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 java.util.BitSet;
23
24 import com.puppycrawl.tools.checkstyle.StatelessCheck;
25 import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
26 import com.puppycrawl.tools.checkstyle.api.DetailAST;
27 import com.puppycrawl.tools.checkstyle.api.TokenTypes;
28 import com.puppycrawl.tools.checkstyle.utils.CommonUtil;
29 import com.puppycrawl.tools.checkstyle.utils.TokenUtil;
30
31 /**
32 * <div>
33 * Checks for assignments in subexpressions, such as in
34 * {@code String s = Integer.toString(i = 2);}.
35 * </div>
36 *
37 * <p>
38 * Rationale: Except for the loop idioms,
39 * all assignments should occur in their own top-level statement to increase readability.
40 * With inner assignments like the one given above, it is difficult to see all places
41 * where a variable is set.
42 * </p>
43 *
44 * <p>
45 * Note: Check allows usage of the popular assignments in loops:
46 * </p>
47 * {@snippet :
48 * String line;
49 * while ((line = bufferedReader.readLine()) != null) { // OK
50 * // process the line
51 * }
52 *
53 * for (;(line = bufferedReader.readLine()) != null;) { // OK
54 * // process the line
55 * }
56 *
57 * do {
58 * // process the line
59 * }
60 * while ((line = bufferedReader.readLine()) != null); // OK
61 * }
62 *
63 * <p>
64 * Assignment inside a condition is not a problem here, as the assignment is surrounded
65 * by an extra pair of parentheses. The comparison is {@code != null} and there is no chance that
66 * intention was to write {@code line == reader.readLine()}.
67 * </p>
68 *
69 * @since 3.0
70 */
71 @StatelessCheck
72 public class InnerAssignmentCheck
73 extends AbstractCheck {
74
75 /**
76 * A key is pointing to the warning message text in "messages.properties"
77 * file.
78 */
79 public static final String MSG_KEY = "assignment.inner.avoid";
80
81 /**
82 * Allowed AST types from an assignment AST node
83 * towards the root.
84 */
85 private static final int[][] ALLOWED_ASSIGNMENT_CONTEXT = {
86 {TokenTypes.EXPR, TokenTypes.SLIST},
87 {TokenTypes.VARIABLE_DEF},
88 {TokenTypes.EXPR, TokenTypes.ELIST, TokenTypes.FOR_INIT},
89 {TokenTypes.EXPR, TokenTypes.ELIST, TokenTypes.FOR_ITERATOR},
90 {TokenTypes.ANNOTATION_MEMBER_VALUE_PAIR}, {
91 TokenTypes.RESOURCE,
92 TokenTypes.RESOURCES,
93 TokenTypes.RESOURCE_SPECIFICATION,
94 },
95 {TokenTypes.EXPR, TokenTypes.LAMBDA},
96 {TokenTypes.EXPR, TokenTypes.SWITCH_RULE, TokenTypes.LITERAL_SWITCH, TokenTypes.SLIST},
97 };
98
99 /**
100 * Allowed AST types from an assignment AST node
101 * towards the root.
102 */
103 private static final int[][] CONTROL_CONTEXT = {
104 {TokenTypes.EXPR, TokenTypes.LITERAL_DO},
105 {TokenTypes.EXPR, TokenTypes.LITERAL_FOR},
106 {TokenTypes.EXPR, TokenTypes.LITERAL_WHILE},
107 {TokenTypes.EXPR, TokenTypes.LITERAL_IF},
108 {TokenTypes.EXPR, TokenTypes.LITERAL_ELSE},
109 };
110
111 /**
112 * Allowed AST types from a comparison node (above an assignment)
113 * towards the root.
114 */
115 private static final int[][] ALLOWED_ASSIGNMENT_IN_COMPARISON_CONTEXT = {
116 {TokenTypes.EXPR, TokenTypes.LITERAL_WHILE},
117 {TokenTypes.EXPR, TokenTypes.FOR_CONDITION},
118 {TokenTypes.EXPR, TokenTypes.LITERAL_DO},
119 };
120
121 /**
122 * The token types that identify comparison operators.
123 */
124 private static final BitSet COMPARISON_TYPES = TokenUtil.asBitSet(
125 TokenTypes.EQUAL,
126 TokenTypes.GE,
127 TokenTypes.GT,
128 TokenTypes.LE,
129 TokenTypes.LT,
130 TokenTypes.NOT_EQUAL
131 );
132
133 /**
134 * The token types that are ignored while checking "loop-idiom".
135 */
136 private static final BitSet LOOP_IDIOM_IGNORED_PARENTS = TokenUtil.asBitSet(
137 TokenTypes.LAND,
138 TokenTypes.LOR,
139 TokenTypes.LNOT,
140 TokenTypes.BOR,
141 TokenTypes.BAND
142 );
143
144 /**
145 * Creates a new {@code InnerAssignmentCheck} instance.
146 */
147 public InnerAssignmentCheck() {
148 // no code by default
149 }
150
151 @Override
152 public int[] getDefaultTokens() {
153 return getRequiredTokens();
154 }
155
156 @Override
157 public int[] getAcceptableTokens() {
158 return getRequiredTokens();
159 }
160
161 @Override
162 public int[] getRequiredTokens() {
163 return new int[] {
164 TokenTypes.ASSIGN, // '='
165 TokenTypes.DIV_ASSIGN, // "/="
166 TokenTypes.PLUS_ASSIGN, // "+="
167 TokenTypes.MINUS_ASSIGN, // "-="
168 TokenTypes.STAR_ASSIGN, // "*="
169 TokenTypes.MOD_ASSIGN, // "%="
170 TokenTypes.SR_ASSIGN, // ">>="
171 TokenTypes.BSR_ASSIGN, // ">>>="
172 TokenTypes.SL_ASSIGN, // "<<="
173 TokenTypes.BXOR_ASSIGN, // "^="
174 TokenTypes.BOR_ASSIGN, // "|="
175 TokenTypes.BAND_ASSIGN, // "&="
176 };
177 }
178
179 @Override
180 public void visitToken(DetailAST ast) {
181 if (!isInContext(ast, ALLOWED_ASSIGNMENT_CONTEXT, CommonUtil.EMPTY_BIT_SET)
182 && !isInNoBraceControlStatement(ast)
183 && !isInLoopIdiom(ast)) {
184 log(ast, MSG_KEY);
185 }
186 }
187
188 /**
189 * Determines if ast is in the body of a flow control statement without
190 * braces. An example of such a statement would be
191 * {@snippet :
192 * if (y > 0)
193 * x = y;
194 * }
195 *
196 * <p>
197 * This leads to the following AST structure:
198 * </p>
199 * {@snippet lang="text" :
200 * LITERAL_IF
201 * LPAREN
202 * EXPR // test
203 * RPAREN
204 * EXPR // body
205 * SEMI
206 * }
207 *
208 * <p>
209 * We need to ensure that ast is in the body and not in the test.
210 * </p>
211 *
212 * @param ast an assignment operator AST
213 * @return whether ast is in the body of a flow control statement
214 */
215 private static boolean isInNoBraceControlStatement(DetailAST ast) {
216 boolean result = false;
217 if (isInContext(ast, CONTROL_CONTEXT, CommonUtil.EMPTY_BIT_SET)) {
218 final DetailAST expr = ast.getParent();
219 final DetailAST exprNext = expr.getNextSibling();
220 result = exprNext.getType() == TokenTypes.SEMI;
221 }
222 return result;
223 }
224
225 /**
226 * Tests whether the given AST is used in the "assignment in loop" idiom.
227 * {@snippet :
228 * String line;
229 * while ((line = bufferedReader.readLine()) != null) {
230 * // process the line
231 * }
232 * for (;(line = bufferedReader.readLine()) != null;) {
233 * // process the line
234 * }
235 * do {
236 * // process the line
237 * }
238 * while ((line = bufferedReader.readLine()) != null);
239 * }
240 * Assignment inside a condition is not a problem here, as the assignment is surrounded by an
241 * extra pair of parentheses. The comparison is {@code != null} and there is no chance that
242 * intention was to write {@code line == reader.readLine()}.
243 *
244 * @param ast assignment AST
245 * @return whether the context of the assignment AST indicates the idiom
246 */
247 private static boolean isInLoopIdiom(DetailAST ast) {
248 return isComparison(ast.getParent())
249 && isInContext(ast.getParent(),
250 ALLOWED_ASSIGNMENT_IN_COMPARISON_CONTEXT,
251 LOOP_IDIOM_IGNORED_PARENTS);
252 }
253
254 /**
255 * Checks if an AST is a comparison operator.
256 *
257 * @param ast the AST to check
258 * @return true iff ast is a comparison operator.
259 */
260 private static boolean isComparison(DetailAST ast) {
261 final int astType = ast.getType();
262 return COMPARISON_TYPES.get(astType);
263 }
264
265 /**
266 * Tests whether the provided AST is in
267 * one of the given contexts.
268 *
269 * @param ast the AST from which to start walking towards root
270 * @param contextSet the contexts to test against.
271 * @param skipTokens parent token types to ignore
272 *
273 * @return whether the parents nodes of ast match one of the allowed type paths.
274 */
275 private static boolean isInContext(DetailAST ast, int[][] contextSet, BitSet skipTokens) {
276 boolean found = false;
277 for (int[] element : contextSet) {
278 DetailAST current = ast;
279 for (int anElement : element) {
280 current = getParent(current, skipTokens);
281 if (current.getType() == anElement) {
282 found = true;
283 }
284 else {
285 found = false;
286 break;
287 }
288 }
289
290 if (found) {
291 break;
292 }
293 }
294 return found;
295 }
296
297 /**
298 * Get ast parent, ignoring token types from {@code skipTokens}.
299 *
300 * @param ast token to get parent
301 * @param skipTokens token types to skip
302 * @return first not ignored parent of ast
303 */
304 private static DetailAST getParent(DetailAST ast, BitSet skipTokens) {
305 DetailAST result = ast.getParent();
306 while (skipTokens.get(result.getType())) {
307 result = result.getParent();
308 }
309 return result;
310 }
311
312 }