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🔔 Subscribe to Ram N JavaThe Problem: Adding Operations to Existing Classes
Imagine an e-commerce shopping cart containing various products—such as books, electronics, and fruits. Each product type has distinct pricing, shipping rules, and tax calculation formulas.
If you add new operations (like calculating discounted prices, export tariffs, or generating itemized receipts) directly into each product class, you violate the Open/Closed Principle. Every time business requirements introduce a new calculation or reporting feature, you are forced to modify and retest every product class.
The Solution: The Visitor Design Pattern
The Visitor Design Pattern separates an algorithm from the object structure on which it operates using a technique known as Double Dispatch:
-
Element Interface (
Visitable): Declares anaccept(Visitor visitor)method implemented by each product (e.g.,Book,Fruit). -
Visitor Interface: Defines dedicated visit methods for each concrete product type, such as
visit(Book book)andvisit(Fruit fruit). -
Concrete Visitor: Encapsulates specific operational algorithms (e.g.,
ShoppingCartVisitor), computing costs and discounts based on the exact product type without polluting the product models.
Key Architectural Benefits
Applying the Visitor pattern to structured collections offers several advantages:
- Open/Closed Principle: Add new operations (like tax auditing or loyalty points calculation) simply by introducing a new visitor class, leaving product classes untouched.
- Single Responsibility Principle: Gathers related operations into a single class rather than scattering them across different element definitions.
- Accumulated State: Visitors can maintain running states (such as running totals or item summaries) as they traverse a complex object structure.
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| Visitor Design pattern - Implementation [Products] |
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| Visitor Design pattern - Implementation [Products] - Class Diagram |
Product.java
/*
* Visitable Class
*/
public abstract class Product
{
protected int price;
public abstract void accept( ShoppingMallVisitor shoppingMallVisitor );
public int getPrice()
{
return price;
}
}
Book.java
public class Book extends Product
{
private String name;
private String isbnNumber;
public Book( String name, String isbnNumber, int price )
{
super();
this.name = name;
this.isbnNumber = isbnNumber;
this.price = price;
}
public String getName()
{
return name;
}
public String getIsbnNumber()
{
return isbnNumber;
}
@Override
public void accept( ShoppingMallVisitor shoppingMallVisitor )
{
shoppingMallVisitor.visit(this);
}
@Override
public String toString()
{
return "Book [name=" + name + ", isbnNumber=" + isbnNumber + ", price=" + price + "]";
}
}
DVD.java
public class DVD extends Product
{
private String name;
public DVD( String name, int price )
{
super();
this.name = name;
this.price = price;
}
public String getName()
{
return name;
}
@Override
public void accept( ShoppingMallVisitor shoppingMallVisitor )
{
shoppingMallVisitor.visit(this);
}
@Override
public String toString()
{
return "DVD [name=" + name + ", price=" + price + "]";
}
}
Shirt.java
public class Shirt extends Product
{
private String brandName;
private String size;
public Shirt( String brandName, String size, int price )
{
super();
this.brandName = brandName;
this.size = size;
this.price = price;
}
public String getBrandName()
{
return brandName;
}
public String getSize()
{
return size;
}
@Override
public void accept( ShoppingMallVisitor shoppingMallVisitor )
{
shoppingMallVisitor.visit(this);
}
@Override
public String toString()
{
return "Shirt [brandName=" + brandName + ", size=" + size + ", price=" + price + "]";
}
}
ShoppingMallVisitor.java
public interface ShoppingMallVisitor
{
public void visit(Product product);
}
Cashier.java
public class Cashier implements ShoppingMallVisitor
{
private String name;
private int totalPrice = 0;
public int getTotalPrice()
{
return totalPrice;
}
public Cashier( String name )
{
super();
this.name = name;
}
public String getName()
{
return name;
}
@Override
public void visit( Product product )
{
totalPrice = totalPrice + product.getPrice();
}
}
ShoppingMallProducts.java
import java.util.ArrayList;
import java.util.LinkedHashMap;
public class ShoppingMallProducts
{
static LinkedHashMap<String, Product> productMap;
static
{
productMap = new LinkedHashMap<String, Product>();
Book weArebackBook = new Book("We are Back", "ISBN-999", 200);
Book jesusStoryBook = new Book("Jesus Story", "ISBN-1999", 500);
Book moralStoryBook = new Book("Moral Story", "ISBN-2999", 600);
DVD dieHard2Dvd = new DVD("Die Hard 2", 100);
DVD terminatorDvd = new DVD("The Terminator", 150);
DVD titanicDvd = new DVD("Titanic", 120);
Shirt zeroShirt = new Shirt("Zero", "42", 1500);
Shirt peterEnglandShirt = new Shirt("Peter England", "42", 2500);
Shirt johnPlayersShirt = new Shirt("John Players", "40", 2500);
productMap.put("weArebackBook", weArebackBook);
productMap.put("jesusStoryBook", jesusStoryBook);
productMap.put("moralStoryBook", moralStoryBook);
productMap.put("dieHard2Dvd", dieHard2Dvd);
productMap.put("terminatorDvd", terminatorDvd);
productMap.put("titanicDvd", titanicDvd);
productMap.put("zeroShirt", zeroShirt);
productMap.put("peterEnglandShirt", peterEnglandShirt);
productMap.put("johnPlayersShirt", johnPlayersShirt);
}
public int getTotalAmount( ArrayList<Product> purchasedProductList )
{
Cashier cashier = new Cashier("John");
System.out.println("Puschased Products details : \n");
for( Product product : purchasedProductList )
{
// Each product accepts the visitor
product.accept(cashier);
System.out.println(product+"\n");
}
return cashier.getTotalPrice();
}
}
Client.java
import java.util.ArrayList;
import java.util.LinkedHashMap;
import java.util.Set;
public class Client
{
public static void main( String[] args )
{
LinkedHashMap<String, Product> productMap = ShoppingMallProducts.productMap;
Set<String> products=productMap.keySet();
System.out.println("Products Available \n");
for( String string : products )
{
System.out.println(productMap.get(string));
}
System.out.println(" \n############################################################# \n");
ArrayList<Product> purchasedProductList = new ArrayList<Product>();
purchasedProductList.add(productMap.get("weArebackBook"));
purchasedProductList.add(productMap.get("jesusStoryBook"));
purchasedProductList.add(productMap.get("dieHard2Dvd"));
purchasedProductList.add(productMap.get("terminatorDvd"));
purchasedProductList.add(productMap.get("zeroShirt"));
purchasedProductList.add(productMap.get("peterEnglandShirt"));
ShoppingMallProducts shoppingMallProducts = new ShoppingMallProducts();
System.out.println("Total Amount : "+shoppingMallProducts.getTotalAmount(purchasedProductList) + "Rs");
}
}
Output
Products Available
Book [name=We are Back, isbnNumber=ISBN-999, price=200]
Book [name=Jesus Story, isbnNumber=ISBN-1999, price=500]
Book [name=Moral Story, isbnNumber=ISBN-2999, price=600]
DVD [name=Die Hard 2, price=100]
DVD [name=The Terminator, price=150]
DVD [name=Titanic, price=120]
Shirt [brandName=Zero, size=42, price=1500]
Shirt [brandName=Peter England, size=42, price=2500]
Shirt [brandName=John Players, size=40, price=2500]
#############################################################
Puschased Products details :
Book [name=We are Back, isbnNumber=ISBN-999, price=200]
Book [name=Jesus Story, isbnNumber=ISBN-1999, price=500]
DVD [name=Die Hard 2, price=100]
DVD [name=The Terminator, price=150]
Shirt [brandName=Zero, size=42, price=1500]
Shirt [brandName=Peter England, size=42, price=2500]
Total Amount : 4950Rs
