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package javax.lang.model.type;
import javax.lang.model.element.*;
import javax.lang.model.util.*;
/**
* A visitor of types, in the style of the
* visitor design pattern. Classes implementing this
* interface are used to operate on a type when the kind of
* type is unknown at compile time. When a visitor is passed to a
* type's {@link TypeMirror#accept accept} method, the visitXyz
* method most applicable to that type 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. 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 a type representing a language construct
* introduced after source version N, a {@code
* FooVisitor}N will throw an {@link UnknownTypeException}
* 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 AbstractTypeVisitor6 AbstractTypeVisitor}s:
* Skeletal visitor implementations.
*
*
- {@link SimpleTypeVisitor6 SimpleTypeVisitor}s: Support
* default actions and a default return value.
*
*
- {@link TypeKindVisitor6 TypeKindVisitor}s: Visit methods
* provided on a {@linkplain TypeMirror#getKind per-kind} granularity
* as some categories of types can have more than one kind.
*
*
*
* @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 TypeVisitor {
/**
* Visits a type.
* @param t the type to visit
* @param p a visitor-specified parameter
* @return a visitor-specified result
*/
R visit(TypeMirror t, P p);
/**
* A convenience method equivalent to {@code visit(t, null)}.
*
* @implSpec The default implementation is {@code visit(t, null)}.
*
* @param t the element to visit
* @return a visitor-specified result
*/
default R visit(TypeMirror t) {
return visit(t, null);
}
/**
* Visits a primitive type.
* @param t the type to visit
* @param p a visitor-specified parameter
* @return a visitor-specified result
*/
R visitPrimitive(PrimitiveType t, P p);
/**
* Visits the null type.
* @param t the type to visit
* @param p a visitor-specified parameter
* @return a visitor-specified result
*/
R visitNull(NullType t, P p);
/**
* Visits an array type.
* @param t the type to visit
* @param p a visitor-specified parameter
* @return a visitor-specified result
*/
R visitArray(ArrayType t, P p);
/**
* Visits a declared type.
* @param t the type to visit
* @param p a visitor-specified parameter
* @return a visitor-specified result
*/
R visitDeclared(DeclaredType t, P p);
/**
* Visits an error type.
* @param t the type to visit
* @param p a visitor-specified parameter
* @return a visitor-specified result
*/
R visitError(ErrorType t, P p);
/**
* Visits a type variable.
* @param t the type to visit
* @param p a visitor-specified parameter
* @return a visitor-specified result
*/
R visitTypeVariable(TypeVariable t, P p);
/**
* Visits a wildcard type.
* @param t the type to visit
* @param p a visitor-specified parameter
* @return a visitor-specified result
*/
R visitWildcard(WildcardType t, P p);
/**
* Visits an executable type.
* @param t the type to visit
* @param p a visitor-specified parameter
* @return a visitor-specified result
*/
R visitExecutable(ExecutableType t, P p);
/**
* Visits a {@link NoType} instance.
* @param t the type to visit
* @param p a visitor-specified parameter
* @return a visitor-specified result
*/
R visitNoType(NoType t, P p);
/**
* Visits an unknown kind of type.
* This can occur if the language evolves and new kinds
* of types are added to the {@code TypeMirror} hierarchy.
* @param t the type to visit
* @param p a visitor-specified parameter
* @return a visitor-specified result
* @throws UnknownTypeException
* a visitor implementation may optionally throw this exception
*/
R visitUnknown(TypeMirror t, P p);
/**
* Visits a union type.
*
* @param t the type to visit
* @param p a visitor-specified parameter
* @return a visitor-specified result
* @since 1.7
*/
R visitUnion(UnionType t, P p);
/**
* Visits an intersection type.
*
* @param t the type to visit
* @param p a visitor-specified parameter
* @return a visitor-specified result
* @since 1.8
*/
R visitIntersection(IntersectionType t, P p);
}