001///////////////////////////////////////////////////////////////////////////////////////////////
002// checkstyle: Checks Java source code and other text files for adherence to a set of rules.
003// Copyright (C) 2001-2024 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 * <pre>
067 * public boolean equals(Foo obj) {...}
068 * </pre>
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 * <p>
086 * Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker}
087 * </p>
088 *
089 * <p>
090 * Violation Message Keys:
091 * </p>
092 * <ul>
093 * <li>
094 * {@code covariant.equals}
095 * </li>
096 * </ul>
097 *
098 * @since 3.2
099 */
100@FileStatefulCheck
101public class CovariantEqualsCheck extends AbstractCheck {
102
103    /**
104     * A key is pointing to the warning message text in "messages.properties"
105     * file.
106     */
107    public static final String MSG_KEY = "covariant.equals";
108
109    /** Set of equals method definitions. */
110    private final Set<DetailAST> equalsMethods = new HashSet<>();
111
112    @Override
113    public int[] getDefaultTokens() {
114        return getRequiredTokens();
115    }
116
117    @Override
118    public int[] getRequiredTokens() {
119        return new int[] {
120            TokenTypes.CLASS_DEF,
121            TokenTypes.LITERAL_NEW,
122            TokenTypes.ENUM_DEF,
123            TokenTypes.RECORD_DEF,
124        };
125    }
126
127    @Override
128    public int[] getAcceptableTokens() {
129        return getRequiredTokens();
130    }
131
132    @Override
133    public void visitToken(DetailAST ast) {
134        equalsMethods.clear();
135
136        // examine method definitions for equals methods
137        final DetailAST objBlock = ast.findFirstToken(TokenTypes.OBJBLOCK);
138        if (objBlock != null) {
139            DetailAST child = objBlock.getFirstChild();
140            boolean hasEqualsObject = false;
141            while (child != null) {
142                if (CheckUtil.isEqualsMethod(child)) {
143                    if (isFirstParameterObject(child)) {
144                        hasEqualsObject = true;
145                    }
146                    else {
147                        equalsMethods.add(child);
148                    }
149                }
150                child = child.getNextSibling();
151            }
152
153            // report equals method definitions
154            if (!hasEqualsObject) {
155                for (DetailAST equalsAST : equalsMethods) {
156                    final DetailAST nameNode = equalsAST
157                            .findFirstToken(TokenTypes.IDENT);
158                    log(nameNode, MSG_KEY);
159                }
160            }
161        }
162    }
163
164    /**
165     * Tests whether a method's first parameter is an Object.
166     *
167     * @param methodDefAst the method definition AST to test.
168     *     Precondition: ast is a TokenTypes.METHOD_DEF node.
169     * @return true if ast has first parameter of type Object.
170     */
171    private static boolean isFirstParameterObject(DetailAST methodDefAst) {
172        final DetailAST paramsNode = methodDefAst.findFirstToken(TokenTypes.PARAMETERS);
173
174        // parameter type "Object"?
175        final DetailAST paramNode =
176            paramsNode.findFirstToken(TokenTypes.PARAMETER_DEF);
177        final DetailAST typeNode = paramNode.findFirstToken(TokenTypes.TYPE);
178        final FullIdent fullIdent = FullIdent.createFullIdentBelow(typeNode);
179        final String name = fullIdent.getText();
180        return "Object".equals(name) || "java.lang.Object".equals(name);
181    }
182
183}