Source code

001///////////////////////////////////////////////////////////////////////////////////////////////
002// checkstyle: Checks Java source code and other text files for adherence to a set of rules.
003// Copyright (C) 2001-2025 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    @Override
088    public int[] getDefaultTokens() {
089        return getRequiredTokens();
090    }
091
092    @Override
093    public int[] getAcceptableTokens() {
094        return getRequiredTokens();
095    }
096
097    @Override
098    public int[] getRequiredTokens() {
099        return new int[] {
100            TokenTypes.LITERAL_CATCH,
101            TokenTypes.IDENT,
102        };
103    }
104
105    @Override
106    public void beginTree(DetailAST rootAST) {
107        catchParameters.clear();
108    }
109
110    @Override
111    public void visitToken(DetailAST ast) {
112        if (ast.getType() == TokenTypes.LITERAL_CATCH) {
113            final CatchParameterDetails catchParameter = new CatchParameterDetails(ast);
114            catchParameters.push(catchParameter);
115        }
116        else if (isCatchParameterIdentifierCandidate(ast) && !isLeftHandOfAssignment(ast)) {
117            // we do not count reassignment as usage
118            catchParameters.stream()
119                    .filter(parameter -> parameter.getName().equals(ast.getText()))
120                    .findFirst()
121                    .ifPresent(CatchParameterDetails::registerAsUsed);
122        }
123    }
124
125    @Override
126    public void leaveToken(DetailAST ast) {
127        if (ast.getType() == TokenTypes.LITERAL_CATCH) {
128            final Optional<CatchParameterDetails> unusedCatchParameter =
129                    Optional.ofNullable(catchParameters.peek())
130                            .filter(parameter -> !parameter.isUsed())
131                            .filter(parameter -> !"_".equals(parameter.getName()));
132
133            unusedCatchParameter.ifPresent(parameter -> {
134                log(parameter.getParameterDefinition(),
135                        MSG_UNUSED_CATCH_PARAMETER,
136                        parameter.getName());
137            });
138            catchParameters.pop();
139        }
140    }
141
142    /**
143     * Visit ast of type {@link TokenTypes#IDENT}
144     * and check if it is a candidate for a catch parameter identifier.
145     *
146     * @param identifierAst token representing {@link TokenTypes#IDENT}
147     * @return true if the given {@link TokenTypes#IDENT} could be a catch parameter identifier
148     */
149    private static boolean isCatchParameterIdentifierCandidate(DetailAST identifierAst) {
150        // we should ignore the ident if it is in the exception declaration
151        return identifierAst.getParent().getParent().getType() != TokenTypes.LITERAL_CATCH
152            && (!TokenUtil.isOfType(identifierAst.getParent(), INVALID_CATCH_PARAM_IDENT_PARENTS)
153                 || isMethodInvocation(identifierAst));
154    }
155
156    /**
157     * Check if the given {@link TokenTypes#IDENT} is a child of a dot operator
158     * and is a candidate for catch parameter.
159     *
160     * @param identAst token representing {@link TokenTypes#IDENT}
161     * @return true if the given {@link TokenTypes#IDENT} is a child of a dot operator
162     *     and a candidate for catch parameter.
163     */
164    private static boolean isMethodInvocation(DetailAST identAst) {
165        final DetailAST parent = identAst.getParent();
166        return parent.getType() == TokenTypes.DOT
167                && identAst.equals(parent.getFirstChild());
168    }
169
170    /**
171     * Check if the given {@link TokenTypes#IDENT} is a left hand side value.
172     *
173     * @param identAst token representing {@link TokenTypes#IDENT}
174     * @return true if the given {@link TokenTypes#IDENT} is a left hand side value.
175     */
176    private static boolean isLeftHandOfAssignment(DetailAST identAst) {
177        final DetailAST parent = identAst.getParent();
178        return parent.getType() == TokenTypes.ASSIGN
179                && !identAst.equals(parent.getLastChild());
180    }
181
182    /**
183     * Maintains information about the catch parameter.
184     */
185    private static final class CatchParameterDetails {
186
187        /**
188         * The name of the catch parameter.
189         */
190        private final String name;
191
192        /**
193         * Ast of type {@link TokenTypes#PARAMETER_DEF} to use it when logging.
194         */
195        private final DetailAST parameterDefinition;
196
197        /**
198         * Is the variable used.
199         */
200        private boolean used;
201
202        /**
203         * Create a new catch parameter instance.
204         *
205         * @param enclosingCatchClause ast of type {@link TokenTypes#LITERAL_CATCH}
206         */
207        private CatchParameterDetails(DetailAST enclosingCatchClause) {
208            parameterDefinition =
209                    enclosingCatchClause.findFirstToken(TokenTypes.PARAMETER_DEF);
210            name = parameterDefinition.findFirstToken(TokenTypes.IDENT).getText();
211        }
212
213        /**
214         * Register the catch parameter as used.
215         */
216        private void registerAsUsed() {
217            used = true;
218        }
219
220        /**
221         * Get the name of the catch parameter.
222         *
223         * @return the name of the catch parameter
224         */
225        private String getName() {
226            return name;
227        }
228
229        /**
230         * Check if the catch parameter is used.
231         *
232         * @return true if the catch parameter is used
233         */
234        private boolean isUsed() {
235            return used;
236        }
237
238        /**
239         * Get the parameter definition token of the catch parameter
240         * represented by ast of type {@link TokenTypes#PARAMETER_DEF}.
241         *
242         * @return the ast of type {@link TokenTypes#PARAMETER_DEF}
243         */
244        private DetailAST getParameterDefinition() {
245            return parameterDefinition;
246        }
247    }
248}