Wed, 23 Jan 2013 20:11:07 -0800
8006264: Add explanation of why default methods cannot be used in JDK 8 javax.lang.model
Reviewed-by: jjg
1 /*
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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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).
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26 package javax.lang.model.util;
28 import javax.lang.model.type.*;
29 import javax.annotation.processing.SupportedSourceVersion;
30 import javax.lang.model.SourceVersion;
31 import static javax.lang.model.SourceVersion.*;
34 /**
35 * A simple visitor of types with default behavior appropriate for the
36 * {@link SourceVersion#RELEASE_6 RELEASE_6} source version.
37 *
38 * Visit methods corresponding to {@code RELEASE_6} language
39 * constructs call {@link #defaultAction defaultAction}, passing their
40 * arguments to {@code defaultAction}'s corresponding parameters.
41 *
42 * For constructs introduced in {@code RELEASE_7} and later, {@code
43 * visitUnknown} is called instead.
44 *
45 * <p> Methods in this class may be overridden subject to their
46 * general contract. Note that annotating methods in concrete
47 * subclasses with {@link java.lang.Override @Override} will help
48 * ensure that methods are overridden as intended.
49 *
50 * <p> <b>WARNING:</b> The {@code TypeVisitor} interface implemented
51 * by this class may have methods added to it in the future to
52 * accommodate new, currently unknown, language structures added to
53 * future versions of the Java™ programming language.
54 * Therefore, methods whose names begin with {@code "visit"} may be
55 * added to this class in the future; to avoid incompatibilities,
56 * classes which extend this class should not declare any instance
57 * methods with names beginning with {@code "visit"}.
58 *
59 * <p>When such a new visit method is added, the default
60 * implementation in this class will be to call the {@link
61 * #visitUnknown visitUnknown} method. A new simple type visitor
62 * class will also be introduced to correspond to the new language
63 * level; this visitor will have different default behavior for the
64 * visit method in question. When the new visitor is introduced, all
65 * or portions of this visitor may be deprecated.
66 *
67 * <p>Note that adding a default implementation of a new visit method
68 * in a visitor class will occur instead of adding a <em>default
69 * method</em> directly in the visitor interface since a Java SE 8
70 * language feature cannot be used to this version of the API since
71 * this version is required to be runnable on Java SE 7
72 * implementations. Future versions of the API that are only required
73 * to run on Java SE 8 and later may take advantage of default methods
74 * in this situation.
75 *
76 * @param <R> the return type of this visitor's methods. Use {@link
77 * Void} for visitors that do not need to return results.
78 * @param <P> the type of the additional parameter to this visitor's
79 * methods. Use {@code Void} for visitors that do not need an
80 * additional parameter.
81 *
82 * @author Joseph D. Darcy
83 * @author Scott Seligman
84 * @author Peter von der Ahé
85 *
86 * @see SimpleTypeVisitor7
87 * @see SimpleTypeVisitor8
88 * @since 1.6
89 */
90 @SupportedSourceVersion(RELEASE_6)
91 public class SimpleTypeVisitor6<R, P> extends AbstractTypeVisitor6<R, P> {
92 /**
93 * Default value to be returned; {@link #defaultAction
94 * defaultAction} returns this value unless the method is
95 * overridden.
96 */
97 protected final R DEFAULT_VALUE;
99 /**
100 * Constructor for concrete subclasses; uses {@code null} for the
101 * default value.
102 */
103 protected SimpleTypeVisitor6(){
104 DEFAULT_VALUE = null;
105 }
107 /**
108 * Constructor for concrete subclasses; uses the argument for the
109 * default value.
110 *
111 * @param defaultValue the value to assign to {@link #DEFAULT_VALUE}
112 */
113 protected SimpleTypeVisitor6(R defaultValue){
114 DEFAULT_VALUE = defaultValue;
115 }
117 /**
118 * The default action for visit methods. The implementation in
119 * this class just returns {@link #DEFAULT_VALUE}; subclasses will
120 * commonly override this method.
121 */
122 protected R defaultAction(TypeMirror e, P p) {
123 return DEFAULT_VALUE;
124 }
126 /**
127 * {@inheritDoc} This implementation calls {@code defaultAction}.
128 *
129 * @param t {@inheritDoc}
130 * @param p {@inheritDoc}
131 * @return the result of {@code defaultAction}
132 */
133 public R visitPrimitive(PrimitiveType t, P p) {
134 return defaultAction(t, p);
135 }
137 /**
138 * {@inheritDoc} This implementation calls {@code defaultAction}.
139 *
140 * @param t {@inheritDoc}
141 * @param p {@inheritDoc}
142 * @return the result of {@code defaultAction}
143 */
144 public R visitNull(NullType t, P p){
145 return defaultAction(t, p);
146 }
148 /**
149 * {@inheritDoc} This implementation calls {@code defaultAction}.
150 *
151 * @param t {@inheritDoc}
152 * @param p {@inheritDoc}
153 * @return the result of {@code defaultAction}
154 */
155 public R visitArray(ArrayType t, P p){
156 return defaultAction(t, p);
157 }
159 /**
160 * {@inheritDoc} This implementation calls {@code defaultAction}.
161 *
162 * @param t {@inheritDoc}
163 * @param p {@inheritDoc}
164 * @return the result of {@code defaultAction}
165 */
166 public R visitDeclared(DeclaredType t, P p){
167 return defaultAction(t, p);
168 }
170 /**
171 * {@inheritDoc} This implementation calls {@code defaultAction}.
172 *
173 * @param t {@inheritDoc}
174 * @param p {@inheritDoc}
175 * @return the result of {@code defaultAction}
176 */
177 public R visitError(ErrorType t, P p){
178 return defaultAction(t, p);
179 }
181 /**
182 * {@inheritDoc} This implementation calls {@code defaultAction}.
183 *
184 * @param t {@inheritDoc}
185 * @param p {@inheritDoc}
186 * @return the result of {@code defaultAction}
187 */
188 public R visitTypeVariable(TypeVariable t, P p){
189 return defaultAction(t, p);
190 }
192 /**
193 * {@inheritDoc} This implementation calls {@code defaultAction}.
194 *
195 * @param t {@inheritDoc}
196 * @param p {@inheritDoc}
197 * @return the result of {@code defaultAction}
198 */
199 public R visitWildcard(WildcardType t, P p){
200 return defaultAction(t, p);
201 }
203 /**
204 * {@inheritDoc} This implementation calls {@code defaultAction}.
205 *
206 * @param t {@inheritDoc}
207 * @param p {@inheritDoc}
208 * @return the result of {@code defaultAction}
209 */
210 public R visitExecutable(ExecutableType t, P p) {
211 return defaultAction(t, p);
212 }
214 /**
215 * {@inheritDoc} This implementation calls {@code defaultAction}.
216 *
217 * @param t {@inheritDoc}
218 * @param p {@inheritDoc}
219 * @return the result of {@code defaultAction}
220 */
221 public R visitNoType(NoType t, P p){
222 return defaultAction(t, p);
223 }
224 }