001///////////////////////////////////////////////////////////////////////////////////////////////
002// checkstyle: Checks Java source code and other text files for adherence to a set of rules.
003// Copyright (C) 2001-2025 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 java.util.HashSet;
023import java.util.Set;
024
025import com.puppycrawl.tools.checkstyle.FileStatefulCheck;
026import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
027import com.puppycrawl.tools.checkstyle.api.DetailAST;
028import com.puppycrawl.tools.checkstyle.api.FullIdent;
029import com.puppycrawl.tools.checkstyle.api.TokenTypes;
030import com.puppycrawl.tools.checkstyle.utils.CheckUtil;
031
032/**
033 * <div>
034 * Checks that classes and records which define a covariant {@code equals()} method
035 * also override method {@code equals(Object)}.
036 * </div>
037 *
038 * <p>
039 * Covariant {@code equals()} - method that is similar to {@code equals(Object)},
040 * but with a covariant parameter type (any subtype of Object).
041 * </p>
042 *
043 * <p>
044 * <strong>Notice</strong>: the enums are also checked,
045 * even though they cannot override {@code equals(Object)}.
046 * The reason is to point out that implementing {@code equals()} in enums
047 * is considered an awful practice: it may cause having two different enum values
048 * that are equal using covariant enum method, and not equal when compared normally.
049 * </p>
050 *
051 * <p>
052 * Inspired by <a href="https://www.cs.jhu.edu/~daveho/pubs/oopsla2004.pdf">
053 * Finding Bugs is Easy, chapter '4.5 Bad Covariant Definition of Equals (Eq)'</a>:
054 * </p>
055 *
056 * <p>
057 * Java classes and records may override the {@code equals(Object)} method to define
058 * a predicate for object equality. This method is used by many of the Java
059 * runtime library classes; for example, to implement generic containers.
060 * </p>
061 *
062 * <p>
063 * Programmers sometimes mistakenly use the type of their class {@code Foo}
064 * as the type of the parameter to {@code equals()}:
065 * </p>
066 * <div class="wrapper"><pre class="prettyprint"><code class="language-java">
067 * public boolean equals(Foo obj) {...}
068 * </code></pre></div>
069 *
070 * <p>
071 * This covariant version of {@code equals()} does not override the version in
072 * the {@code Object} class, and it may lead to unexpected behavior at runtime,
073 * especially if the class is used with one of the standard collection classes
074 * which expect that the standard {@code equals(Object)} method is overridden.
075 * </p>
076 *
077 * <p>
078 * This kind of bug is not obvious because it looks correct, and in circumstances
079 * where the class is accessed through the references of the class type (rather
080 * than a supertype), it will work correctly. However, the first time it is used
081 * in a container, the behavior might be mysterious. For these reasons, this type
082 * of bug can elude testing and code inspections.
083 * </p>
084 *
085 * @since 3.2
086 */
087@FileStatefulCheck
088public class CovariantEqualsCheck extends AbstractCheck {
089
090    /**
091     * A key is pointing to the warning message text in "messages.properties"
092     * file.
093     */
094    public static final String MSG_KEY = "covariant.equals";
095
096    /** Set of equals method definitions. */
097    private final Set<DetailAST> equalsMethods = new HashSet<>();
098
099    @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}