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.ArrayList;
023import java.util.Collections;
024import java.util.List;
025
026import com.puppycrawl.tools.checkstyle.StatelessCheck;
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.CheckUtil;
031
032/**
033 * <div>
034 * Ensures that the enhanced switch (using {@code ->} for case labels) is used
035 * instead of the traditional switch (using {@code :} for case labels) where possible.
036 * </div>
037 *
038 * <p>
039 * Rationale: Java 14 has introduced enhancements for switch statements and expressions
040 * that disallow fall-through behavior. The enhanced switch syntax using {@code ->}
041 * for case labels typically leads to more concise and readable code, reducing the likelihood
042 * of errors associated with fall-through cases.
043 * </p>
044 *
045 * <p>
046 * See the <a href="https://docs.oracle.com/javase/specs/jls/se22/html/jls-14.html#jls-14.11.1-200">
047 * Java Language Specification</a> for more information about {@code ->} case labels, also known as
048 * "switch rules".
049 * </p>
050 *
051 * @since 13.3.0
052 */
053@StatelessCheck
054public class UseEnhancedSwitchCheck extends AbstractCheck {
055
056    /**
057     * A key is pointing to the warning message text in "messages.properties" file.
058     */
059    public static final String MSG_KEY = "use.enhanced.switch";
060
061    /**
062     * Creates a new {@code UseEnhancedSwitchCheck} instance.
063     */
064    public UseEnhancedSwitchCheck() {
065        // no code by default
066    }
067
068    @Override
069    public int[] getDefaultTokens() {
070        return getRequiredTokens();
071    }
072
073    @Override
074    public int[] getAcceptableTokens() {
075        return getRequiredTokens();
076    }
077
078    @Override
079    public int[] getRequiredTokens() {
080        return new int[] {TokenTypes.LITERAL_SWITCH};
081    }
082
083    @Override
084    public void visitToken(DetailAST ast) {
085        final List<DetailAST> caseGroups = getCaseGroups(ast);
086        if (!caseGroups.isEmpty() && allCaseGroupsTerminate(caseGroups)) {
087            log(ast, MSG_KEY);
088        }
089    }
090
091    /**
092     * Check if all case groups terminate (i.e. do not fall through), except the last one which
093     * terminates regardless of its content.
094     *
095     * @param caseGroups the list of case groups to check
096     * @return {@code true} if all case groups terminate, {@code false} otherwise
097     */
098    private static boolean allCaseGroupsTerminate(List<DetailAST> caseGroups) {
099        boolean allCaseGroupsTerminate = true;
100        for (int index = 0; index < caseGroups.size() - 1; index++) {
101            final DetailAST statementList = caseGroups.get(index).findFirstToken(TokenTypes.SLIST);
102
103            if (!CheckUtil.isTerminated(statementList)) {
104                allCaseGroupsTerminate = false;
105                break;
106            }
107        }
108        return allCaseGroupsTerminate;
109    }
110
111    /**
112     * Get all case groups from the switch.
113     *
114     * @param switchAst the AST node representing a {@code LITERAL_SWITCH}
115     * @return all {@code CASE_GROUP} nodes within the switch
116     */
117    private static List<DetailAST> getCaseGroups(DetailAST switchAst) {
118        final List<DetailAST> caseGroups = new ArrayList<>();
119        DetailAST ast = switchAst.getFirstChild();
120        while (ast != null) {
121            if (ast.getType() == TokenTypes.CASE_GROUP) {
122                caseGroups.add(ast);
123            }
124            ast = ast.getNextSibling();
125        }
126        return Collections.unmodifiableList(caseGroups);
127    }
128
129}