package org.apidesign.visitor;

 public final class Language {

     private Language() { }

     public static abstract class Expression {

         Expression() {}

         public abstract void visit(Visitor v);

     }

     public static final class Plus extends Expression {

         private final Expression first;

         private final Expression second;

         public Plus(Expression first, Expression second) {

             this.first = first;

             this.second = second;

         }

         public Expression getFirst() { return first; }

         public Expression getSecond() { return second; }

         @Override

         public void visit(Visitor v) { v.visitPlus(this); }

     }

     public static final class Number extends Expression {

         private final int value;

         public Number(int value) { this.value = value; }

         public int getValue() { return value; }

         @Override

         public void visit(Visitor v) { v.visitNumber(this); }

     }

     public interface Visitor {

         public void visitUnknown(Expression e);

         public void visitPlus(Plus s);

         public void visitNumber(Number n);

     }

 }