1 /*
2 * Copyright (c) 2005, 2017, 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
8 * particular file as subject to the "Classpath" exception as provided
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 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
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 * <p> <b>WARNING:</b> It is possible that methods will be added to
44 * this interface to accommodate new, currently unknown, language
45 * structures added to future versions of the Java™ programming
46 * language. Therefore, visitor classes directly implementing this
47 * interface may be source incompatible with future versions of the
48 * platform. To avoid this source incompatibility, visitor
49 * implementations are encouraged to instead extend the appropriate
50 * abstract visitor class that implements this interface. However, an
51 * API should generally use this visitor interface as the type for
52 * parameters, return type, etc. rather than one of the abstract
53 * classes.
54 *
55 * <p>Note that methods to accommodate new language constructs could
56 * be added in a source <em>compatible</em> way if they were added as
57 * <em>default methods</em>. However, default methods are only
58 * available on Java SE 8 and higher releases and the {@code
59 * javax.lang.model.*} packages bundled in Java SE 8 were required to
60 * also be runnable on Java SE 7. Therefore, default methods
61 * were <em>not</em> used when extending {@code javax.lang.model.*}
62 * to cover Java SE 8 language features. However, default methods
63 * are used in subsequent revisions of the {@code javax.lang.model.*}
64 * packages that are only required to run on Java SE 8 and higher
65 * platform versions.
66 *
67 * @param <R> the return type of this visitor's methods. Use {@link
68 * Void} for visitors that do not need to return results.
69 * @param <P> the type of the additional parameter to this visitor's
70 * methods. Use {@code Void} for visitors that do not need an
71 * additional parameter.
72 *
73 * @author Joseph D. Darcy
74 * @author Scott Seligman
75 * @author Peter von der Ahé
76 * @since 1.6
77 */
78 public interface ElementVisitor<R, P> {
79 /**
80 * Visits an element.
81 * @param e the element to visit
82 * @param p a visitor-specified parameter
83 * @return a visitor-specified result
84 */
85 R visit(Element e, P p);
86
87 /**
88 * A convenience method equivalent to {@code visit(e, null)}.
89 *
90 * @implSpec The default implementation is {@code visit(e, null)}.
91 *
92 * @param e the element to visit
93 * @return a visitor-specified result
94 */
95 default R visit(Element e) {
96 return visit(e, null);
97 }
98
99 /**
100 * Visits a package element.
101 * @param e the element to visit
102 * @param p a visitor-specified parameter
103 * @return a visitor-specified result
104 */
105 R visitPackage(PackageElement e, P p);
106
107 /**
108 * Visits a type element.
109 * @param e the element to visit
110 * @param p a visitor-specified parameter
111 * @return a visitor-specified result
112 */
113 R visitType(TypeElement e, P p);
114
115 /**
116 * Visits a variable element.
117 * @param e the element to visit
118 * @param p a visitor-specified parameter
119 * @return a visitor-specified result
120 */
121 R visitVariable(VariableElement e, P p);
122
123 /**
124 * Visits an executable element.
125 * @param e the element to visit
126 * @param p a visitor-specified parameter
127 * @return a visitor-specified result
128 */
129 R visitExecutable(ExecutableElement e, P p);
130
131 /**
132 * Visits a type parameter element.
133 * @param e the element to visit
134 * @param p a visitor-specified parameter
135 * @return a visitor-specified result
136 */
137 R visitTypeParameter(TypeParameterElement e, P p);
138
139 /**
140 * Visits an unknown kind of element.
141 * This can occur if the language evolves and new kinds
142 * of elements are added to the {@code Element} hierarchy.
143 *
144 * @param e the element to visit
145 * @param p a visitor-specified parameter
146 * @return a visitor-specified result
147 * @throws UnknownElementException
148 * a visitor implementation may optionally throw this exception
149 */
150 R visitUnknown(Element e, P p);
151
152 /**
153 * Visits a module element.
154 *
155 * @implSpec Visits a {@code ModuleElement} by calling {@code
156 * visitUnknown(e, p)}.
157 *
158 * @param e the element to visit
159 * @param p a visitor-specified parameter
160 * @return a visitor-specified result
161 * @since 9
162 */
163 default R visitModule(ModuleElement e, P p) {
164 return visitUnknown(e, p);
165 }
166 }