Module java.compiler
Package javax.lang.model.element

Interface ElementVisitor<R,P>

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.
All Known Implementing Classes:
AbstractElementVisitor14, AbstractElementVisitor6, AbstractElementVisitor7, AbstractElementVisitor8, AbstractElementVisitor9, ElementKindVisitor14, ElementKindVisitor6, ElementKindVisitor7, ElementKindVisitor8, ElementKindVisitor9, ElementScanner14, ElementScanner6, ElementScanner7, ElementScanner8, ElementScanner9, SimpleElementVisitor14, SimpleElementVisitor6, SimpleElementVisitor7, SimpleElementVisitor8, SimpleElementVisitor9
public interface ElementVisitor<R,P>
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.

API Note:
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 visitModule and visitRecordComponent. The implementations of the default methods will in turn call 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 util package. The families follow a naming pattern along the lines of FooVisitorN where N indicates the source version the visitor is appropriate for. In particular, a FooVisitorN 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), FooVisitorN may also handle that later source version; in that case, the SupportedSourceVersion annotation on the FooVisitorN class will indicate a later version. When visiting an element representing a language construct introduced after source version N, a FooVisitorN will throw an 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:

Since:
1.6

  • Method Details

    • visit

      R visit(Element e, P p)
      Visits an element.
      Parameters:
      e - the element to visit
      p - a visitor-specified parameter
      Returns:
      a visitor-specified result

    • visit

      default R visit(Element e)
      A convenience method equivalent to visit(e, null).
      Implementation Requirements:
      The default implementation is visit(e, null).
      Parameters:
      e - the element to visit
      Returns:
      a visitor-specified result

    • visitPackage

      R visitPackage(PackageElement e, P p)
      Visits a package element.
      Parameters:
      e - the element to visit
      p - a visitor-specified parameter
      Returns:
      a visitor-specified result

    • visitType

      R visitType(TypeElement e, P p)
      Visits a type element.
      Parameters:
      e - the element to visit
      p - a visitor-specified parameter
      Returns:
      a visitor-specified result

    • visitVariable

      R visitVariable(VariableElement e, P p)
      Visits a variable element.
      Parameters:
      e - the element to visit
      p - a visitor-specified parameter
      Returns:
      a visitor-specified result

    • visitExecutable

      R visitExecutable(ExecutableElement e, P p)
      Visits an executable element.
      Parameters:
      e - the element to visit
      p - a visitor-specified parameter
      Returns:
      a visitor-specified result

    • visitTypeParameter

      R visitTypeParameter(TypeParameterElement e, P p)
      Visits a type parameter element.
      Parameters:
      e - the element to visit
      p - a visitor-specified parameter
      Returns:
      a visitor-specified result

    • visitUnknown

      R visitUnknown(Element 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.
      Parameters:
      e - the element to visit
      p - a visitor-specified parameter
      Returns:
      a visitor-specified result
      Throws:
      UnknownElementException - a visitor implementation may optionally throw this exception

    • visitModule

      default R visitModule(ModuleElement e, P p)
      Visits a module element.
      Implementation Requirements:
      The default implementation visits a ModuleElement by calling visitUnknown(e, p).
      Parameters:
      e - the element to visit
      p - a visitor-specified parameter
      Returns:
      a visitor-specified result
      Since:
      9

    • visitRecordComponent

      default R visitRecordComponent(RecordComponentElement e, P p)
      Visits a record component element.
      Implementation Requirements:
      The default implementation visits a RecordComponentElement by calling visitUnknown(e, p).
      Parameters:
      e - the element to visit
      p - a visitor-specified parameter
      Returns:
      a visitor-specified result
      Since:
      16