </li>
039 * <li>
040 * The Cloneable interface forces the Object's clone method to work
041 * correctly. Without implementing it, the Object's clone method will
042 * throw a CloneNotSupportedException.
043 * </li>
044 * <li>
045 * Non-final classes must return the object returned from a call to
046 * super.clone().
047 * </li>
048 * <li>
049 * Final classes can use a constructor to create a clone which is different
050 * from non-final classes.
051 * </li>
052 * <li>
053 * If a super class implements the clone method incorrectly all subclasses
054 * calling super.clone() are doomed to failure.
055 * </li>
056 * <li>
057 * If a class has references to mutable objects then those object
058 * references must be replaced with copies in the clone method
059 * after calling super.clone().
060 * </li>
061 * <li>
062 * The clone method does not work correctly with final mutable object
063 * references because final references cannot be reassigned.
064 * </li>
065 * <li>
066 * If a super class overrides the clone method then all subclasses must
067 * provide a correct clone implementation.
068 * </li>
069 * </ul>
070 *
071 * <p>Two alternatives to the clone method, in some cases, is a copy constructor
072 * or a static factory method to return copies of an object. Both of these
073 * approaches are simpler and do not conflict with final fields. They do not
074 * force the calling client to handle a CloneNotSupportedException.  They also
075 * are typed therefore no casting is necessary. Finally, they are more
076 * flexible since they can take interface types rather than concrete classes.
077 *
078 * <p>Sometimes a copy constructor or static factory is not an acceptable
079 * alternative to the clone method.  The example below highlights the
080 * limitation of a copy constructor (or static factory). Assume
081 * Square is a subclass for Shape.
082 *
083 * <pre>
084 * Shape s1 = new Square();
085 * System.out.println(s1 instanceof Square); //true
086 * </pre>
087 * ...assume at this point the code knows nothing of s1 being a Square
088 *    that's the beauty of polymorphism but the code wants to copy
089 *    the Square which is declared as a Shape, its super type...
090 *
091 * <pre>
092 * Shape s2 = new Shape(s1); //using the copy constructor
093 * System.out.println(s2 instanceof Square); //false
094 * </pre>
095 * The working solution (without knowing about all subclasses and doing many
096 * casts) is to do the following (assuming correct clone implementation).
097 *
098 * <pre>
099 * Shape s2 = s1.clone();
100 * System.out.println(s2 instanceof Square); //true
101 * </pre>
102 * Just keep in mind if this type of polymorphic cloning is required
103 * then a properly implemented clone method may be the best choice.
104 *
105 * <p>Much of this information was taken from Effective Java:
106 * Programming Language Guide First Edition by Joshua Bloch
107 * pages 45-52.  Give Bloch credit for writing an excellent book.
108 * </p>
109 *
110 * <p>This check is almost exactly the same as the {@link NoFinalizerCheck}
111 *
112 * @author Travis Schneeberger
113 * @see Object#clone()
114 */
115public class NoCloneCheck extends AbstractIllegalMethodCheck {
116
117    /**
118     * A key is pointing to the warning message text in "messages.properties"
119     * file.
120     */
121    public static final String MSG_KEY = "avoid.clone.method";
122
123    /**
124     * Creates an instance.
125     */
126    public NoCloneCheck() {
127        super("clone", MSG_KEY);
128    }
129}