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"
058     * file.
059     */
060    public static final String MSG_KEY = "use.enhanced.switch";
061
062    /**
063     * Creates a new {@code UseEnhancedSwitchCheck} instance.
064     */
065    public UseEnhancedSwitchCheck() {
066        // no code by default
067    }
068
069    @Override
070    public int[] getDefaultTokens() {
071        return getRequiredTokens();
072    }
073
074    @Override
075    public int[] getAcceptableTokens() {
076        return getRequiredTokens();
077    }
078
079    @Override
080    public int[] getRequiredTokens() {
081        return new int[] {TokenTypes.LITERAL_SWITCH};
082    }
083
084    @Override
085    public void visitToken(DetailAST ast) {
086        final List<DetailAST> caseGroups = getCaseGroups(ast);
087        if (!caseGroups.isEmpty() && allCaseGroupsTerminate(caseGroups)) {
088            log(ast, MSG_KEY);
089        }
090    }
091
092    /**
093     * Check if all case groups terminate (i.e. do not fall through), except the last one which
094     * terminates regardless of its content.
095     *
096     * @param caseGroups the list of case groups to check
097     * @return {@code true} if all case groups terminate, {@code false} otherwise
098     */
099    private static boolean allCaseGroupsTerminate(List<DetailAST> caseGroups) {
100        boolean allCaseGroupsTerminate = true;
101        for (int index = 0; index < caseGroups.size() - 1; index++) {
102            final DetailAST statementList = caseGroups.get(index).findFirstToken(TokenTypes.SLIST);
103
104            if (!CheckUtil.isTerminated(statementList)) {
105                allCaseGroupsTerminate = false;
106                break;
107            }
108        }
109        return allCaseGroupsTerminate;
110    }
111
112    /**
113     * Get all case groups from the switch.
114     *
115     * @param switchAst the AST node representing a {@code LITERAL_SWITCH}
116     * @return all {@code CASE_GROUP} nodes within the switch
117     */
118    private static List<DetailAST> getCaseGroups(DetailAST switchAst) {
119        final List<DetailAST> caseGroups = new ArrayList<>();
120        DetailAST ast = switchAst.getFirstChild();
121        while (ast != null) {
122            if (ast.getType() == TokenTypes.CASE_GROUP) {
123                caseGroups.add(ast);
124            }
125            ast = ast.getNextSibling();
126        }
127        return Collections.unmodifiableList(caseGroups);
128    }
129
130}