javax.lang.model.element.ElementVisitor Maven / Gradle / Ivy
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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 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.
*
* @apiNote
* 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.
*
* 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.
*
*
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}. 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.
*
*
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}N where N indicates the
* {@linkplain javax.lang.model.SourceVersion source version} the
* visitor is appropriate for.
*
* In particular, a {@code FooVisitor}N is expected to handle
* all language constructs present in source version N. If
* there are no new language constructs added in version
* N + 1 (or subsequent releases), {@code
* FooVisitor}N may also handle that later source version; in
* that case, the {@link
* javax.annotation.processing.SupportedSourceVersion
* SupportedSourceVersion} annotation on the {@code
* FooVisitor}N class will indicate a later version.
*
* When visiting an element representing a language construct
* introduced after source version N, a {@code
* FooVisitor}N will throw an {@link UnknownElementException}
* unless that behavior is overridden.
*
*
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:
*
*
*
* - {@link AbstractElementVisitor6 AbstractElementVisitor}s:
* Skeletal visitor implementations.
*
*
- {@link SimpleElementVisitor6 SimpleElementVisitor}s: Support
* default actions and a default return value.
*
*
- {@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.
*
*
- {@link ElementScanner6 ElementScanner}s: Scanners are visitors
* which traverse an element and the elements {@linkplain
* Element#getEnclosedElements enclosed} by it and associated with it.
*
*
*
* @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
* @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(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);
}
}