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
6 * under the terms of the GNU General Public License version 2 only, as
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 * <strong>WARNING:</strong> It is possible that methods will be added
45 * to this interface to accommodate new, currently unknown, language
46 * structures added to future versions of the Java™ programming
47 * language.
48 *
49 * Such additions have already occurred to support language features
50 * added after this API was introduced.
51 *
52 * Visitor classes directly implementing this interface may be source
53 * incompatible with future versions of the platform. To avoid this
54 * source incompatibility, visitor implementations are encouraged to
55 * instead extend the appropriate abstract visitor class that
56 * implements this interface. However, an API should generally use
57 * this visitor interface as the type for parameters, return type,
58 * etc. rather than one of the abstract classes.
59 *
60 * <p>Methods to accommodate new language constructs are expected to
61 * be added as default methods to provide strong source compatibility,
62 * as done for {@link visitModule visitModule}. The implementations of
63 * the default methods will in turn call {@link visitUnknown
64 * visitUnknown}, behavior that will be overridden in concrete
65 * visitors supporting the source version with the new language
66 * construct.
67 *
68 * <p>There are several families of classes implementing this visitor
69 * interface in the {@linkplain javax.lang.model.util util
70 * package}. The families follow a naming pattern along the lines of
71 * {@code FooVisitor}<i>N</i> where <i>N</i> indicates the
72 * {@linkplain javax.lang.model.SourceVersion source version} the
73 * visitor is appropriate for.
74 *
75 * In particular, a {@code FooVisitor}<i>N</i> is expected to handle
76 * all language constructs present in source version <i>N</i>. If
77 * there are no new language constructs added in version
78 * <i>N</i> + 1 (or subsequent releases), {@code
79 * FooVisitor}<i>N</i> may also handle that later source version; in
80 * that case, the {@link
81 * javax.annotation.processing.SupportedSourceVersion
82 * SupportedSourceVersion} annotation on the {@code
83 * FooVisitor}<i>N</i> class will indicate a later version.
84 *
85 * When visiting an element representing a language construct
86 * introduced <strong>after</strong> source version <i>N</i>, a {@code
87 * FooVisitor}<i>N</i> will throw an {@link UnknownElementException}
88 * unless that behavior is overridden.
89 *
90 * <p>When choosing which member of a visitor family to subclass,
91 * subclassing the most recent one increases the range of source
92 * versions covered. When choosing which visitor family to subclass,
93 * consider their built-in capabilities:
94 *
95 * <ul>
96 *
97 * <li>{@link AbstractElementVisitor6 AbstractElementVisitor}s:
98 * Skeletal visitor implementations.
99 *
100 * <li>{@link SimpleElementVisitor6 SimpleElementVisitor}s: Support
101 * default actions and a default return value.
102 *
103 * <li>{@link ElementKindVisitor6 ElementKindVisitor}s: Visit methods
104 * provided on a {@linkplain Element#getKind per-kind} granularity as
105 * some categories of elements can have more than one kind.
106 *
107 * <li>{@link ElementScanner6 ElementScanner}s: Scanners are visitors
108 * which traverse an element and the elements {@linkplain
109 * Element#getEnclosedElements enclosed} by it and associated with it.
110 *
111 * </ul>
112 *
113 * @param <R> the return type of this visitor's methods. Use {@link
114 * Void} for visitors that do not need to return results.
115 * @param <P> the type of the additional parameter to this visitor's
116 * methods. Use {@code Void} for visitors that do not need an
117 * additional parameter.
118 *
119 * @author Joseph D. Darcy
120 * @author Scott Seligman
121 * @author Peter von der Ahé
122 * @since 1.6
123 */
124 public interface ElementVisitor<R, P> {
125 /**
126 * Visits an element.
127 * @param e the element to visit
128 * @param p a visitor-specified parameter
129 * @return a visitor-specified result
130 */
131 R visit(Element e, P p);
132
133 /**
134 * A convenience method equivalent to {@code visit(e, null)}.
135 *
136 * @implSpec The default implementation is {@code visit(e, null)}.
137 *
138 * @param e the element to visit
139 * @return a visitor-specified result
140 */
141 default R visit(Element e) {
142 return visit(e, null);
143 }
144
145 /**
146 * Visits a package element.
147 * @param e the element to visit
148 * @param p a visitor-specified parameter
149 * @return a visitor-specified result
150 */
151 R visitPackage(PackageElement e, P p);
152
153 /**
154 * Visits a type element.
155 * @param e the element to visit
156 * @param p a visitor-specified parameter
157 * @return a visitor-specified result
158 */
159 R visitType(TypeElement e, P p);
160
161 /**
162 * Visits a variable element.
163 * @param e the element to visit
164 * @param p a visitor-specified parameter
165 * @return a visitor-specified result
166 */
167 R visitVariable(VariableElement e, P p);
168
169 /**
170 * Visits an executable element.
171 * @param e the element to visit
172 * @param p a visitor-specified parameter
173 * @return a visitor-specified result
174 */
175 R visitExecutable(ExecutableElement e, P p);
176
177 /**
178 * Visits a type parameter element.
179 * @param e the element to visit
180 * @param p a visitor-specified parameter
181 * @return a visitor-specified result
182 */
183 R visitTypeParameter(TypeParameterElement e, P p);
184
185 /**
186 * Visits an unknown kind of element.
187 * This can occur if the language evolves and new kinds
188 * of elements are added to the {@code Element} hierarchy.
189 *
190 * @param e the element to visit
191 * @param p a visitor-specified parameter
192 * @return a visitor-specified result
193 * @throws UnknownElementException
194 * a visitor implementation may optionally throw this exception
195 */
196 R visitUnknown(Element e, P p);
197
198 /**
199 * Visits a module element.
200 *
201 * @implSpec The default implementation visits a {@code
202 * ModuleElement} by calling {@code visitUnknown(e, p)}.
203 *
204 * @param e the element to visit
205 * @param p a visitor-specified parameter
206 * @return a visitor-specified result
207 * @since 9
208 * @spec JPMS
209 */
210 default R visitModule(ModuleElement e, P p) {
211 return visitUnknown(e, p);
212 }
213
214 /**
215 * {@preview Associated with records, a preview feature of the Java language.
216 *
217 * This method is associated with <i>records</i>, a preview
218 * feature of the Java language. Preview features
219 * may be removed in a future release, or upgraded to permanent
220 * features of the Java language.}
221 *
222 * Visits a record component element.
223 *
224 * @implSpec The default implementation visits a {@code
225 * RecordComponentElement} by calling {@code visitUnknown(e, p)}.
226 *
227 * @param e the element to visit
228 * @param p a visitor-specified parameter
229 * @return a visitor-specified result
230 * @since 14
231 */
232 @jdk.internal.PreviewFeature(feature=jdk.internal.PreviewFeature.Feature.RECORDS,
233 essentialAPI=false)
234 @SuppressWarnings("preview")
235 default R visitRecordComponent(RecordComponentElement e, P p) {
236 return visitUnknown(e, p);
237 }
238 }