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 may
63 * be 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 v.visit(e, null)}.
89 * @param e the element to visit
90 * @return a visitor-specified result
91 */
92 R visit(Element e);
93
94 /**
95 * Visits a package element.
96 * @param e the element to visit
97 * @param p a visitor-specified parameter
98 * @return a visitor-specified result
99 */
100 R visitPackage(PackageElement e, P p);
101
102 /**
103 * Visits a type element.
104 * @param e the element to visit
105 * @param p a visitor-specified parameter
106 * @return a visitor-specified result
107 */
108 R visitType(TypeElement e, P p);
109
110 /**
111 * Visits a variable element.
112 * @param e the element to visit
129 * @param p a visitor-specified parameter
130 * @return a visitor-specified result
131 */
132 R visitTypeParameter(TypeParameterElement e, P p);
133
134 /**
135 * Visits an unknown kind of element.
136 * This can occur if the language evolves and new kinds
137 * of elements are added to the {@code Element} hierarchy.
138 *
139 * @param e the element to visit
140 * @param p a visitor-specified parameter
141 * @return a visitor-specified result
142 * @throws UnknownElementException
143 * a visitor implementation may optionally throw this exception
144 */
145 R visitUnknown(Element e, P p);
146
147 /**
148 * Visits a module element.
149 * @param e the element to visit
150 * @param p a visitor-specified parameter
151 * @return a visitor-specified result
152 * @since 9
153 */
154 R visitModule(ModuleElement e, P p);
155 }
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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
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 }
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