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 * <div class="wrapper"><pre class="prettyprint"><code class="language-java">
041 * if (x == "something")
042 * </code></pre></div>
043 *
044 * <p>
045 * when they mean
046 * </p>
047 * <div class="wrapper"><pre class="prettyprint"><code class="language-java">
048 * if ("something".equals(x))
049 * </code></pre></div>
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"
058     * file.
059     */
060    public static final String MSG_KEY = "string.literal.equality";
061
062    @Override
063    public int[] getDefaultTokens() {
064        return getRequiredTokens();
065    }
066
067    @Override
068    public int[] getAcceptableTokens() {
069        return getRequiredTokens();
070    }
071
072    @Override
073    public int[] getRequiredTokens() {
074        return new int[] {TokenTypes.EQUAL, TokenTypes.NOT_EQUAL};
075    }
076
077    @Override
078    public void visitToken(DetailAST ast) {
079        final boolean hasStringLiteralChild = hasStringLiteralChild(ast);
080
081        if (hasStringLiteralChild) {
082            log(ast, MSG_KEY, ast.getText());
083        }
084    }
085
086    /**
087     * Checks whether string literal or text block literals are concatenated.
088     *
089     * @param ast ast
090     * @return {@code true} if string literal or text block literals are concatenated
091     */
092    private static boolean hasStringLiteralChild(DetailAST ast) {
093        DetailAST currentAst = ast;
094        boolean result = false;
095        while (currentAst != null) {
096            if (currentAst.findFirstToken(TokenTypes.STRING_LITERAL) == null
097                    && currentAst.findFirstToken(TokenTypes.TEXT_BLOCK_LITERAL_BEGIN) == null) {
098                currentAst = currentAst.findFirstToken(TokenTypes.PLUS);
099            }
100            else {
101                result = true;
102                break;
103            }
104        }
105        return result;
106    }
107
108}