1 ///////////////////////////////////////////////////////////////////////////////////////////////
2 // checkstyle: Checks Java source code and other text files for adherence to a set of rules.
3 // Copyright (C) 2001-2025 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 java.util.HashSet;
23 import java.util.Set;
24
25 import com.puppycrawl.tools.checkstyle.FileStatefulCheck;
26 import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
27 import com.puppycrawl.tools.checkstyle.api.DetailAST;
28 import com.puppycrawl.tools.checkstyle.api.FullIdent;
29 import com.puppycrawl.tools.checkstyle.api.TokenTypes;
30 import com.puppycrawl.tools.checkstyle.utils.CheckUtil;
31
32 /**
33 * <div>
34 * Checks that classes and records which define a covariant {@code equals()} method
35 * also override method {@code equals(Object)}.
36 * </div>
37 *
38 * <p>
39 * Covariant {@code equals()} - method that is similar to {@code equals(Object)},
40 * but with a covariant parameter type (any subtype of Object).
41 * </p>
42 *
43 * <p>
44 * <strong>Notice</strong>: the enums are also checked,
45 * even though they cannot override {@code equals(Object)}.
46 * The reason is to point out that implementing {@code equals()} in enums
47 * is considered an awful practice: it may cause having two different enum values
48 * that are equal using covariant enum method, and not equal when compared normally.
49 * </p>
50 *
51 * <p>
52 * Inspired by <a href="https://www.cs.jhu.edu/~daveho/pubs/oopsla2004.pdf">
53 * Finding Bugs is Easy, chapter '4.5 Bad Covariant Definition of Equals (Eq)'</a>:
54 * </p>
55 *
56 * <p>
57 * Java classes and records may override the {@code equals(Object)} method to define
58 * a predicate for object equality. This method is used by many of the Java
59 * runtime library classes; for example, to implement generic containers.
60 * </p>
61 *
62 * <p>
63 * Programmers sometimes mistakenly use the type of their class {@code Foo}
64 * as the type of the parameter to {@code equals()}:
65 * </p>
66 * <div class="wrapper"><pre class="prettyprint"><code class="language-java">
67 * public boolean equals(Foo obj) {...}
68 * </code></pre></div>
69 *
70 * <p>
71 * This covariant version of {@code equals()} does not override the version in
72 * the {@code Object} class, and it may lead to unexpected behavior at runtime,
73 * especially if the class is used with one of the standard collection classes
74 * which expect that the standard {@code equals(Object)} method is overridden.
75 * </p>
76 *
77 * <p>
78 * This kind of bug is not obvious because it looks correct, and in circumstances
79 * where the class is accessed through the references of the class type (rather
80 * than a supertype), it will work correctly. However, the first time it is used
81 * in a container, the behavior might be mysterious. For these reasons, this type
82 * of bug can elude testing and code inspections.
83 * </p>
84 *
85 * @since 3.2
86 */
87 @FileStatefulCheck
88 public class CovariantEqualsCheck extends AbstractCheck {
89
90 /**
91 * A key is pointing to the warning message text in "messages.properties"
92 * file.
93 */
94 public static final String MSG_KEY = "covariant.equals";
95
96 /** Set of equals method definitions. */
97 private final Set<DetailAST> equalsMethods = new HashSet<>();
98
99 @Override
100 public int[] getDefaultTokens() {
101 return getRequiredTokens();
102 }
103
104 @Override
105 public int[] getRequiredTokens() {
106 return new int[] {
107 TokenTypes.CLASS_DEF,
108 TokenTypes.LITERAL_NEW,
109 TokenTypes.ENUM_DEF,
110 TokenTypes.RECORD_DEF,
111 };
112 }
113
114 @Override
115 public int[] getAcceptableTokens() {
116 return getRequiredTokens();
117 }
118
119 @Override
120 public void visitToken(DetailAST ast) {
121 equalsMethods.clear();
122
123 // examine method definitions for equals methods
124 final DetailAST objBlock = ast.findFirstToken(TokenTypes.OBJBLOCK);
125 if (objBlock != null) {
126 DetailAST child = objBlock.getFirstChild();
127 boolean hasEqualsObject = false;
128 while (child != null) {
129 if (CheckUtil.isEqualsMethod(child)) {
130 if (isFirstParameterObject(child)) {
131 hasEqualsObject = true;
132 }
133 else {
134 equalsMethods.add(child);
135 }
136 }
137 child = child.getNextSibling();
138 }
139
140 // report equals method definitions
141 if (!hasEqualsObject) {
142 for (DetailAST equalsAST : equalsMethods) {
143 final DetailAST nameNode = equalsAST
144 .findFirstToken(TokenTypes.IDENT);
145 log(nameNode, MSG_KEY);
146 }
147 }
148 }
149 }
150
151 /**
152 * Tests whether a method's first parameter is an Object.
153 *
154 * @param methodDefAst the method definition AST to test.
155 * Precondition: ast is a TokenTypes.METHOD_DEF node.
156 * @return true if ast has first parameter of type Object.
157 */
158 private static boolean isFirstParameterObject(DetailAST methodDefAst) {
159 final DetailAST paramsNode = methodDefAst.findFirstToken(TokenTypes.PARAMETERS);
160
161 // parameter type "Object"?
162 final DetailAST paramNode =
163 paramsNode.findFirstToken(TokenTypes.PARAMETER_DEF);
164 final DetailAST typeNode = paramNode.findFirstToken(TokenTypes.TYPE);
165 final FullIdent fullIdent = FullIdent.createFullIdentBelow(typeNode);
166 final String name = fullIdent.getText();
167 return "Object".equals(name) || "java.lang.Object".equals(name);
168 }
169
170 }