1 /*
2 * Copyright (c) 2005, 2020, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
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7 * published by the Free Software Foundation. Oracle designates this
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9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
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24 */
25
26 package javax.lang.model.element;
27
28 import javax.lang.model.util.*;
29
30 /**
31 * A visitor of program elements, in the style of the visitor design
32 * pattern. Classes implementing this interface are used to operate
33 * on an element when the kind of element is unknown at compile time.
34 * When a visitor is passed to an element's {@link Element#accept
35 * accept} method, the <code>visit<i>Xyz</i></code> method most applicable
36 * to that element is invoked.
37 *
38 * <p> Classes implementing this interface may or may not throw a
39 * {@code NullPointerException} if the additional parameter {@code p}
40 * is {@code null}; see documentation of the implementing class for
41 * details.
42 *
43 * @apiNote
44 *
45 * WARNING: It is possible that methods will be added to
46 * this interface to accommodate new, currently unknown, language
47 * structures added to future versions of the Java™ programming
48 * language.
49 *
50 * Such additions have already occurred to support language features
51 * added after this API was introduced.
52 *
53 * Visitor classes directly implementing this
54 * interface may be source incompatible with future versions of the
55 * platform. To avoid this source incompatibility, visitor
56 * implementations are encouraged to instead extend the appropriate
57 * abstract visitor class that implements this interface. However, an
58 * API should generally use this visitor interface as the type for
59 * parameters, return type, etc. rather than one of the abstract
60 * classes.
61 *
62 * <p>Methods to accommodate new language constructs are expected to
63 * be added as default methods to provide strong source compatibility,
64 * as done for {@link visitModule visitModule}. The implementations of
65 * the default methods will in turn call {@link visitUnknown
66 * visitUnknown}, behavior that will be overridden in concrete
67 * visitors supporting the source version with the new language
68 * construct.
69 *
70 * <p>There are several families of classes implementing this visitor
71 * interface in the {@linkplain javax.lang.model.util util
72 * package}. The families follow a naming pattern along the lines of
73 * {@code FooVisitor}<i>N</i> where <i>N</i> indicates the
74 * {@linkplain javax.lang.model.SourceVersion source version} the
75 * visitor is appropriate for.
76 *
77 * In particular, a {@code FooVisitor}<i>N</i> is expected to handle
78 * all language constructs present in source version <i>N</i>. If
79 * there are no new language constructs added in version
80 * <i>N</i> + 1 (or subsequent releases), {@code
81 * FooVisitor}<i>N</i> may also handle that later source version; in
82 * that case, the {@link
83 * javax.annotation.processing.SupportedSourceVersion
84 * SupportedSourceVersion} annotation on the {@code
85 * FooVisitor}<i>N</i> class will indicate a later version.
86 *
87 * When visiting an element representing a language construct
88 * introduced <strong>after</strong> source version <i>N</i>, a {@code
89 * FooVisitor}<i>N</i> will throw an {@link UnknownElementException}
90 * unless that behavior is overridden.
91 *
92 * <p>When choosing which member of a visitor family to subclass,
93 * subclassing the most recent one increases the range of source
94 * versions covered. When choosing which visitor family to subclass,
95 * consider their built-in capabilities:
96 *
97 * <ul>
98 *
99 * <li>{@link AbstractElementVisitor6 AbstractElementVisitor}s:
100 * Skeletal visitor implementations.
101 *
102 * <li>{@link SimpleElementVisitor6 SimpleElementVisitor}s: Support
103 * default actions and a default return value.
104 *
105 * <li>{@link ElementKindVisitor6 ElementKindVisitor}s: Visit methods
106 * provided on a {@linkplain Element#getKind per-kind} granularity as
107 * some categories of elements can have more than one kind.
108 *
109 * <li>{@link ElementScanner6 ElementScanner}s: Scanners are visitors
110 * which traverse an element and the elements {@linkplain
111 * Element#getEnclosedElements enclosed} by it and associated with it.
112 *
113 * </ul>
114 *
115 * @param <R> the return type of this visitor's methods. Use {@link
116 * Void} for visitors that do not need to return results.
117 * @param <P> the type of the additional parameter to this visitor's
118 * methods. Use {@code Void} for visitors that do not need an
119 * additional parameter.
120 *
121 * @author Joseph D. Darcy
122 * @author Scott Seligman
123 * @author Peter von der Ahé
124 * @since 1.6
125 */
126 public interface ElementVisitor<R, P> {
127 /**
128 * Visits an element.
129 * @param e the element to visit
130 * @param p a visitor-specified parameter
131 * @return a visitor-specified result
132 */
133 R visit(Element e, P p);
134
135 /**
136 * A convenience method equivalent to {@code visit(e, null)}.
137 *
138 * @implSpec The default implementation is {@code visit(e, null)}.
139 *
140 * @param e the element to visit
141 * @return a visitor-specified result
142 */
143 default R visit(Element e) {
144 return visit(e, null);
145 }
146
147 /**
148 * Visits a package element.
149 * @param e the element to visit
150 * @param p a visitor-specified parameter
151 * @return a visitor-specified result
152 */
153 R visitPackage(PackageElement e, P p);
154
155 /**
156 * Visits a type element.
157 * @param e the element to visit
158 * @param p a visitor-specified parameter
159 * @return a visitor-specified result
160 */
161 R visitType(TypeElement e, P p);
162
163 /**
164 * Visits a variable element.
165 * @param e the element to visit
166 * @param p a visitor-specified parameter
167 * @return a visitor-specified result
168 */
169 R visitVariable(VariableElement e, P p);
170
171 /**
172 * Visits an executable element.
173 * @param e the element to visit
174 * @param p a visitor-specified parameter
175 * @return a visitor-specified result
176 */
177 R visitExecutable(ExecutableElement e, P p);
178
179 /**
180 * Visits a type parameter element.
181 * @param e the element to visit
182 * @param p a visitor-specified parameter
183 * @return a visitor-specified result
184 */
185 R visitTypeParameter(TypeParameterElement e, P p);
186
187 /**
188 * Visits an unknown kind of element.
189 * This can occur if the language evolves and new kinds
190 * of elements are added to the {@code Element} hierarchy.
191 *
192 * @param e the element to visit
193 * @param p a visitor-specified parameter
194 * @return a visitor-specified result
195 * @throws UnknownElementException
196 * a visitor implementation may optionally throw this exception
197 */
198 R visitUnknown(Element e, P p);
199
200 /**
201 * Visits a module element.
202 *
203 * @implSpec The default implementation visits a {@code
204 * ModuleElement} by calling {@code visitUnknown(e, p)}.
205 *
206 * @param e the element to visit
207 * @param p a visitor-specified parameter
208 * @return a visitor-specified result
209 * @since 9
210 * @spec JPMS
211 */
212 default R visitModule(ModuleElement e, P p) {
213 return visitUnknown(e, p);
214 }
215
216 /**
217 * {@preview Associated with records, a preview feature of the Java language.
218 *
219 * This method is associated with <i>records</i>, a preview
220 * feature of the Java language. Preview features
221 * may be removed in a future release, or upgraded to permanent
222 * features of the Java language.}
223 *
224 * Visits a record component element.
225 *
226 * @implSpec The default implementation visits a {@code
227 * RecordComponentElement} by calling {@code visitUnknown(e, p)}.
228 *
229 * @param e the element to visit
230 * @param p a visitor-specified parameter
231 * @return a visitor-specified result
232 * @since 14
233 */
234 @jdk.internal.PreviewFeature(feature=jdk.internal.PreviewFeature.Feature.RECORDS,
235 essentialAPI=false)
236 @SuppressWarnings("preview")
237 default R visitRecordComponent(RecordComponentElement e, P p) {
238 return visitUnknown(e, p);
239 }
240 }