Source code

001///////////////////////////////////////////////////////////////////////////////////////////////
002// checkstyle: Checks Java source code and other text files for adherence to a set of rules.
003// Copyright (C) 2001-2026 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.ArrayDeque;
023import java.util.Deque;
024import java.util.Optional;
025
026import com.puppycrawl.tools.checkstyle.FileStatefulCheck;
027import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
028import com.puppycrawl.tools.checkstyle.api.DetailAST;
029import com.puppycrawl.tools.checkstyle.api.TokenTypes;
030import com.puppycrawl.tools.checkstyle.utils.TokenUtil;
031
032/**
033 * <div>
034 * Ensures that catch parameters that are not used are declared as an unnamed variable.
035 * </div>
036 *
037 * <p>
038 * Rationale:
039 * </p>
040 * <ul>
041 *     <li>
042 *         Improves code readability by clearly indicating which parameters are unused.
043 *     </li>
044 *     <li>
045 *         Follows Java conventions for denoting unused parameters with an underscore ({@code _}).
046 *     </li>
047 * </ul>
048 *
049 * <p>
050 * See the <a href="https://docs.oracle.com/en/java/javase/21/docs/specs/unnamed-jls.html">
051 * Java Language Specification</a> for more information about unnamed variables.
052 * </p>
053 *
054 * <p>
055 * <b>Attention</b>: This check should be activated only on source code
056 * that is compiled by jdk21 or higher;
057 * unnamed catch parameters came out as the first preview in Java 21.
058 * </p>
059 *
060 * @since 10.18.0
061 */
062
063@FileStatefulCheck
064public class UnusedCatchParameterShouldBeUnnamedCheck extends AbstractCheck {
065
066    /**
067     * A key is pointing to the warning message text in "messages.properties"
068     * file.
069     */
070    public static final String MSG_UNUSED_CATCH_PARAMETER = "unused.catch.parameter";
071
072    /**
073     * Invalid parents of the catch parameter identifier.
074     */
075    private static final int[] INVALID_CATCH_PARAM_IDENT_PARENTS = {
076        TokenTypes.DOT,
077        TokenTypes.LITERAL_NEW,
078        TokenTypes.METHOD_CALL,
079        TokenTypes.TYPE,
080    };
081
082    /**
083     * Keeps track of the catch parameters in a block.
084     */
085    private final Deque<CatchParameterDetails> catchParameters = new ArrayDeque<>();
086
087    /**
088     * Creates a new {@code UnusedCatchParameterShouldBeUnnamedCheck} instance.
089     */
090    public UnusedCatchParameterShouldBeUnnamedCheck() {
091        // no code by default
092    }
093
094    @Override
095    public int[] getDefaultTokens() {
096        return getRequiredTokens();
097    }
098
099    @Override
100    public int[] getAcceptableTokens() {
101        return getRequiredTokens();
102    }
103
104    @Override
105    public int[] getRequiredTokens() {
106        return new int[] {
107            TokenTypes.LITERAL_CATCH,
108            TokenTypes.IDENT,
109        };
110    }
111
112    @Override
113    public void beginTree(DetailAST rootAST) {
114        catchParameters.clear();
115    }
116
117    @Override
118    public void visitToken(DetailAST ast) {
119        if (ast.getType() == TokenTypes.LITERAL_CATCH) {
120            final CatchParameterDetails catchParameter = new CatchParameterDetails(ast);
121            catchParameters.push(catchParameter);
122        }
123        else if (isCatchParameterIdentifierCandidate(ast) && !isLeftHandOfAssignment(ast)) {
124            // we do not count reassignment as usage
125            catchParameters.stream()
126                    .filter(parameter -> parameter.getName().equals(ast.getText()))
127                    .findFirst()
128                    .ifPresent(CatchParameterDetails::registerAsUsed);
129        }
130    }
131
132    @Override
133    public void leaveToken(DetailAST ast) {
134        if (ast.getType() == TokenTypes.LITERAL_CATCH) {
135            final Optional<CatchParameterDetails> unusedCatchParameter =
136                    Optional.ofNullable(catchParameters.peek())
137                            .filter(parameter -> !parameter.isUsed())
138                            .filter(parameter -> !"_".equals(parameter.getName()));
139
140            unusedCatchParameter.ifPresent(parameter -> {
141                log(parameter.getParameterDefinition(),
142                        MSG_UNUSED_CATCH_PARAMETER,
143                        parameter.getName());
144            });
145            catchParameters.pop();
146        }
147    }
148
149    /**
150     * Visit ast of type {@link TokenTypes#IDENT}
151     * and check if it is a candidate for a catch parameter identifier.
152     *
153     * @param identifierAst token representing {@link TokenTypes#IDENT}
154     * @return true if the given {@link TokenTypes#IDENT} could be a catch parameter identifier
155     */
156    private static boolean isCatchParameterIdentifierCandidate(DetailAST identifierAst) {
157        // we should ignore the ident if it is in the exception declaration
158        return identifierAst.getParent().getParent().getType() != TokenTypes.LITERAL_CATCH
159            && (!TokenUtil.isOfType(identifierAst.getParent(), INVALID_CATCH_PARAM_IDENT_PARENTS)
160                 || isMethodInvocation(identifierAst));
161    }
162
163    /**
164     * Check if the given {@link TokenTypes#IDENT} is a child of a dot operator
165     * and is a candidate for catch parameter.
166     *
167     * @param identAst token representing {@link TokenTypes#IDENT}
168     * @return true if the given {@link TokenTypes#IDENT} is a child of a dot operator
169     *     and a candidate for catch parameter.
170     */
171    private static boolean isMethodInvocation(DetailAST identAst) {
172        final DetailAST parent = identAst.getParent();
173        return parent.getType() == TokenTypes.DOT
174                && identAst.equals(parent.getFirstChild());
175    }
176
177    /**
178     * Check if the given {@link TokenTypes#IDENT} is a left hand side value.
179     *
180     * @param identAst token representing {@link TokenTypes#IDENT}
181     * @return true if the given {@link TokenTypes#IDENT} is a left hand side value.
182     */
183    private static boolean isLeftHandOfAssignment(DetailAST identAst) {
184        final DetailAST parent = identAst.getParent();
185        return parent.getType() == TokenTypes.ASSIGN
186                && !identAst.equals(parent.getLastChild());
187    }
188
189    /**
190     * Maintains information about the catch parameter.
191     */
192    private static final class CatchParameterDetails {
193
194        /**
195         * The name of the catch parameter.
196         */
197        private final String name;
198
199        /**
200         * Ast of type {@link TokenTypes#PARAMETER_DEF} to use it when logging.
201         */
202        private final DetailAST parameterDefinition;
203
204        /**
205         * Is the variable used.
206         */
207        private boolean used;
208
209        /**
210         * Create a new catch parameter instance.
211         *
212         * @param enclosingCatchClause ast of type {@link TokenTypes#LITERAL_CATCH}
213         */
214        private CatchParameterDetails(DetailAST enclosingCatchClause) {
215            parameterDefinition =
216                    enclosingCatchClause.findFirstToken(TokenTypes.PARAMETER_DEF);
217            name = parameterDefinition.findFirstToken(TokenTypes.IDENT).getText();
218        }
219
220        /**
221         * Register the catch parameter as used.
222         */
223        private void registerAsUsed() {
224            used = true;
225        }
226
227        /**
228         * Get the name of the catch parameter.
229         *
230         * @return the name of the catch parameter
231         */
232        private String getName() {
233            return name;
234        }
235
236        /**
237         * Check if the catch parameter is used.
238         *
239         * @return true if the catch parameter is used
240         */
241        private boolean isUsed() {
242            return used;
243        }
244
245        /**
246         * Get the parameter definition token of the catch parameter
247         * represented by ast of type {@link TokenTypes#PARAMETER_DEF}.
248         *
249         * @return the ast of type {@link TokenTypes#PARAMETER_DEF}
250         */
251        private DetailAST getParameterDefinition() {
252            return parameterDefinition;
253        }
254    }
255
256}