1 /*
   2  * Copyright (c) 2005, 2013, 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 <tt>visit<i>XYZ</i></tt> 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&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 are required to
  60  * also be runnable on Java SE 7.  Therefore, default methods
  61  * <em>cannot</em> be 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  * @see AbstractElementVisitor6
  77  * @see AbstractElementVisitor7
  78  * @since 1.6
  79  */
  80 public interface ElementVisitor<R, P> {
  81     /**
  82      * Visits an element.
  83      * @param e  the element to visit
  84      * @param p  a visitor-specified parameter
  85      * @return a visitor-specified result
  86      */
  87     R visit(Element e, P p);
  88 
  89     /**
  90      * A convenience method equivalent to {@code v.visit(e, null)}.
  91      * @param e  the element to visit
  92      * @return a visitor-specified result
  93      */
  94     R visit(Element e);
  95 
  96     /**
  97      * Visits a package element.
  98      * @param e  the element to visit
  99      * @param p  a visitor-specified parameter
 100      * @return a visitor-specified result
 101      */
 102     R visitPackage(PackageElement e, P p);
 103 
 104     /**
 105      * Visits a type element.
 106      * @param e  the element to visit
 107      * @param p  a visitor-specified parameter
 108      * @return a visitor-specified result
 109      */
 110     R visitType(TypeElement e, P p);
 111 
 112     /**
 113      * Visits a variable element.
 114      * @param e  the element to visit
 115      * @param p  a visitor-specified parameter
 116      * @return a visitor-specified result
 117      */
 118     R visitVariable(VariableElement e, P p);
 119 
 120     /**
 121      * Visits an executable element.
 122      * @param e  the element to visit
 123      * @param p  a visitor-specified parameter
 124      * @return a visitor-specified result
 125      */
 126     R visitExecutable(ExecutableElement e, P p);
 127 
 128     /**
 129      * Visits a type parameter element.
 130      * @param e  the element to visit
 131      * @param p  a visitor-specified parameter
 132      * @return a visitor-specified result
 133      */
 134     R visitTypeParameter(TypeParameterElement e, P p);
 135 
 136     /**
 137      * Visits an unknown kind of element.
 138      * This can occur if the language evolves and new kinds
 139      * of elements are added to the {@code Element} hierarchy.
 140      *
 141      * @param e  the element to visit
 142      * @param p  a visitor-specified parameter
 143      * @return a visitor-specified result
 144      * @throws UnknownElementException
 145      *  a visitor implementation may optionally throw this exception
 146      */
 147     R visitUnknown(Element e, P p);
 148 }