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/*
 * Copyright (C) 2021 ZeoFlow SRL
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package javax.lang.model.element;

import javax.lang.model.util.AbstractElementVisitor6;
import javax.lang.model.util.AbstractElementVisitor7;

/**
 * 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 javax.lang.model.element.Element#accept
 * accept} method, the visitXYZ method most applicable
 * to that element is invoked.
 *
 *  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.
 *
 * 
 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.  Therefore, 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.
 *
 * 
Note that methods to accommodate new language constructs could
 * be added in a source compatible way if they were added as
 * default methods.  However, default methods are only
 * available on Java SE 8 and higher releases and the {@code
 * jx.lang.model.*} packages bundled in Java SE 8 are required to
 * also be runnable on Java SE 7.  Therefore, default methods
 * cannot be used when extending {@code jx.lang.model.*}
 * to cover Java SE 8 language features.  However, default methods may
 * be used in subsequent revisions of the {@code jx.lang.model.*}
 * packages that are only required to run on Java SE 8 and higher
 * platform versions.
 *
 * @param  the return type of this visitor's methods.  Use {@link
 *            Void} for visitors that do not need to return results.
 * @param  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 Ahé
 * @see AbstractElementVisitor6
 * @see AbstractElementVisitor7
 * @since 1.6
 */
public interface ElementVisitor
{

    /**
     * Visits an element.
     *
     * @param e the element to visit
     * @param p a visitor-specified parameter
     *
     * @return a visitor-specified result
     */
    R visit(javax.lang.model.element.Element e, P p);

    /**
     * A convenience method equivalent to {@code v.visit(e, null)}.
     *
     * @param e the element to visit
     *
     * @return a visitor-specified result
     */
    R visit(javax.lang.model.element.Element e);

    /**
     * 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);

}