package org.apidesign.visitor;

 public final class Language {

     private Language() { }

     public interface Expression {

         public abstract void visit(Visitor v);

     }

     public interface Plus extends Expression {

         public Expression getFirst();

         public Expression getSecond();

     }

     public interface Number extends Expression {

         public int getValue();

     }

     // BEGIN: visitor.clientprovider.v2

     /** @since 2.0 */

     public interface Minus extends Expression {

         public Expression getFirst();

         public Expression getSecond();

     }

     public static abstract class Visitor {

         Visitor() {}

         /** @since 2.0 */

         public static Visitor create(Version20 v) {

             return create20(v);

         }

         /** @since 2.0 */

         public interface Version20 extends Version10 {

             public void visitMinus(Minus m);

         }

         /** @since 2.0 */

         public abstract void dispatchNumber(Number n);

     // FINISH: visitor.clientprovider.v2

         public static Visitor create(Version10 v) {

             return create10(v);

         }

         public interface Version10 {

             public boolean visitUnknown(Expression e);

             public void visitPlus(Plus s);

             public void visitNumber(Number n);

         }

         public abstract void dispatchPlus(Plus p);

         public abstract void dispatchMinus(Minus m);

     }

     static Visitor create10(final Visitor.Version10 v) {

         return new Visitor() {

             @Override

             public void dispatchPlus(Plus p) {

                 v.visitPlus(p);

             }

             @Override

             public void dispatchNumber(Number n) {

                 v.visitNumber(n);

             }

             @Override

             public void dispatchMinus(Minus m) {

                 if (v.visitUnknown(m)) {

                     m.getFirst().visit(this);

                     m.getSecond().visit(this);

                 }

             }

         };

     }

     static Visitor create20(final Visitor.Version20 v) {

         return new Visitor() {

             @Override

             public void dispatchPlus(Plus p) {

                 v.visitPlus(p);

             }

             @Override

             public void dispatchNumber(Number n) {

                 v.visitNumber(n);

             }

             @Override

             public void dispatchMinus(Minus m) {

                 v.visitMinus(m);

             }

         };

     }

 }