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package javax.lang.model.element;

import javax.lang.model.util.*;

A visitor of program elements, in the style of the visitor design pattern. Classes implementing this interface are used to operate on an element when the kind of element is unknown at compile time. When a visitor is passed to an element's 
accept method, the visitXyz method most applicable
to that element is invoked.
 Classes implementing this interface may or may not throw a NullPointerException if the additional parameter p is null; see documentation of the implementing class for details. 
Author:
Joseph D. Darcy, Scott Seligman, Peter von der Ahé
Type parameters:
<R> – the return type of this visitor's methods. Use Void for visitors that do not need to return results.
<P> – the type of the additional parameter to this visitor's methods. Use Void for visitors that do not need an additional parameter.
API Note:

WARNING: It is possible that methods will be added
to this interface to accommodate new, currently unknown, language
structures added to future versions of the Java programming
language.
Such additions have already occurred to support language features
added after this API was introduced.
Visitor classes directly implementing this interface may be source
incompatible with future versions of the platform.  To avoid this
source incompatibility, visitor implementations are encouraged to
instead extend the appropriate abstract visitor class that
implements this interface.  However, an API should generally use
this visitor interface as the type for parameters, return type,
etc. rather than one of the abstract classes.
Methods to accommodate new language constructs are expected to be added as default methods to provide strong source compatibility, as done for visitModule. The implementations of the default methods will in turn call 
visitUnknown, behavior that will be overridden in concrete visitors supporting the source version with the new language construct. 
There are several families of classes implementing this visitor interface in the util
package. The families follow a naming pattern along the lines of FooVisitorN where N indicates the source version the visitor is appropriate for. In particular, a FooVisitorN is expected to handle
all language constructs present in source version N. If
there are no new language constructs added in version
N + 1 (or subsequent releases), 
FooVisitorN may also handle that later source version; in that case, the 
SupportedSourceVersion annotation on the 
FooVisitorN class will indicate a later version.
When visiting an element representing a language construct
introduced after source version N, a 
FooVisitorN will throw an UnknownElementException unless that behavior is overridden. 
When choosing which member of a visitor family to subclass,
subclassing the most recent one increases the range of source
versions covered. When choosing which visitor family to subclass,
consider their built-in capabilities:

AbstractElementVisitors: Skeletal visitor implementations. 
SimpleElementVisitors: Support default actions and a default return value. 
ElementKindVisitors: Visit methods provided on a per-kind granularity as some categories of elements can have more than one kind. 
ElementScanners: Scanners are visitors which traverse an element and the elements enclosed by it and associated with it. 
Since:
1.6/**
 * A visitor of program elements, in the style of the visitor design
 * pattern.  Classes implementing this interface are used to operate
 * on an element when the kind of element is unknown at compile time.
 * When a visitor is passed to an element's {@link Element#accept
 * accept} method, the <code>visit<i>Xyz</i></code> method most applicable
 * to that element is invoked.
 *
 * <p> Classes implementing this interface may or may not throw a
 * {@code NullPointerException} if the additional parameter {@code p}
 * is {@code null}; see documentation of the implementing class for
 * details.
 *
 * @apiNote
 * <strong>WARNING:</strong> It is possible that methods will be added
 * to this interface to accommodate new, currently unknown, language
 * structures added to future versions of the Java programming
 * language.
 *
 * Such additions have already occurred to support language features
 * added after this API was introduced.
 *
 * Visitor classes directly implementing this interface may be source
 * incompatible with future versions of the platform.  To avoid this
 * source incompatibility, visitor implementations are encouraged to
 * instead extend the appropriate abstract visitor class that
 * implements this interface.  However, an API should generally use
 * this visitor interface as the type for parameters, return type,
 * etc. rather than one of the abstract classes.
 *
 * <p>Methods to accommodate new language constructs are expected to
 * be added as default methods to provide strong source compatibility,
 * as done for {@link visitModule visitModule}. The implementations of
 * the default methods will in turn call {@link visitUnknown
 * visitUnknown}, behavior that will be overridden in concrete
 * visitors supporting the source version with the new language
 * construct.
 *
 * <p>There are several families of classes implementing this visitor
 * interface in the {@linkplain javax.lang.model.util util
 * package}. The families follow a naming pattern along the lines of
 * {@code FooVisitor}<i>N</i> where <i>N</i> indicates the
 * {@linkplain javax.lang.model.SourceVersion source version} the
 * visitor is appropriate for.
 *
 * In particular, a {@code FooVisitor}<i>N</i> is expected to handle
 * all language constructs present in source version <i>N</i>. If
 * there are no new language constructs added in version
 * <i>N</i>&nbsp;+&nbsp;1 (or subsequent releases), {@code
 * FooVisitor}<i>N</i> may also handle that later source version; in
 * that case, the {@link
 * javax.annotation.processing.SupportedSourceVersion
 * SupportedSourceVersion} annotation on the {@code
 * FooVisitor}<i>N</i> class will indicate a later version.
 *
 * When visiting an element representing a language construct
 * introduced <strong>after</strong> source version <i>N</i>, a {@code
 * FooVisitor}<i>N</i> will throw an {@link UnknownElementException}
 * unless that behavior is overridden.
 *
 * <p>When choosing which member of a visitor family to subclass,
 * subclassing the most recent one increases the range of source
 * versions covered. When choosing which visitor family to subclass,
 * consider their built-in capabilities:
 *
 * <ul>
 *
 * <li>{@link AbstractElementVisitor6 AbstractElementVisitor}s:
 * Skeletal visitor implementations.
 *
 * <li>{@link SimpleElementVisitor6 SimpleElementVisitor}s: Support
 * default actions and a default return value.
 *
 * <li>{@link ElementKindVisitor6 ElementKindVisitor}s: Visit methods
 * provided on a {@linkplain Element#getKind per-kind} granularity as
 * some categories of elements can have more than one kind.
 *
 * <li>{@link ElementScanner6 ElementScanner}s: Scanners are visitors
 * which traverse an element and the elements {@linkplain
 * Element#getEnclosedElements enclosed} by it and associated with it.
 *
 * </ul>
 *
 * @param <R> the return type of this visitor's methods.  Use {@link
 *            Void} for visitors that do not need to return results.
 * @param <P> the type of the additional parameter to this visitor's
 *            methods.  Use {@code Void} for visitors that do not need an
 *            additional parameter.
 *
 * @author Joseph D. Darcy
 * @author Scott Seligman
 * @author Peter von der Ah&eacute;
 * @since 1.6
 */
public interface ElementVisitor<R, P> {
    
Visits an element.
Params:
e –  the element to visit
p –  a visitor-specified parameter
Returns:
a visitor-specified result/**
     * Visits an element.
     * @param e  the element to visit
     * @param p  a visitor-specified parameter
     * @return a visitor-specified result
     */
    R visit(Element e, P p);

    
A convenience method equivalent to visit(e, null). 
Params:
e –  the element to visit
Implementation Requirements:
The default implementation is visit(e, null).
Returns:
a visitor-specified result/**
     * A convenience method equivalent to {@code visit(e, null)}.
     *
     * @implSpec The default implementation is {@code visit(e, null)}.
     *
     * @param e  the element to visit
     * @return a visitor-specified result
     */
    default R visit(Element e) {
        return visit(e, null);
    }

    
Visits a package element.
Params:
e –  the element to visit
p –  a visitor-specified parameter
Returns:
a visitor-specified result/**
     * Visits a package element.
     * @param e  the element to visit
     * @param p  a visitor-specified parameter
     * @return a visitor-specified result
     */
    R visitPackage(PackageElement e, P p);

    
Visits a type element.
Params:
e –  the element to visit
p –  a visitor-specified parameter
Returns:
a visitor-specified result/**
     * Visits a type element.
     * @param e  the element to visit
     * @param p  a visitor-specified parameter
     * @return a visitor-specified result
     */
    R visitType(TypeElement e, P p);

    
Visits a variable element.
Params:
e –  the element to visit
p –  a visitor-specified parameter
Returns:
a visitor-specified result/**
     * Visits a variable element.
     * @param e  the element to visit
     * @param p  a visitor-specified parameter
     * @return a visitor-specified result
     */
    R visitVariable(VariableElement e, P p);

    
Visits an executable element.
Params:
e –  the element to visit
p –  a visitor-specified parameter
Returns:
a visitor-specified result/**
     * Visits an executable element.
     * @param e  the element to visit
     * @param p  a visitor-specified parameter
     * @return a visitor-specified result
     */
    R visitExecutable(ExecutableElement e, P p);

    
Visits a type parameter element.
Params:
e –  the element to visit
p –  a visitor-specified parameter
Returns:
a visitor-specified result/**
     * Visits a type parameter element.
     * @param e  the element to visit
     * @param p  a visitor-specified parameter
     * @return a visitor-specified result
     */
    R visitTypeParameter(TypeParameterElement e, P p);

    
Visits an unknown kind of element. This can occur if the language evolves and new kinds of elements are added to the Element hierarchy. 
Params:
e –  the element to visit
p –  a visitor-specified parameter
Throws:
UnknownElementException – 
 a visitor implementation may optionally throw this exception
Returns:
a visitor-specified result/**
     * Visits an unknown kind of element.
     * This can occur if the language evolves and new kinds
     * of elements are added to the {@code Element} hierarchy.
     *
     * @param e  the element to visit
     * @param p  a visitor-specified parameter
     * @return a visitor-specified result
     * @throws UnknownElementException
     *  a visitor implementation may optionally throw this exception
     */
    R visitUnknown(Element e, P p);

    
Visits a module element.
Params:
e –  the element to visit
p –  a visitor-specified parameter
Implementation Requirements:
The default implementation visits a 
ModuleElement by calling visitUnknown(e, p).
Returns:
a visitor-specified result
Since:
9
@spec
JPMS/**
     * Visits a module element.
     *
     * @implSpec The default implementation visits a {@code
     * ModuleElement} by calling {@code visitUnknown(e, p)}.
     *
     * @param e  the element to visit
     * @param p  a visitor-specified parameter
     * @return a visitor-specified result
     * @since 9
     * @spec JPMS
     */
    default R visitModule(ModuleElement e, P p) {
        return visitUnknown(e, p);
    }

    
{@preview Associated with records, a preview feature of the Java language.
          This method is associated with records, a preview
          feature of the Java language. Preview features
          may be removed in a future release, or upgraded to permanent
          features of the Java language.}
Visits a record component element.
Params:
e –  the element to visit
p –  a visitor-specified parameter
Implementation Requirements:
The default implementation visits a 
RecordComponentElement by calling visitUnknown(e, p).
Returns:
a visitor-specified result
Since:
14/**
     * {@preview Associated with records, a preview feature of the Java language.
     *
     *           This method is associated with <i>records</i>, a preview
     *           feature of the Java language. Preview features
     *           may be removed in a future release, or upgraded to permanent
     *           features of the Java language.}
     *
     * Visits a record component element.
     *
     * @implSpec The default implementation visits a {@code
     * RecordComponentElement} by calling {@code visitUnknown(e, p)}.
     *
     * @param e  the element to visit
     * @param p  a visitor-specified parameter
     * @return a visitor-specified result
     * @since 14
     */
    @jdk.internal.PreviewFeature(feature=jdk.internal.PreviewFeature.Feature.RECORDS,
                                 essentialAPI=false)
    @SuppressWarnings("preview")
    default R visitRecordComponent(RecordComponentElement e, P p) {
        return visitUnknown(e, p);
    }
}