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 com.puppycrawl.tools.checkstyle.StatelessCheck;
023import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
024import com.puppycrawl.tools.checkstyle.api.DetailAST;
025import com.puppycrawl.tools.checkstyle.api.TokenTypes;
026
027/**
028 * <div>
029 * Checks that string literals are not used with {@code ==} or {@code !=}.
030 * Since {@code ==} will compare the object references, not the actual value of the strings,
031 * {@code String.equals()} should be used.
032 * More information can be found
033 * <a href="https://stackoverflow.com/questions/513832/how-do-i-compare-strings-in-java/">
034 * in this article</a>.
035 * </div>
036 *
037 * <p>
038 * Rationale: Novice Java programmers often use code like:
039 * </p>
040 * {@snippet lang="text" :
041 * if (x == "something")
042 * }
043 *
044 * <p>
045 * when they mean
046 * </p>
047 * {@snippet lang="text" :
048 * if ("something".equals(x))
049 * }
050 *
051 * @since 3.2
052 */
053@StatelessCheck
054public class StringLiteralEqualityCheck 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 = "string.literal.equality";
060
061    /**
062     * Creates a new {@code StringLiteralEqualityCheck} instance.
063     */
064    public StringLiteralEqualityCheck() {
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.EQUAL, TokenTypes.NOT_EQUAL};
081    }
082
083    @Override
084    public void visitToken(DetailAST ast) {
085        final boolean hasStringLiteralChild = hasStringLiteralChild(ast);
086
087        if (hasStringLiteralChild) {
088            log(ast, MSG_KEY, ast.getText());
089        }
090    }
091
092    /**
093     * Checks whether string literal or text block literals are concatenated.
094     *
095     * @param ast ast
096     * @return {@code true} if string literal or text block literals are concatenated
097     */
098    private static boolean hasStringLiteralChild(DetailAST ast) {
099        DetailAST currentAst = ast;
100        boolean result = false;
101        while (currentAst != null) {
102            if (currentAst.findFirstToken(TokenTypes.STRING_LITERAL) == null
103                    && currentAst.findFirstToken(TokenTypes.TEXT_BLOCK_LITERAL_BEGIN) == null) {
104                currentAst = currentAst.findFirstToken(TokenTypes.PLUS);
105            }
106            else {
107                result = true;
108                break;
109            }
110        }
111        return result;
112    }
113
114}