001////////////////////////////////////////////////////////////////////////////////
002// checkstyle: Checks Java source code for adherence to a set of rules.
003// Copyright (C) 2001-2020 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 * <p>
034 * Checks that classes which define a covariant {@code equals()} method
035 * also override method {@code equals(Object)}.
036 * </p>
037 * <p>
038 * Covariant {@code equals()} - method that is similar to {@code equals(Object)},
039 * but with a covariant parameter type (any subtype of Object).
040 * </p>
041 * <p>
042 * <strong>Notice</strong>: the enums are also checked,
043 * even though they cannot override {@code equals(Object)}.
044 * The reason is to point out that implementing {@code equals()} in enums
045 * is considered an awful practice: it may cause having two different enum values
046 * that are equal using covariant enum method, and not equal when compared normally.
047 * </p>
048 * <p>
049 * Inspired by <a href="https://www.cs.jhu.edu/~daveho/pubs/oopsla2004.pdf">
050 * Finding Bugs is Easy, chapter '4.5 Bad Covariant Definition of Equals (Eq)'</a>:
051 * </p>
052 * <p>
053 * Java classes may override the {@code equals(Object)} method to define
054 * a predicate for object equality. This method is used by many of the Java
055 * runtime library classes; for example, to implement generic containers.
056 * </p>
057 * <p>
058 * Programmers sometimes mistakenly use the type of their class {@code Foo}
059 * as the type of the parameter to {@code equals()}:
060 * </p>
061 * <pre>
062 * public boolean equals(Foo obj) {...}
063 * </pre>
064 * <p>
065 * This covariant version of {@code equals()} does not override the version in
066 * the {@code Object} class, and it may lead to unexpected behavior at runtime,
067 * especially if the class is used with one of the standard collection classes
068 * which expect that the standard {@code equals(Object)} method is overridden.
069 * </p>
070 * <p>
071 * This kind of bug is not obvious because it looks correct, and in circumstances
072 * where the class is accessed through the references of the class type (rather
073 * than a supertype), it will work correctly. However, the first time it is used
074 * in a container, the behavior might be mysterious. For these reasons, this type
075 * of bug can elude testing and code inspections.
076 * </p>
077 * <p>
078 * To configure the check:
079 * </p>
080 * <pre>
081 * &lt;module name=&quot;CovariantEquals&quot;/&gt;
082 * </pre>
083 * <p>
084 * For example:
085 * </p>
086 * <pre>
087 * class Test {
088 *   public boolean equals(Test i) {  // violation
089 *     return false;
090 *   }
091 * }
092 * </pre>
093 * <p>
094 * The same class without violations:
095 * </p>
096 * <pre>
097 * class Test {
098 *   public boolean equals(Test i) {  // no violation
099 *     return false;
100 *   }
101 *
102 *   public boolean equals(Object i) {
103 *     return false;
104 *   }
105 * }
106 * </pre>
107 *
108 * @since 3.2
109 */
110@FileStatefulCheck
111public class CovariantEqualsCheck extends AbstractCheck {
112
113    /**
114     * A key is pointing to the warning message text in "messages.properties"
115     * file.
116     */
117    public static final String MSG_KEY = "covariant.equals";
118
119    /** Set of equals method definitions. */
120    private final Set<DetailAST> equalsMethods = new HashSet<>();
121
122    @Override
123    public int[] getDefaultTokens() {
124        return getRequiredTokens();
125    }
126
127    @Override
128    public int[] getRequiredTokens() {
129        return new int[] {TokenTypes.CLASS_DEF, TokenTypes.LITERAL_NEW, TokenTypes.ENUM_DEF, };
130    }
131
132    @Override
133    public int[] getAcceptableTokens() {
134        return getRequiredTokens();
135    }
136
137    @Override
138    public void visitToken(DetailAST ast) {
139        equalsMethods.clear();
140
141        // examine method definitions for equals methods
142        final DetailAST objBlock = ast.findFirstToken(TokenTypes.OBJBLOCK);
143        if (objBlock != null) {
144            DetailAST child = objBlock.getFirstChild();
145            boolean hasEqualsObject = false;
146            while (child != null) {
147                if (CheckUtil.isEqualsMethod(child)) {
148                    if (isFirstParameterObject(child)) {
149                        hasEqualsObject = true;
150                    }
151                    else {
152                        equalsMethods.add(child);
153                    }
154                }
155                child = child.getNextSibling();
156            }
157
158            // report equals method definitions
159            if (!hasEqualsObject) {
160                for (DetailAST equalsAST : equalsMethods) {
161                    final DetailAST nameNode = equalsAST
162                            .findFirstToken(TokenTypes.IDENT);
163                    log(nameNode, MSG_KEY);
164                }
165            }
166        }
167    }
168
169    /**
170     * Tests whether a method's first parameter is an Object.
171     * @param methodDefAst the method definition AST to test.
172     *     Precondition: ast is a TokenTypes.METHOD_DEF node.
173     * @return true if ast has first parameter of type Object.
174     */
175    private static boolean isFirstParameterObject(DetailAST methodDefAst) {
176        final DetailAST paramsNode = methodDefAst.findFirstToken(TokenTypes.PARAMETERS);
177
178        // parameter type "Object"?
179        final DetailAST paramNode =
180            paramsNode.findFirstToken(TokenTypes.PARAMETER_DEF);
181        final DetailAST typeNode = paramNode.findFirstToken(TokenTypes.TYPE);
182        final FullIdent fullIdent = FullIdent.createFullIdentBelow(typeNode);
183        final String name = fullIdent.getText();
184        return "Object".equals(name) || "java.lang.Object".equals(name);
185    }
186
187}