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.type; 27 28 import javax.lang.model.element.*; 29 30 /** 31 * A visitor of types, in the style of the 32 * visitor design pattern. Classes implementing this 33 * interface are used to operate on a type when the kind of 34 * type is unknown at compile time. When a visitor is passed to a 35 * type's {@link TypeMirror#accept accept} method, the <code>visit<i>Xyz</i></code> 36 * method most applicable to that type 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 TypeVisitor<R, P> { 79 /** 80 * Visits a type. 81 * @param t the type to visit 82 * @param p a visitor-specified parameter 83 * @return a visitor-specified result 84 */ visit(TypeMirror t, P p)85 R visit(TypeMirror t, P p); 86 87 /** 88 * A convenience method equivalent to {@code visit(t, null)}. 89 * 90 * @implSpec The default implementation is {@code visit(t, null)}. 91 * 92 * @param t the element to visit 93 * @return a visitor-specified result 94 */ visit(TypeMirror t)95 default R visit(TypeMirror t) { 96 return visit(t, null); 97 } 98 99 /** 100 * Visits a primitive type. 101 * @param t the type to visit 102 * @param p a visitor-specified parameter 103 * @return a visitor-specified result 104 */ visitPrimitive(PrimitiveType t, P p)105 R visitPrimitive(PrimitiveType t, P p); 106 107 /** 108 * Visits the null type. 109 * @param t the type to visit 110 * @param p a visitor-specified parameter 111 * @return a visitor-specified result 112 */ visitNull(NullType t, P p)113 R visitNull(NullType t, P p); 114 115 /** 116 * Visits an array type. 117 * @param t the type to visit 118 * @param p a visitor-specified parameter 119 * @return a visitor-specified result 120 */ visitArray(ArrayType t, P p)121 R visitArray(ArrayType t, P p); 122 123 /** 124 * Visits a declared type. 125 * @param t the type to visit 126 * @param p a visitor-specified parameter 127 * @return a visitor-specified result 128 */ visitDeclared(DeclaredType t, P p)129 R visitDeclared(DeclaredType t, P p); 130 131 /** 132 * Visits an error type. 133 * @param t the type to visit 134 * @param p a visitor-specified parameter 135 * @return a visitor-specified result 136 */ visitError(ErrorType t, P p)137 R visitError(ErrorType t, P p); 138 139 /** 140 * Visits a type variable. 141 * @param t the type to visit 142 * @param p a visitor-specified parameter 143 * @return a visitor-specified result 144 */ visitTypeVariable(TypeVariable t, P p)145 R visitTypeVariable(TypeVariable t, P p); 146 147 /** 148 * Visits a wildcard type. 149 * @param t the type to visit 150 * @param p a visitor-specified parameter 151 * @return a visitor-specified result 152 */ visitWildcard(WildcardType t, P p)153 R visitWildcard(WildcardType t, P p); 154 155 /** 156 * Visits an executable type. 157 * @param t the type to visit 158 * @param p a visitor-specified parameter 159 * @return a visitor-specified result 160 */ visitExecutable(ExecutableType t, P p)161 R visitExecutable(ExecutableType t, P p); 162 163 /** 164 * Visits a {@link NoType} instance. 165 * @param t the type to visit 166 * @param p a visitor-specified parameter 167 * @return a visitor-specified result 168 */ visitNoType(NoType t, P p)169 R visitNoType(NoType t, P p); 170 171 /** 172 * Visits an unknown kind of type. 173 * This can occur if the language evolves and new kinds 174 * of types are added to the {@code TypeMirror} hierarchy. 175 * @param t the type to visit 176 * @param p a visitor-specified parameter 177 * @return a visitor-specified result 178 * @throws UnknownTypeException 179 * a visitor implementation may optionally throw this exception 180 */ visitUnknown(TypeMirror t, P p)181 R visitUnknown(TypeMirror t, P p); 182 183 /** 184 * Visits a union type. 185 * 186 * @param t the type to visit 187 * @param p a visitor-specified parameter 188 * @return a visitor-specified result 189 * @since 1.7 190 */ visitUnion(UnionType t, P p)191 R visitUnion(UnionType t, P p); 192 193 /** 194 * Visits an intersection type. 195 * 196 * @param t the type to visit 197 * @param p a visitor-specified parameter 198 * @return a visitor-specified result 199 * @since 1.8 200 */ visitIntersection(IntersectionType t, P p)201 R visitIntersection(IntersectionType t, P p); 202 } 203