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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 {@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} and {@link
 * visitRecordComponent visitRecordComponent}. 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.
 *
 * @since 1.6
 */
public interface ElementVisitor<R, P> {
    /**
     * 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 {@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.
     * @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.
     * @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.
     * @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.
     * @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.
     * @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 {@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.
     *
     * @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
     */
    default R visitModule(ModuleElement e, P p) {
        return visitUnknown(e, p);
    }

    /**
     * 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 16
     */
    default R visitRecordComponent(RecordComponentElement e, P p) {
        return visitUnknown(e, p);
    }
}