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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: * *
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