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 * This code is distributed in the hope that it will be useful, but WITHOUT

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 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

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 * accompanied this code).

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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 <tt>visit<i>XYZ</i></tt> 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.

 *

 * <p> <b>WARNING:</b> 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.

 *

 * <p>Note that methods to accommodate new language constructs could

 * be added in a source <em>compatible</em> way if they were added as

 * <em>default methods</em>.  However, default methods are only

 * available on Java SE 8 and higher releases and the {@code

 * javax.lang.model.*} packages bundled in Java SE 8 are required to

 * also be runnable on Java SE 7.  Therefore, default methods

 * <em>cannot</em> be used when extending {@code javax.lang.model.*}

 * to cover Java SE 8 language features.  However, default methods may

 * be used in subsequent revisions of the {@code javax.lang.model.*}

 * packages that are only required to run on Java SE 8 and higher

 * platform versions.

 *

 * @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 Ahé

 * @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 v.visit(e, null)}.

     * @param e  the element to visit

     * @return a visitor-specified result

     */

    R visit(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);

}