# Interface TypeVisitor<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:
  `AbstractTypeVisitor14, AbstractTypeVisitor6, AbstractTypeVisitor7, AbstractTypeVisitor8, AbstractTypeVisitor9, AbstractTypeVisitorPreviewPREVIEW, SimpleTypeVisitor14, SimpleTypeVisitor6, SimpleTypeVisitor7, SimpleTypeVisitor8, SimpleTypeVisitor9, SimpleTypeVisitorPreviewPREVIEW, TypeKindVisitor14, TypeKindVisitor6, TypeKindVisitor7, TypeKindVisitor8, TypeKindVisitor9, TypeKindVisitorPreviewPREVIEW`

public interface TypeVisitor<R,P>
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 `accept` method, the `visitXyz` method most applicable to that type 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. 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 `FooVisitor` N where N indicates the source version the visitor is appropriate for. In particular, a `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), `FooVisitor` N may also handle that later source version; in that case, the `SupportedSourceVersion` annotation on the `FooVisitor` N class will indicate a later version. When visiting a type representing a language construct introduced after source version N, a `FooVisitor` N will throw an `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:

  - `AbstractTypeVisitor`s: Skeletal visitor implementations.
  - `SimpleTypeVisitor`s: Support default actions and a default return value.
  - `TypeKindVisitor`s: Visit methods provided on a per-kind granularity as some categories of types can have more than one kind.
Since:
  1.6

- ## Method Summary

  Modifier and Type
  Method
  Description
  `default R`
  `visit(TypeMirror t)`
  A convenience method equivalent to `visit(t, null)`.
  `R`
  `visit(TypeMirror t, P p)`
  Visits a type.
  `R`
  `visitArray(ArrayType t, P p)`
  Visits an array type.
  `R`
  `visitDeclared(DeclaredType t, P p)`
  Visits a declared type.
  `R`
  `visitError(ErrorType t, P p)`
  Visits an error type.
  `R`
  `visitExecutable(ExecutableType t, P p)`
  Visits an executable type.
  `R`
  `visitIntersection(IntersectionType t, P p)`
  Visits an intersection type.
  `R`
  `visitNoType(NoType t, P p)`
  Visits a `NoType` instance.
  `R`
  `visitNull(NullType t, P p)`
  Visits the null type.
  `R`
  `visitPrimitive(PrimitiveType t, P p)`
  Visits a primitive type.
  `R`
  `visitTypeVariable(TypeVariable t, P p)`
  Visits a type variable.
  `R`
  `visitUnion(UnionType t, P p)`
  Visits a union type.
  `R`
  `visitUnknown(TypeMirror t, P p)`
  Visits an unknown kind of type.
  `R`
  `visitWildcard(WildcardType t, P p)`
  Visits a wildcard type.

- ## Method Details

  - ### visit

    R visit(TypeMirror t, P p)
    Visits a type.
    Parameters:
      `t` - the type to visit
      `p` - a visitor-specified parameter
    Returns:
      a visitor-specified result

  - ### visit

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

  - ### visitPrimitive

    R visitPrimitive(PrimitiveType t, P p)
    Visits a primitive type.
    Parameters:
      `t` - the type to visit
      `p` - a visitor-specified parameter
    Returns:
      a visitor-specified result

  - ### visitNull

    R visitNull(NullType t, P p)
    Visits the null type.
    Parameters:
      `t` - the type to visit
      `p` - a visitor-specified parameter
    Returns:
      a visitor-specified result

  - ### visitArray

    R visitArray(ArrayType t, P p)
    Visits an array type.
    Parameters:
      `t` - the type to visit
      `p` - a visitor-specified parameter
    Returns:
      a visitor-specified result

  - ### visitDeclared

    R visitDeclared(DeclaredType t, P p)
    Visits a declared type.
    Parameters:
      `t` - the type to visit
      `p` - a visitor-specified parameter
    Returns:
      a visitor-specified result

  - ### visitError

    R visitError(ErrorType t, P p)
    Visits an error type.
    Parameters:
      `t` - the type to visit
      `p` - a visitor-specified parameter
    Returns:
      a visitor-specified result

  - ### visitTypeVariable

    R visitTypeVariable(TypeVariable t, P p)
    Visits a type variable.
    Parameters:
      `t` - the type to visit
      `p` - a visitor-specified parameter
    Returns:
      a visitor-specified result

  - ### visitWildcard

    R visitWildcard(WildcardType t, P p)
    Visits a wildcard type.
    Parameters:
      `t` - the type to visit
      `p` - a visitor-specified parameter
    Returns:
      a visitor-specified result

  - ### visitExecutable

    R visitExecutable(ExecutableType t, P p)
    Visits an executable type.
    Parameters:
      `t` - the type to visit
      `p` - a visitor-specified parameter
    Returns:
      a visitor-specified result

  - ### visitNoType

    R visitNoType(NoType t, P p)
    Visits a `NoType` instance.
    Parameters:
      `t` - the type to visit
      `p` - a visitor-specified parameter
    Returns:
      a visitor-specified result

  - ### visitUnknown

    R visitUnknown(TypeMirror 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 `TypeMirror` hierarchy.
    Parameters:
      `t` - the type to visit
      `p` - a visitor-specified parameter
    Returns:
      a visitor-specified result
    Throws:
      `UnknownTypeException` - a visitor implementation may optionally throw this exception

  - ### visitUnion

    R visitUnion(UnionType t, P p)
    Visits a union type.
    Parameters:
      `t` - the type to visit
      `p` - a visitor-specified parameter
    Returns:
      a visitor-specified result
    Since:
      1.7

  - ### visitIntersection

    R visitIntersection(IntersectionType t, P p)
    Visits an intersection type.
    Parameters:
      `t` - the type to visit
      `p` - a visitor-specified parameter
    Returns:
      a visitor-specified result
    Since:
      1.8