Coverage Report - com.puppycrawl.tools.checkstyle.checks.coding.NoCloneCheck
 
Classes in this File Line Coverage Branch Coverage Complexity
NoCloneCheck
100%
13/13
100%
6/6
1.5
 
 1  
 ////////////////////////////////////////////////////////////////////////////////
 2  
 // checkstyle: Checks Java source code for adherence to a set of rules.
 3  
 // Copyright (C) 2001-2017 the original author or authors.
 4  
 //
 5  
 // This library is free software; you can redistribute it and/or
 6  
 // modify it under the terms of the GNU Lesser General Public
 7  
 // License as published by the Free Software Foundation; either
 8  
 // version 2.1 of the License, or (at your option) any later version.
 9  
 //
 10  
 // This library is distributed in the hope that it will be useful,
 11  
 // but WITHOUT ANY WARRANTY; without even the implied warranty of
 12  
 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 13  
 // Lesser General Public License for more details.
 14  
 //
 15  
 // You should have received a copy of the GNU Lesser General Public
 16  
 // License along with this library; if not, write to the Free Software
 17  
 // Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 18  
 ////////////////////////////////////////////////////////////////////////////////
 19  
 
 20  
 package com.puppycrawl.tools.checkstyle.checks.coding;
 21  
 
 22  
 import com.puppycrawl.tools.checkstyle.StatelessCheck;
 23  
 import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
 24  
 import com.puppycrawl.tools.checkstyle.api.DetailAST;
 25  
 import com.puppycrawl.tools.checkstyle.api.TokenTypes;
 26  
 
 27  
 /**
 28  
  * <p>
 29  
  * Checks that the clone method is not overridden from the
 30  
  * Object class.
 31  
  * </p>
 32  
  *
 33  
  * <p>Rationale: The clone method relies on strange/hard to follow rules that
 34  
  * do not work it all situations.  Consequently, it is difficult to
 35  
  * override correctly.  Below are some of the rules/reasons why the clone
 36  
  * method should be avoided.
 37  
  *
 38  
  * <ul>
 39  
  * <li>
 40  
  * Classes supporting the clone method should implement the Cloneable
 41  
  * interface but the Cloneable interface does not include the clone method.
 42  
  * As a result, it doesn't enforce the method override.
 43  
  * </li>
 44  
  * <li>
 45  
  * The Cloneable interface forces the Object's clone method to work
 46  
  * correctly. Without implementing it, the Object's clone method will
 47  
  * throw a CloneNotSupportedException.
 48  
  * </li>
 49  
  * <li>
 50  
  * Non-final classes must return the object returned from a call to
 51  
  * super.clone().
 52  
  * </li>
 53  
  * <li>
 54  
  * Final classes can use a constructor to create a clone which is different
 55  
  * from non-final classes.
 56  
  * </li>
 57  
  * <li>
 58  
  * If a super class implements the clone method incorrectly all subclasses
 59  
  * calling super.clone() are doomed to failure.
 60  
  * </li>
 61  
  * <li>
 62  
  * If a class has references to mutable objects then those object
 63  
  * references must be replaced with copies in the clone method
 64  
  * after calling super.clone().
 65  
  * </li>
 66  
  * <li>
 67  
  * The clone method does not work correctly with final mutable object
 68  
  * references because final references cannot be reassigned.
 69  
  * </li>
 70  
  * <li>
 71  
  * If a super class overrides the clone method then all subclasses must
 72  
  * provide a correct clone implementation.
 73  
  * </li>
 74  
  * </ul>
 75  
  *
 76  
  * <p>Two alternatives to the clone method, in some cases, is a copy constructor
 77  
  * or a static factory method to return copies of an object. Both of these
 78  
  * approaches are simpler and do not conflict with final fields. They do not
 79  
  * force the calling client to handle a CloneNotSupportedException.  They also
 80  
  * are typed therefore no casting is necessary. Finally, they are more
 81  
  * flexible since they can take interface types rather than concrete classes.
 82  
  *
 83  
  * <p>Sometimes a copy constructor or static factory is not an acceptable
 84  
  * alternative to the clone method.  The example below highlights the
 85  
  * limitation of a copy constructor (or static factory). Assume
 86  
  * Square is a subclass for Shape.
 87  
  *
 88  
  * <pre>
 89  
  * Shape s1 = new Square();
 90  
  * System.out.println(s1 instanceof Square); //true
 91  
  * </pre>
 92  
  * ...assume at this point the code knows nothing of s1 being a Square
 93  
  *    that's the beauty of polymorphism but the code wants to copy
 94  
  *    the Square which is declared as a Shape, its super type...
 95  
  *
 96  
  * <pre>
 97  
  * Shape s2 = new Shape(s1); //using the copy constructor
 98  
  * System.out.println(s2 instanceof Square); //false
 99  
  * </pre>
 100  
  * The working solution (without knowing about all subclasses and doing many
 101  
  * casts) is to do the following (assuming correct clone implementation).
 102  
  *
 103  
  * <pre>
 104  
  * Shape s2 = s1.clone();
 105  
  * System.out.println(s2 instanceof Square); //true
 106  
  * </pre>
 107  
  * Just keep in mind if this type of polymorphic cloning is required
 108  
  * then a properly implemented clone method may be the best choice.
 109  
  *
 110  
  * <p>Much of this information was taken from Effective Java:
 111  
  * Programming Language Guide First Edition by Joshua Bloch
 112  
  * pages 45-52.  Give Bloch credit for writing an excellent book.
 113  
  * </p>
 114  
  *
 115  
  * <p>This check is almost exactly the same as the {@link NoFinalizerCheck}
 116  
  *
 117  
  * @author Travis Schneeberger
 118  
  * @see Object#clone()
 119  
  */
 120  
 @StatelessCheck
 121  9
 public class NoCloneCheck extends AbstractCheck {
 122  
 
 123  
     /**
 124  
      * A key is pointing to the warning message text in "messages.properties"
 125  
      * file.
 126  
      */
 127  
     public static final String MSG_KEY = "avoid.clone.method";
 128  
 
 129  
     @Override
 130  
     public int[] getDefaultTokens() {
 131  11
         return getRequiredTokens();
 132  
     }
 133  
 
 134  
     @Override
 135  
     public int[] getAcceptableTokens() {
 136  5
         return getRequiredTokens();
 137  
     }
 138  
 
 139  
     @Override
 140  
     public int[] getRequiredTokens() {
 141  27
         return new int[] {TokenTypes.METHOD_DEF};
 142  
     }
 143  
 
 144  
     @Override
 145  
     public void visitToken(DetailAST aAST) {
 146  11
         final DetailAST mid = aAST.findFirstToken(TokenTypes.IDENT);
 147  11
         final String name = mid.getText();
 148  
 
 149  11
         if ("clone".equals(name)) {
 150  
 
 151  9
             final DetailAST params = aAST.findFirstToken(TokenTypes.PARAMETERS);
 152  9
             final boolean hasEmptyParamList =
 153  9
                 params.findFirstToken(TokenTypes.PARAMETER_DEF) == null;
 154  
 
 155  9
             if (hasEmptyParamList) {
 156  7
                 log(aAST.getLineNo(), MSG_KEY);
 157  
             }
 158  
         }
 159  11
     }
 160  
 }