1 /////////////////////////////////////////////////////////////////////////////////////////////// 2 // checkstyle: Checks Java source code and other text files for adherence to a set of rules. 3 // Copyright (C) 2001-2024 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 * <p> 33 * This check is almost exactly the same as the {@code NoFinalizerCheck}. 34 * </p> 35 * <p> 36 * See 37 * <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html#clone()"> 38 * Object.clone()</a> 39 * </p> 40 * <p> 41 * Rationale: The clone method relies on strange, hard to follow rules that 42 * are difficult to get right and do not work in all situations. In some cases, 43 * either a copy constructor or a static factory method can be used instead of 44 * the clone method to return copies of an object. For more information on rules 45 * for the clone method and its issues, see Effective Java: 46 * Programming Language Guide First Edition by Joshua Bloch pages 45-52. 47 * </p> 48 * <p> 49 * Below are some rules/reasons why the clone method should be avoided. 50 * </p> 51 * <ul> 52 * <li> 53 * Classes supporting the clone method should implement the Cloneable 54 * interface but the Cloneable interface does not include the clone method. 55 * As a result, it doesn't enforce the method override. 56 * </li> 57 * <li> 58 * The Cloneable interface forces the Object's clone method to work 59 * correctly. Without implementing it, the Object's clone method will 60 * throw a CloneNotSupportedException. 61 * </li> 62 * <li> 63 * Non-final classes must return the object returned from a call to 64 * super.clone(). 65 * </li> 66 * <li> 67 * Final classes can use a constructor to create a clone which is different 68 * from non-final classes. 69 * </li> 70 * <li> 71 * If a super class implements the clone method incorrectly all subclasses 72 * calling super.clone() are doomed to failure. 73 * </li> 74 * <li> 75 * If a class has references to mutable objects then those object 76 * references must be replaced with copies in the clone method 77 * after calling super.clone(). 78 * </li> 79 * <li> 80 * The clone method does not work correctly with final mutable object 81 * references because final references cannot be reassigned. 82 * </li> 83 * <li> 84 * If a super class overrides the clone method then all subclasses must 85 * provide a correct clone implementation. 86 * </li> 87 * </ul> 88 * 89 * <p>Two alternatives to the clone method, in some cases, is a copy constructor 90 * or a static factory method to return copies of an object. Both of these 91 * approaches are simpler and do not conflict with final fields. They do not 92 * force the calling client to handle a CloneNotSupportedException. They also 93 * are typed therefore no casting is necessary. Finally, they are more 94 * flexible since they can take interface types rather than concrete classes. 95 * </p> 96 * <p>Sometimes a copy constructor or static factory is not an acceptable 97 * alternative to the clone method. The example below highlights the 98 * limitation of a copy constructor (or static factory). Assume 99 * Square is a subclass for Shape. 100 * </p> 101 * <pre> 102 * Shape s1 = new Square(); 103 * System.out.println(s1 instanceof Square); //true 104 * </pre> 105 * <p> 106 * ...assume at this point the code knows nothing of s1 being a Square 107 * that's the beauty of polymorphism but the code wants to copy 108 * the Square which is declared as a Shape, its super type... 109 * </p> 110 * <pre> 111 * Shape s2 = new Shape(s1); //using the copy constructor 112 * System.out.println(s2 instanceof Square); //false 113 * </pre> 114 * <p> 115 * The working solution (without knowing about all subclasses and doing many 116 * casts) is to do the following (assuming correct clone implementation). 117 * </p> 118 * <pre> 119 * Shape s2 = s1.clone(); 120 * System.out.println(s2 instanceof Square); //true 121 * </pre> 122 * <p> 123 * Just keep in mind if this type of polymorphic cloning is required 124 * then a properly implemented clone method may be the best choice. 125 * </p> 126 * <p>Much of this information was taken from Effective Java: 127 * Programming Language Guide First Edition by Joshua Bloch 128 * pages 45-52. Give Bloch credit for writing an excellent book. 129 * </p> 130 * <p> 131 * Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker} 132 * </p> 133 * <p> 134 * Violation Message Keys: 135 * </p> 136 * <ul> 137 * <li> 138 * {@code avoid.clone.method} 139 * </li> 140 * </ul> 141 * 142 * @since 5.0 143 */ 144 @StatelessCheck 145 public class NoCloneCheck extends AbstractCheck { 146 147 /** 148 * A key is pointing to the warning message text in "messages.properties" 149 * file. 150 */ 151 public static final String MSG_KEY = "avoid.clone.method"; 152 153 @Override 154 public int[] getDefaultTokens() { 155 return getRequiredTokens(); 156 } 157 158 @Override 159 public int[] getAcceptableTokens() { 160 return getRequiredTokens(); 161 } 162 163 @Override 164 public int[] getRequiredTokens() { 165 return new int[] {TokenTypes.METHOD_DEF}; 166 } 167 168 @Override 169 public void visitToken(DetailAST ast) { 170 final DetailAST mid = ast.findFirstToken(TokenTypes.IDENT); 171 final String name = mid.getText(); 172 173 if ("clone".equals(name)) { 174 final DetailAST params = ast.findFirstToken(TokenTypes.PARAMETERS); 175 final boolean hasEmptyParamList = 176 params.findFirstToken(TokenTypes.PARAMETER_DEF) == null; 177 178 if (hasEmptyParamList) { 179 log(ast, MSG_KEY); 180 } 181 } 182 } 183 184 }