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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.
*
* @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
* @see AbstractElementVisitor6
* @see AbstractElementVisitor7
* @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);
}