1 ///////////////////////////////////////////////////////////////////////////////////////////////
2 // checkstyle: Checks Java source code and other text files for adherence to a set of rules.
3 // Copyright (C) 2001-2024 the original author or authors.
4 //
5 // This library is free software; you can redistribute it and/or
6 // modify it under the terms of the GNU Lesser General Public
7 // License as published by the Free Software Foundation; either
8 // version 2.1 of the License, or (at your option) any later version.
9 //
10 // This library is distributed in the hope that it will be useful,
11 // but WITHOUT ANY WARRANTY; without even the implied warranty of
12 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 // Lesser General Public License for more details.
14 //
15 // You should have received a copy of the GNU Lesser General Public
16 // License along with this library; if not, write to the Free Software
17 // Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
18 ///////////////////////////////////////////////////////////////////////////////////////////////
19
20 package com.puppycrawl.tools.checkstyle.checks.coding;
21
22 import com.puppycrawl.tools.checkstyle.StatelessCheck;
23 import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
24 import com.puppycrawl.tools.checkstyle.api.DetailAST;
25 import com.puppycrawl.tools.checkstyle.api.TokenTypes;
26
27 /**
28 * <p>
29 * Checks that the clone method is not overridden from the
30 * Object class.
31 * </p>
32 * <p>
33 * This check is almost exactly the same as the {@code NoFinalizerCheck}.
34 * </p>
35 * <p>
36 * See
37 * <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html#clone()">
38 * Object.clone()</a>
39 * </p>
40 * <p>
41 * Rationale: The clone method relies on strange, hard to follow rules that
42 * are difficult to get right and do not work in all situations. In some cases,
43 * either a copy constructor or a static factory method can be used instead of
44 * the clone method to return copies of an object. For more information on rules
45 * for the clone method and its issues, see Effective Java:
46 * Programming Language Guide First Edition by Joshua Bloch pages 45-52.
47 * </p>
48 * <p>
49 * Below are some rules/reasons why the clone method should be avoided.
50 * </p>
51 * <ul>
52 * <li>
53 * Classes supporting the clone method should implement the Cloneable
54 * interface but the Cloneable interface does not include the clone method.
55 * As a result, it doesn't enforce the method override.
56 * </li>
57 * <li>
58 * The Cloneable interface forces the Object's clone method to work
59 * correctly. Without implementing it, the Object's clone method will
60 * throw a CloneNotSupportedException.
61 * </li>
62 * <li>
63 * Non-final classes must return the object returned from a call to
64 * super.clone().
65 * </li>
66 * <li>
67 * Final classes can use a constructor to create a clone which is different
68 * from non-final classes.
69 * </li>
70 * <li>
71 * If a super class implements the clone method incorrectly all subclasses
72 * calling super.clone() are doomed to failure.
73 * </li>
74 * <li>
75 * If a class has references to mutable objects then those object
76 * references must be replaced with copies in the clone method
77 * after calling super.clone().
78 * </li>
79 * <li>
80 * The clone method does not work correctly with final mutable object
81 * references because final references cannot be reassigned.
82 * </li>
83 * <li>
84 * If a super class overrides the clone method then all subclasses must
85 * provide a correct clone implementation.
86 * </li>
87 * </ul>
88 *
89 * <p>Two alternatives to the clone method, in some cases, is a copy constructor
90 * or a static factory method to return copies of an object. Both of these
91 * approaches are simpler and do not conflict with final fields. They do not
92 * force the calling client to handle a CloneNotSupportedException. They also
93 * are typed therefore no casting is necessary. Finally, they are more
94 * flexible since they can take interface types rather than concrete classes.
95 * </p>
96 * <p>Sometimes a copy constructor or static factory is not an acceptable
97 * alternative to the clone method. The example below highlights the
98 * limitation of a copy constructor (or static factory). Assume
99 * Square is a subclass for Shape.
100 * </p>
101 * <pre>
102 * Shape s1 = new Square();
103 * System.out.println(s1 instanceof Square); //true
104 * </pre>
105 * <p>
106 * ...assume at this point the code knows nothing of s1 being a Square
107 * that's the beauty of polymorphism but the code wants to copy
108 * the Square which is declared as a Shape, its super type...
109 * </p>
110 * <pre>
111 * Shape s2 = new Shape(s1); //using the copy constructor
112 * System.out.println(s2 instanceof Square); //false
113 * </pre>
114 * <p>
115 * The working solution (without knowing about all subclasses and doing many
116 * casts) is to do the following (assuming correct clone implementation).
117 * </p>
118 * <pre>
119 * Shape s2 = s1.clone();
120 * System.out.println(s2 instanceof Square); //true
121 * </pre>
122 * <p>
123 * Just keep in mind if this type of polymorphic cloning is required
124 * then a properly implemented clone method may be the best choice.
125 * </p>
126 * <p>Much of this information was taken from Effective Java:
127 * Programming Language Guide First Edition by Joshua Bloch
128 * pages 45-52. Give Bloch credit for writing an excellent book.
129 * </p>
130 * <p>
131 * Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker}
132 * </p>
133 * <p>
134 * Violation Message Keys:
135 * </p>
136 * <ul>
137 * <li>
138 * {@code avoid.clone.method}
139 * </li>
140 * </ul>
141 *
142 * @since 5.0
143 */
144 @StatelessCheck
145 public class NoCloneCheck extends AbstractCheck {
146
147 /**
148 * A key is pointing to the warning message text in "messages.properties"
149 * file.
150 */
151 public static final String MSG_KEY = "avoid.clone.method";
152
153 @Override
154 public int[] getDefaultTokens() {
155 return getRequiredTokens();
156 }
157
158 @Override
159 public int[] getAcceptableTokens() {
160 return getRequiredTokens();
161 }
162
163 @Override
164 public int[] getRequiredTokens() {
165 return new int[] {TokenTypes.METHOD_DEF};
166 }
167
168 @Override
169 public void visitToken(DetailAST ast) {
170 final DetailAST mid = ast.findFirstToken(TokenTypes.IDENT);
171 final String name = mid.getText();
172
173 if ("clone".equals(name)) {
174 final DetailAST params = ast.findFirstToken(TokenTypes.PARAMETERS);
175 final boolean hasEmptyParamList =
176 params.findFirstToken(TokenTypes.PARAMETER_DEF) == null;
177
178 if (hasEmptyParamList) {
179 log(ast, MSG_KEY);
180 }
181 }
182 }
183
184 }