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+/* AbstractTypeVisitor6.java -- A visitor of types for 1.6.
+   Copyright (C) 2015  Free Software Foundation, Inc.
+
+This file is part of GNU Classpath.
+
+GNU Classpath is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 2, or (at your option)
+any later version.
+
+GNU Classpath 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 for more details.
+
+You should have received a copy of the GNU General Public License
+along with GNU Classpath; see the file COPYING.  If not, write to the
+Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+02110-1301 USA.
+
+Linking this library statically or dynamically with other modules is
+making a combined work based on this library.  Thus, the terms and
+conditions of the GNU General Public License cover the whole
+combination.
+
+As a special exception, the copyright holders of this library give you
+permission to link this library with independent modules to produce an
+executable, regardless of the license terms of these independent
+modules, and to copy and distribute the resulting executable under
+terms of your choice, provided that you also meet, for each linked
+independent module, the terms and conditions of the license of that
+module.  An independent module is a module which is not derived from
+or based on this library.  If you modify this library, you may extend
+this exception to your version of the library, but you are not
+obligated to do so.  If you do not wish to do so, delete this
+exception statement from your version. */
+
+package javax.lang.model.util;
+
+import javax.lang.model.SourceVersion;
+
+import javax.lang.model.type.TypeMirror;
+import javax.lang.model.type.TypeVisitor;
+import javax.lang.model.type.UnionType;
+import javax.lang.model.type.UnknownTypeException;
+
+/**
+ * <p>A skeletal implementation of {@link TypeVisitor} for the
+ * 1.6 version of the Java programming language
+ * ({@link SourceVersion#RELEASE_6}).  Implementors can extend this
+ * class and need provide only implementations of the
+ * {@code visitXYZ} methods appropriate to 1.6.</p>
+ * <p>As the interface this class implements may be extended in future,
+ * in order to support later language versions, methods beginning with
+ * the phrase {@code "visit"} should be avoided in subclasses.  This
+ * class itself will be extended to direct these new methods to the
+ * {@link #visitUnknown(TypeMirror,P) method and a new class will be
+ * added to provide implementations for the new language version.
+ * At this time, all or some of this class may be deprecated.</p>
+ *
+ * @param <R> the return type of the visitor's methods.  {@code Void}
+ *            can be used where there is no return value.
+ * @param <P> the type of the additional parameter supplied to the visitor's
+ *            methods. {@code Void} can be used where no additional
+ *            parameter is required.
+ *
+ * @author Andrew John Hughes (gnu_andrew@member.fsf.org)
+ * @since 1.6
+ */
+public abstract class AbstractTypeVisitor6<R,P> implements TypeVisitor<R,P>
+{
+
+  /**
+   * Constructs a new {@code AbstractTypeVisitor6}.
+   */
+  protected AbstractTypeVisitor6() {}
+
+  /**
+   * Visits a type mirror by passing itself to the type
+   * mirror's {@code accept} method, with {@code null} for the additional
+   * parameter i.e. {@code{v.visit(typeMirror)} is equivalent to 
+   * {@code{typeMirror.accept(v, null)}.
+   *
+   * @param typeMirror the type to visit.
+   * @return the return value specific to the visitor.
+   */
+  @Override
+  public final R visit(TypeMirror typeMirror)
+  {
+    return typeMirror.accept(this, null);
+  }
+
+  /**
+   * Visits a type mirror by passing itself to the type
+   * mirror's {@code accept} method, with the specified value as the additional
+   * parameter i.e. {@code{v.visit(typeMirror, parameter)} is equivalent to 
+   * {@code{typeMirror.accept(v, parameter)}.
+   *
+   * @param typeMirror the type to visit.
+   * @param parameter the value to use as the additional parameter.
+   * @return the return value specific to the visitor.
+   */
+  @Override
+  public final R visit(TypeMirror typeMirror, P parameter)
+  {
+    return typeMirror.accept(this, parameter);
+  }
+
+  /**
+   * Visits an unknown type mirror.  This method is called if
+   * this visitor is used in a version of the language later
+   * than 1.6, where new types have been added.  This
+   * implementation always throws a {@link UnknownTypeException}
+   * but this is not required of subclasses.
+   *
+   * @param typeMirror the type to visit.
+   * @param parameter the additional parameter, specific to the visitor.
+   * @return the return value specific to the visitor.
+   * @throws UnknownTypeException by default.
+   */
+  @Override
+  public R visitUnknown(TypeMirror typeMirror, P parameter)
+  {
+    throw new UnknownTypeException(typeMirror, parameter);
+  }
+
+  /**
+   * Provides a default implementation for the {@code visitUnion} method
+   * added in 1.7, so that existing subclasses continue to compile. It
+   * simply redirects to {@link #visitUnknown(TypeMirror,P)}.
+   *
+   * @param type the union type to visit.
+   * @param param the additional parameter, specific to the visitor.
+   *        May be {@code null} if permitted by the visitor.
+   * @return the return value specific to the visitor.
+   * @since 1.7
+   */
+  @Override
+  public R visitUnion(UnionType type, P parameter)
+  {
+    return visitUnknown(type, parameter);
+  }
+
+}