001///////////////////////////////////////////////////////////////////////////////////////////////
002// checkstyle: Checks Java source code and other text files for adherence to a set of rules.
003// Copyright (C) 2001-2024 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.TokenUtil;
031
032/**
033 * <p>
034 * Ensures that {@code when} is used instead of a single {@code if}
035 * statement inside a case block.
036 * </p>
037 * <p>
038 * Rationale: Java 21 has introduced enhancements for switch statements and expressions
039 * that allow the use of patterns in case labels. The {@code when} keyword is used to specify
040 * condition for a case label, also called as guarded case labels. This syntax is more readable
041 * and concise than the single {@code if} statement inside the pattern match block.
042 * </p>
043 * <p>
044 * See the <a href="https://docs.oracle.com/javase/specs/jls/se22/html/jls-14.html#jls-Guard">
045 * Java Language Specification</a> for more information about guarded case labels.
046 * </p>
047 * <p>
048 * See the <a href="https://docs.oracle.com/javase/specs/jls/se22/html/jls-14.html#jls-14.30">
049 * Java Language Specification</a> for more information about patterns.
050 * </p>
051 * <p>
052 * Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker}
053 * </p>
054 * <p>
055 * Violation Message Keys:
056 * </p>
057 * <ul>
058 * <li>
059 * {@code when.should.be.used}
060 * </li>
061 * </ul>
062 *
063 * @since 10.18.0
064 */
065
066@StatelessCheck
067public class WhenShouldBeUsedCheck extends AbstractCheck {
068
069    /**
070     * A key is pointing to the warning message text in "messages.properties"
071     * file.
072     */
073    public static final String MSG_KEY = "when.should.be.used";
074
075    @Override
076    public int[] getDefaultTokens() {
077        return getRequiredTokens();
078    }
079
080    @Override
081    public int[] getAcceptableTokens() {
082        return getRequiredTokens();
083    }
084
085    @Override
086    public int[] getRequiredTokens() {
087        return new int[] {TokenTypes.LITERAL_CASE};
088    }
089
090    @Override
091    public void visitToken(DetailAST ast) {
092        final boolean hasPatternLabel = hasPatternLabel(ast);
093        final DetailAST statementList = getStatementList(ast);
094        // until https://github.com/checkstyle/checkstyle/issues/15270
095        final boolean isInSwitchRule = ast.getParent().getType() == TokenTypes.SWITCH_RULE;
096
097        if (hasPatternLabel && statementList != null && isInSwitchRule) {
098            final List<DetailAST> blockStatements = getBlockStatements(statementList);
099
100            final boolean hasAcceptableStatementsOnly = blockStatements.stream()
101                    .allMatch(WhenShouldBeUsedCheck::isAcceptableStatement);
102
103            final boolean hasSingleIfWithNoElse = blockStatements.stream()
104                    .filter(WhenShouldBeUsedCheck::isSingleIfWithNoElse)
105                    .count() == 1;
106
107            if (hasAcceptableStatementsOnly && hasSingleIfWithNoElse) {
108                log(ast, MSG_KEY);
109            }
110        }
111    }
112
113    /**
114     * Get the statement list token of the case block.
115     *
116     * @param caseAST the AST node representing {@code LITERAL_CASE}
117     * @return the AST node representing {@code SLIST} of the current case
118     */
119    private static DetailAST getStatementList(DetailAST caseAST) {
120        final DetailAST caseParent = caseAST.getParent();
121        return caseParent.findFirstToken(TokenTypes.SLIST);
122    }
123
124    /**
125     * Get all statements inside the case block.
126     *
127     * @param statementList the AST node representing {@code SLIST} of the current case
128     * @return statements inside the current case block
129     */
130    private static List<DetailAST> getBlockStatements(DetailAST statementList) {
131        final List<DetailAST> blockStatements = new ArrayList<>();
132        DetailAST ast = statementList.getFirstChild();
133        while (ast != null) {
134            blockStatements.add(ast);
135            ast = ast.getNextSibling();
136        }
137        return Collections.unmodifiableList(blockStatements);
138    }
139
140    /**
141     * Check if the statement is an acceptable statement inside the case block.
142     * If these statements are the only ones in the case block, this case
143     * can be considered a violation. If at least one of the statements
144     * is not acceptable, this case can not be a violation.
145     *
146     * @param ast the AST node representing the statement
147     * @return true if the statement is an acceptable statement, false otherwise
148     */
149    private static boolean isAcceptableStatement(DetailAST ast) {
150        final int[] acceptableChildrenOfSlist = {
151            TokenTypes.LITERAL_IF,
152            TokenTypes.LITERAL_BREAK,
153            TokenTypes.EMPTY_STAT,
154            TokenTypes.RCURLY,
155        };
156        return TokenUtil.isOfType(ast, acceptableChildrenOfSlist);
157    }
158
159    /**
160     * Check if the case block has a pattern variable definition
161     * or a record pattern definition.
162     *
163     * @param caseAST the AST node representing {@code LITERAL_CASE}
164     * @return true if the case block has a pattern label, false otherwise
165     */
166    private static boolean hasPatternLabel(DetailAST caseAST) {
167        return caseAST.findFirstToken(TokenTypes.PATTERN_VARIABLE_DEF) != null
168                || caseAST.findFirstToken(TokenTypes.RECORD_PATTERN_DEF) != null
169                || caseAST.findFirstToken(TokenTypes.PATTERN_DEF) != null;
170    }
171
172    /**
173     * Check if the case block statement is a single if statement with no else branch.
174     *
175     * @param statement statement to check inside the current case block
176     * @return true if the statement is a single if statement with no else branch, false otherwise
177     */
178    private static boolean isSingleIfWithNoElse(DetailAST statement) {
179        return statement.getType() == TokenTypes.LITERAL_IF
180                && statement.findFirstToken(TokenTypes.LITERAL_ELSE) == null;
181    }
182
183}