CovariantEqualsCheck.java
////////////////////////////////////////////////////////////////////////////////
// checkstyle: Checks Java source code for adherence to a set of rules.
// Copyright (C) 2001-2021 the original author or authors.
//
// This library is free software; you can redistribute it and/or
// modify it under the terms of the GNU Lesser General Public
// License as published by the Free Software Foundation; either
// version 2.1 of the License, or (at your option) any later version.
//
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
// Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public
// License along with this library; if not, write to the Free Software
// Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
////////////////////////////////////////////////////////////////////////////////
package com.puppycrawl.tools.checkstyle.checks.coding;
import java.util.HashSet;
import java.util.Set;
import com.puppycrawl.tools.checkstyle.FileStatefulCheck;
import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
import com.puppycrawl.tools.checkstyle.api.DetailAST;
import com.puppycrawl.tools.checkstyle.api.FullIdent;
import com.puppycrawl.tools.checkstyle.api.TokenTypes;
import com.puppycrawl.tools.checkstyle.utils.CheckUtil;
/**
* <p>
* Checks that classes and records which define a covariant {@code equals()} method
* also override method {@code equals(Object)}.
* </p>
* <p>
* Covariant {@code equals()} - method that is similar to {@code equals(Object)},
* but with a covariant parameter type (any subtype of Object).
* </p>
* <p>
* <strong>Notice</strong>: the enums are also checked,
* even though they cannot override {@code equals(Object)}.
* The reason is to point out that implementing {@code equals()} in enums
* is considered an awful practice: it may cause having two different enum values
* that are equal using covariant enum method, and not equal when compared normally.
* </p>
* <p>
* Inspired by <a href="https://www.cs.jhu.edu/~daveho/pubs/oopsla2004.pdf">
* Finding Bugs is Easy, chapter '4.5 Bad Covariant Definition of Equals (Eq)'</a>:
* </p>
* <p>
* Java classes and records may override the {@code equals(Object)} method to define
* a predicate for object equality. This method is used by many of the Java
* runtime library classes; for example, to implement generic containers.
* </p>
* <p>
* Programmers sometimes mistakenly use the type of their class {@code Foo}
* as the type of the parameter to {@code equals()}:
* </p>
* <pre>
* public boolean equals(Foo obj) {...}
* </pre>
* <p>
* This covariant version of {@code equals()} does not override the version in
* the {@code Object} class, and it may lead to unexpected behavior at runtime,
* especially if the class is used with one of the standard collection classes
* which expect that the standard {@code equals(Object)} method is overridden.
* </p>
* <p>
* This kind of bug is not obvious because it looks correct, and in circumstances
* where the class is accessed through the references of the class type (rather
* than a supertype), it will work correctly. However, the first time it is used
* in a container, the behavior might be mysterious. For these reasons, this type
* of bug can elude testing and code inspections.
* </p>
* <p>
* To configure the check:
* </p>
* <pre>
* <module name="CovariantEquals"/>
* </pre>
* <p>
* For example:
* </p>
* <pre>
* class Test {
* public boolean equals(Test i) { // violation
* return false;
* }
* }
* </pre>
* <p>
* The same class without violations:
* </p>
* <pre>
* class Test {
* public boolean equals(Test i) { // no violation
* return false;
* }
*
* public boolean equals(Object i) {
* return false;
* }
* }
* </pre>
* <p>
* Another example:
* </p>
* <pre>
* record Test(String str) {
* public boolean equals(Test r) { // violation
* return false;
* }
* }
* </pre>
* <p>
* The same record without violations:
* </p>
* <pre>
* record Test(String str) {
* public boolean equals(Test r) { // no violation
* return false;
* }
*
* public boolean equals(Object r) {
* return false;
* }
* }
* </pre>
* <p>
* Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker}
* </p>
* <p>
* Violation Message Keys:
* </p>
* <ul>
* <li>
* {@code covariant.equals}
* </li>
* </ul>
*
* @since 3.2
*/
@FileStatefulCheck
public class CovariantEqualsCheck extends AbstractCheck {
/**
* A key is pointing to the warning message text in "messages.properties"
* file.
*/
public static final String MSG_KEY = "covariant.equals";
/** Set of equals method definitions. */
private final Set<DetailAST> equalsMethods = new HashSet<>();
@Override
public int[] getDefaultTokens() {
return getRequiredTokens();
}
@Override
public int[] getRequiredTokens() {
return new int[] {
TokenTypes.CLASS_DEF,
TokenTypes.LITERAL_NEW,
TokenTypes.ENUM_DEF,
TokenTypes.RECORD_DEF,
};
}
@Override
public int[] getAcceptableTokens() {
return getRequiredTokens();
}
@Override
public void visitToken(DetailAST ast) {
equalsMethods.clear();
// examine method definitions for equals methods
final DetailAST objBlock = ast.findFirstToken(TokenTypes.OBJBLOCK);
if (objBlock != null) {
DetailAST child = objBlock.getFirstChild();
boolean hasEqualsObject = false;
while (child != null) {
if (CheckUtil.isEqualsMethod(child)) {
if (isFirstParameterObject(child)) {
hasEqualsObject = true;
}
else {
equalsMethods.add(child);
}
}
child = child.getNextSibling();
}
// report equals method definitions
if (!hasEqualsObject) {
for (DetailAST equalsAST : equalsMethods) {
final DetailAST nameNode = equalsAST
.findFirstToken(TokenTypes.IDENT);
log(nameNode, MSG_KEY);
}
}
}
}
/**
* Tests whether a method's first parameter is an Object.
*
* @param methodDefAst the method definition AST to test.
* Precondition: ast is a TokenTypes.METHOD_DEF node.
* @return true if ast has first parameter of type Object.
*/
private static boolean isFirstParameterObject(DetailAST methodDefAst) {
final DetailAST paramsNode = methodDefAst.findFirstToken(TokenTypes.PARAMETERS);
// parameter type "Object"?
final DetailAST paramNode =
paramsNode.findFirstToken(TokenTypes.PARAMETER_DEF);
final DetailAST typeNode = paramNode.findFirstToken(TokenTypes.TYPE);
final FullIdent fullIdent = FullIdent.createFullIdentBelow(typeNode);
final String name = fullIdent.getText();
return "Object".equals(name) || "java.lang.Object".equals(name);
}
}