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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. 
 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 
javax.lang.model.* packages bundled in Java SE 8 were required to also be runnable on Java SE 7. Therefore, default methods were not used when extending javax.lang.model.* to cover Java SE 8 language features. However, default methods are used in subsequent revisions of the javax.lang.model.* packages that are only required to run on Java SE 8 and higher platform versions. 
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.
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.
 *
 * <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&trade; 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 were required to
 * also be runnable on Java SE 7.  Therefore, default methods
 * were <em>not</em> used when extending {@code javax.lang.model.*}
 * to cover Java SE 8 language features.  However, default methods
 * are 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 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:
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:Visits a ModuleElement by calling 
visitUnknown(e, p).
Returns:a visitor-specified result
Since:9
@specJPMS
/**
     * Visits a module element.
     *
     * @implSpec 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);
    }
}