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&trade; 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 Ah&eacute;
  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      */
  85     R visit(TypeMirror t, P p);
  86 
  87     /**
  88      * A convenience method equivalent to {@code v.visit(t, null)}.
  89      * @param t the element to visit
  90      * @return  a visitor-specified result
  91      */
  92     R visit(TypeMirror t);
  93 
  94     /**
  95      * Visits a primitive type.
  96      * @param t the type to visit
  97      * @param p a visitor-specified parameter
  98      * @return  a visitor-specified result
  99      */
 100     R visitPrimitive(PrimitiveType t, P p);
 101 
 102     /**
 103      * Visits the null type.
 104      * @param t the type to visit
 105      * @param p a visitor-specified parameter
 106      * @return  a visitor-specified result
 107      */
 108     R visitNull(NullType t, P p);
 109 
 110     /**
 111      * Visits an array type.
 112      * @param t the type to visit
 113      * @param p a visitor-specified parameter
 114      * @return  a visitor-specified result
 115      */
 116     R visitArray(ArrayType t, P p);
 117 
 118     /**
 119      * Visits a declared type.
 120      * @param t the type to visit
 121      * @param p a visitor-specified parameter
 122      * @return  a visitor-specified result
 123      */
 124     R visitDeclared(DeclaredType t, P p);
 125 
 126     /**
 127      * Visits an error type.
 128      * @param t the type to visit
 129      * @param p a visitor-specified parameter
 130      * @return  a visitor-specified result
 131      */
 132     R visitError(ErrorType t, P p);
 133 
 134     /**
 135      * Visits a type variable.
 136      * @param t the type to visit
 137      * @param p a visitor-specified parameter
 138      * @return  a visitor-specified result
 139      */
 140     R visitTypeVariable(TypeVariable t, P p);
 141 
 142     /**
 143      * Visits a wildcard type.
 144      * @param t the type to visit
 145      * @param p a visitor-specified parameter
 146      * @return  a visitor-specified result
 147      */
 148     R visitWildcard(WildcardType t, P p);
 149 
 150     /**
 151      * Visits an executable type.
 152      * @param t the type to visit
 153      * @param p a visitor-specified parameter
 154      * @return  a visitor-specified result
 155      */
 156     R visitExecutable(ExecutableType t, P p);
 157 
 158     /**
 159      * Visits a {@link NoType} instance.
 160      * @param t the type to visit
 161      * @param p a visitor-specified parameter
 162      * @return  a visitor-specified result
 163      */
 164     R visitNoType(NoType t, P p);
 165 
 166     /**
 167      * Visits an unknown kind of type.
 168      * This can occur if the language evolves and new kinds
 169      * of types are added to the {@code TypeMirror} hierarchy.
 170      * @param t the type to visit
 171      * @param p a visitor-specified parameter
 172      * @return  a visitor-specified result
 173      * @throws UnknownTypeException
 174      *  a visitor implementation may optionally throw this exception
 175      */
 176     R visitUnknown(TypeMirror t, P p);
 177 
 178     /**
 179      * Visits a union type.
 180      *
 181      * @param t the type to visit
 182      * @param p a visitor-specified parameter
 183      * @return  a visitor-specified result
 184      * @since 1.7
 185      */
 186     R visitUnion(UnionType t, P p);
 187 
 188     /**
 189      * Visits an intersection type.
 190      *
 191      * @param t the type to visit
 192      * @param p a visitor-specified parameter
 193      * @return  a visitor-specified result
 194      * @since 1.8
 195      */
 196     R visitIntersection(IntersectionType t, P p);
 197 }