001////////////////////////////////////////////////////////////////////////////////
002// checkstyle: Checks Java source code for adherence to a set of rules.
003// Copyright (C) 2001-2017 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
022import com.puppycrawl.tools.checkstyle.StatelessCheck;
023import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
024import com.puppycrawl.tools.checkstyle.api.DetailAST;
025import com.puppycrawl.tools.checkstyle.api.TokenTypes;
026
027/**
028 * <p>
029 * Checks that the clone method is not overridden from the
030 * Object class.
031 * </p>
032 *
033 * <p>Rationale: The clone method relies on strange/hard to follow rules that
034 * do not work it all situations.  Consequently, it is difficult to
035 * override correctly.  Below are some of the rules/reasons why the clone
036 * method should be avoided.
037 *
038 * <ul>
039 * <li>
040 * Classes supporting the clone method should implement the Cloneable
041 * interface but the Cloneable interface does not include the clone method.
042 * As a result, it doesn't enforce the method override.
043 * </li>
044 * <li>
045 * The Cloneable interface forces the Object's clone method to work
046 * correctly. Without implementing it, the Object's clone method will
047 * throw a CloneNotSupportedException.
048 * </li>
049 * <li>
050 * Non-final classes must return the object returned from a call to
051 * super.clone().
052 * </li>
053 * <li>
054 * Final classes can use a constructor to create a clone which is different
055 * from non-final classes.
056 * </li>
057 * <li>
058 * If a super class implements the clone method incorrectly all subclasses
059 * calling super.clone() are doomed to failure.
060 * </li>
061 * <li>
062 * If a class has references to mutable objects then those object
063 * references must be replaced with copies in the clone method
064 * after calling super.clone().
065 * </li>
066 * <li>
067 * The clone method does not work correctly with final mutable object
068 * references because final references cannot be reassigned.
069 * </li>
070 * <li>
071 * If a super class overrides the clone method then all subclasses must
072 * provide a correct clone implementation.
073 * </li>
074 * </ul>
075 *
076 * <p>Two alternatives to the clone method, in some cases, is a copy constructor
077 * or a static factory method to return copies of an object. Both of these
078 * approaches are simpler and do not conflict with final fields. They do not
079 * force the calling client to handle a CloneNotSupportedException.  They also
080 * are typed therefore no casting is necessary. Finally, they are more
081 * flexible since they can take interface types rather than concrete classes.
082 *
083 * <p>Sometimes a copy constructor or static factory is not an acceptable
084 * alternative to the clone method.  The example below highlights the
085 * limitation of a copy constructor (or static factory). Assume
086 * Square is a subclass for Shape.
087 *
088 * <pre>
089 * Shape s1 = new Square();
090 * System.out.println(s1 instanceof Square); //true
091 * </pre>
092 * ...assume at this point the code knows nothing of s1 being a Square
093 *    that's the beauty of polymorphism but the code wants to copy
094 *    the Square which is declared as a Shape, its super type...
095 *
096 * <pre>
097 * Shape s2 = new Shape(s1); //using the copy constructor
098 * System.out.println(s2 instanceof Square); //false
099 * </pre>
100 * The working solution (without knowing about all subclasses and doing many
101 * casts) is to do the following (assuming correct clone implementation).
102 *
103 * <pre>
104 * Shape s2 = s1.clone();
105 * System.out.println(s2 instanceof Square); //true
106 * </pre>
107 * Just keep in mind if this type of polymorphic cloning is required
108 * then a properly implemented clone method may be the best choice.
109 *
110 * <p>Much of this information was taken from Effective Java:
111 * Programming Language Guide First Edition by Joshua Bloch
112 * pages 45-52.  Give Bloch credit for writing an excellent book.
113 * </p>
114 *
115 * <p>This check is almost exactly the same as the {@link NoFinalizerCheck}
116 *
117 * @author Travis Schneeberger
118 * @see Object#clone()
119 */
120@StatelessCheck
121public class NoCloneCheck extends AbstractCheck {
122
123    /**
124     * A key is pointing to the warning message text in "messages.properties"
125     * file.
126     */
127    public static final String MSG_KEY = "avoid.clone.method";
128
129    @Override
130    public int[] getDefaultTokens() {
131        return getRequiredTokens();
132    }
133
134    @Override
135    public int[] getAcceptableTokens() {
136        return getRequiredTokens();
137    }
138
139    @Override
140    public int[] getRequiredTokens() {
141        return new int[] {TokenTypes.METHOD_DEF};
142    }
143
144    @Override
145    public void visitToken(DetailAST aAST) {
146        final DetailAST mid = aAST.findFirstToken(TokenTypes.IDENT);
147        final String name = mid.getText();
148
149        if ("clone".equals(name)) {
150
151            final DetailAST params = aAST.findFirstToken(TokenTypes.PARAMETERS);
152            final boolean hasEmptyParamList =
153                params.findFirstToken(TokenTypes.PARAMETER_DEF) == null;
154
155            if (hasEmptyParamList) {
156                log(aAST.getLineNo(), MSG_KEY);
157            }
158        }
159    }
160}