NoCloneCheck.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 com.puppycrawl.tools.checkstyle.StatelessCheck;
import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
import com.puppycrawl.tools.checkstyle.api.DetailAST;
import com.puppycrawl.tools.checkstyle.api.TokenTypes;
/**
* <p>
* Checks that the clone method is not overridden from the
* Object class.
* </p>
* <p>
* This check is almost exactly the same as the {@code NoFinalizerCheck}.
* </p>
* <p>
* See
* <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html#clone()">
* Object.clone()</a>
* </p>
* <p>
* Rationale: The clone method relies on strange, hard to follow rules that
* are difficult to get right and do not work in all situations. In some cases,
* either a copy constructor or a static factory method can be used instead of
* the clone method to return copies of an object. For more information on rules
* for the clone method and its issues, see Effective Java:
* Programming Language Guide First Edition by Joshua Bloch pages 45-52.
* </p>
* <p>
* Below are some of the rules/reasons why the clone method should be avoided.
* </p>
* <ul>
* <li>
* Classes supporting the clone method should implement the Cloneable
* interface but the Cloneable interface does not include the clone method.
* As a result, it doesn't enforce the method override.
* </li>
* <li>
* The Cloneable interface forces the Object's clone method to work
* correctly. Without implementing it, the Object's clone method will
* throw a CloneNotSupportedException.
* </li>
* <li>
* Non-final classes must return the object returned from a call to
* super.clone().
* </li>
* <li>
* Final classes can use a constructor to create a clone which is different
* from non-final classes.
* </li>
* <li>
* If a super class implements the clone method incorrectly all subclasses
* calling super.clone() are doomed to failure.
* </li>
* <li>
* If a class has references to mutable objects then those object
* references must be replaced with copies in the clone method
* after calling super.clone().
* </li>
* <li>
* The clone method does not work correctly with final mutable object
* references because final references cannot be reassigned.
* </li>
* <li>
* If a super class overrides the clone method then all subclasses must
* provide a correct clone implementation.
* </li>
* </ul>
*
* <p>Two alternatives to the clone method, in some cases, is a copy constructor
* or a static factory method to return copies of an object. Both of these
* approaches are simpler and do not conflict with final fields. They do not
* force the calling client to handle a CloneNotSupportedException. They also
* are typed therefore no casting is necessary. Finally, they are more
* flexible since they can take interface types rather than concrete classes.
* </p>
* <p>Sometimes a copy constructor or static factory is not an acceptable
* alternative to the clone method. The example below highlights the
* limitation of a copy constructor (or static factory). Assume
* Square is a subclass for Shape.
* </p>
* <pre>
* Shape s1 = new Square();
* System.out.println(s1 instanceof Square); //true
* </pre>
* <p>
* ...assume at this point the code knows nothing of s1 being a Square
* that's the beauty of polymorphism but the code wants to copy
* the Square which is declared as a Shape, its super type...
* </p>
* <pre>
* Shape s2 = new Shape(s1); //using the copy constructor
* System.out.println(s2 instanceof Square); //false
* </pre>
* <p>
* The working solution (without knowing about all subclasses and doing many
* casts) is to do the following (assuming correct clone implementation).
* </p>
* <pre>
* Shape s2 = s1.clone();
* System.out.println(s2 instanceof Square); //true
* </pre>
* <p>
* Just keep in mind if this type of polymorphic cloning is required
* then a properly implemented clone method may be the best choice.
* </p>
* <p>Much of this information was taken from Effective Java:
* Programming Language Guide First Edition by Joshua Bloch
* pages 45-52. Give Bloch credit for writing an excellent book.
* </p>
* <p>
* To configure the check:
* </p>
* <pre>
* <module name="NoClone"/>
* </pre>
* <p>Example: </p>
* <pre>
* public class Foo {
*
* public Object clone() {return null;} // violation, overrides the clone method
*
* public Foo clone() {return null;} // violation, overrides the clone method
*
* public static Object clone(Object o) {return null;} // OK
*
* public static Foo clone(Foo o) {return null;} // OK
*
* }
* </pre>
* <p>
* Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker}
* </p>
* <p>
* Violation Message Keys:
* </p>
* <ul>
* <li>
* {@code avoid.clone.method}
* </li>
* </ul>
*
* @since 5.0
*/
@StatelessCheck
public class NoCloneCheck extends AbstractCheck {
/**
* A key is pointing to the warning message text in "messages.properties"
* file.
*/
public static final String MSG_KEY = "avoid.clone.method";
@Override
public int[] getDefaultTokens() {
return getRequiredTokens();
}
@Override
public int[] getAcceptableTokens() {
return getRequiredTokens();
}
@Override
public int[] getRequiredTokens() {
return new int[] {TokenTypes.METHOD_DEF};
}
@Override
public void visitToken(DetailAST ast) {
final DetailAST mid = ast.findFirstToken(TokenTypes.IDENT);
final String name = mid.getText();
if ("clone".equals(name)) {
final DetailAST params = ast.findFirstToken(TokenTypes.PARAMETERS);
final boolean hasEmptyParamList =
params.findFirstToken(TokenTypes.PARAMETER_DEF) == null;
if (hasEmptyParamList) {
log(ast, MSG_KEY);
}
}
}
}