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   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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   * This code is free software; you can redistribute it and/or modify it
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   * published by the Free Software Foundation.  Oracle designates this
   * particular file as subject to the "Classpath" exception as provided
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   * This code is distributed in the hope that it will be useful, but WITHOUT
   * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   * version 2 for more details (a copy is included in the LICENSE file that
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   * You should have received a copy of the GNU General Public License version
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  package jdkx.lang.model.type;
  
  import jdkx.lang.model.element.*;
  import jdkx.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 <code>visit<i>Xyz</i></code>
   * method most applicable to that type is invoked.
   *
   * <p> 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
   * <strong>WARNING:</strong> 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.
   *
   * <p>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.
   *
   * <p>There are several families of classes implementing this visitor
   * interface in the {@linkplain jdkx.lang.model.util util
   * package}. The families follow a naming pattern along the lines of
   * {@code FooVisitor}<i>N</i> where <i>N</i> indicates the
   * {@linkplain jdkx.lang.model.SourceVersion source version} the
   * visitor is appropriate for.
   *
   * In particular, a {@code FooVisitor}<i>N</i> is expected to handle
   * all language constructs present in source version <i>N</i>. If
   * there are no new language constructs added in version
   * <i>N</i>&nbsp;+&nbsp;1 (or subsequent releases), {@code
   * FooVisitor}<i>N</i> may also handle that later source version; in
   * that case, the {@link
   * jdkx.annotation.processing.SupportedSourceVersion
   * SupportedSourceVersion} annotation on the {@code
   * FooVisitor}<i>N</i> class will indicate a later version.
   *
   * When visiting a type representing a language construct
   * introduced <strong>after</strong> source version <i>N</i>, a {@code
   * FooVisitor}<i>N</i> will throw an {@link UnknownTypeException}
   * unless that behavior is overridden.
   *
   * <p>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:
   *
   * <ul>
   *
   * <li>{@link AbstractTypeVisitor6 AbstractTypeVisitor}s:
   * Skeletal visitor implementations.
   *
   * <li>{@link SimpleTypeVisitor6 SimpleTypeVisitor}s: Support
   * default actions and a default return value.
   *
   * <li>{@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.
   *
   * </ul>
   *
   * @param <R> the return type of this visitor's methods.  Use {@link
   *            Void} for visitors that do not need to return results.
   * @param <P> 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 Ah&eacute;
   * @since 1.6
   */
  public interface TypeVisitor<R, P> {
      /**
       * 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);
  }
  