1 ///////////////////////////////////////////////////////////////////////////////////////////////
2 // checkstyle: Checks Java source code and other text files for adherence to a set of rules.
3 // Copyright (C) 2001-2026 the original author or authors.
4 //
5 // This library is free software; you can redistribute it and/or
6 // modify it under the terms of the GNU Lesser General Public
7 // License as published by the Free Software Foundation; either
8 // version 2.1 of the License, or (at your option) any later version.
9 //
10 // This library is distributed in the hope that it will be useful,
11 // but WITHOUT ANY WARRANTY; without even the implied warranty of
12 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 // Lesser General Public License for more details.
14 //
15 // You should have received a copy of the GNU Lesser General Public
16 // License along with this library; if not, write to the Free Software
17 // Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
18 ///////////////////////////////////////////////////////////////////////////////////////////////
19
20 package com.puppycrawl.tools.checkstyle.checks.coding;
21
22 import com.puppycrawl.tools.checkstyle.StatelessCheck;
23 import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
24 import com.puppycrawl.tools.checkstyle.api.DetailAST;
25 import com.puppycrawl.tools.checkstyle.api.TokenTypes;
26
27 /**
28 * <div>
29 * Checks that string literals are not used with {@code ==} or {@code !=}.
30 * Since {@code ==} will compare the object references, not the actual value of the strings,
31 * {@code String.equals()} should be used.
32 * More information can be found
33 * <a href="https://stackoverflow.com/questions/513832/how-do-i-compare-strings-in-java/">
34 * in this article</a>.
35 * </div>
36 *
37 * <p>
38 * Rationale: Novice Java programmers often use code like:
39 * </p>
40 * {@snippet lang="text" :
41 * if (x == "something")
42 * }
43 *
44 * <p>
45 * when they mean
46 * </p>
47 * {@snippet lang="text" :
48 * if ("something".equals(x))
49 * }
50 *
51 * @since 3.2
52 */
53 @StatelessCheck
54 public class StringLiteralEqualityCheck extends AbstractCheck {
55
56 /**
57 * A key is pointing to the warning message text in "messages.properties"
58 * file.
59 */
60 public static final String MSG_KEY = "string.literal.equality";
61
62 /**
63 * Creates a new {@code StringLiteralEqualityCheck} instance.
64 */
65 public StringLiteralEqualityCheck() {
66 // no code by default
67 }
68
69 @Override
70 public int[] getDefaultTokens() {
71 return getRequiredTokens();
72 }
73
74 @Override
75 public int[] getAcceptableTokens() {
76 return getRequiredTokens();
77 }
78
79 @Override
80 public int[] getRequiredTokens() {
81 return new int[] {TokenTypes.EQUAL, TokenTypes.NOT_EQUAL};
82 }
83
84 @Override
85 public void visitToken(DetailAST ast) {
86 final boolean hasStringLiteralChild = hasStringLiteralChild(ast);
87
88 if (hasStringLiteralChild) {
89 log(ast, MSG_KEY, ast.getText());
90 }
91 }
92
93 /**
94 * Checks whether string literal or text block literals are concatenated.
95 *
96 * @param ast ast
97 * @return {@code true} if string literal or text block literals are concatenated
98 */
99 private static boolean hasStringLiteralChild(DetailAST ast) {
100 DetailAST currentAst = ast;
101 boolean result = false;
102 while (currentAst != null) {
103 if (currentAst.findFirstToken(TokenTypes.STRING_LITERAL) == null
104 && currentAst.findFirstToken(TokenTypes.TEXT_BLOCK_LITERAL_BEGIN) == null) {
105 currentAst = currentAst.findFirstToken(TokenTypes.PLUS);
106 }
107 else {
108 result = true;
109 break;
110 }
111 }
112 return result;
113 }
114
115 }