The Visitor pattern

Problem: Determine the postage for the items in a shopping cart.
Solution: The Visitor pattern creates an external class that will use the data of the cart items to compute the postage.

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public interface Visitable {     void accept(Visitor visitor); // implicitly public }

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public class Book implements Visitable {     @Override public void accept(Visitor visitor) {         if (...) visitor.visit(this);     } }

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public interface Visitor {     void visit(Book book); // implicitly public     void visit(CD cd); }

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public class PostageVisitor implements Visitor {     @Override public void visit(Book book) {         this.totalPostage += ...;     }     @Override public void visit(CD cd) {         this.totalPostage += ...;     }     public void getTotalPostage() {         return this.totalPostage;     } }

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public class Client {     public double calculatePostage() {         PostageVisitor visitor = new PostageVisitor();         for (Visitable item : this.items) {             item.accept(visitor);         }         return visitor.getTotalPostage();     } }