Visitor Pattern

The Visitor pattern is a behavioral design pattern that allows you to add new actions to existing object structures without altering them. It consists of two important components: the items being operated on and the visitors who conduct operations on these elements.

Consider an online shopping platform that offers different discount schemes based on the kind of things bought (e.g., groceries, electronics, or apparel) and the customer status (e.g., new customer, loyal customer, or VIP customer).

Implementing and maintaining these discount strategies directly inside the customer and product classes might result in a complicated and tightly coupled system, especially when new discount strategies or customer groups are routinely added.

The Visitor pattern is used to externalize discount strategies from the customer and product classes. This is achieved by:

Creating Visitors for Discounts: Separate classes (visitors) are created for each discount strategy. These visitors encapsulate the logic to calculate discounts based on different criteria.
Visitable Elements (Customers and Products): Customer and product classes are designed to accept visitors. These classes have a method that allows a visitor to perform operations on them.
Applying Discounts: When a discount needs to be applied, the appropriate visitor is passed to the customer or product. The visitor then computes the discount based on its internal logic and the type of customer or product it is dealing with.

This method decouples the discount calculation logic from the customer and product classes, making the system more flexible and easier to extend with new types of discounts or customer categories.

Real-Life Example

In software development, the Visitor pattern is often used in Integrated Development Environments (IDEs) for code analysis and refactoring. Consider an IDE that needs to handle various programming elements like classes, methods, and variables. Each element has its structure and properties.

Specialized visitors, like a Code Analysis Tool visitor, traverse these elements to perform actions like detecting code smells or suggesting optimizations. Another visitor, such as a Refactoring Tool, might visit the same elements to perform refactoring operations, like renaming variables or reorganizing classes.

Each visitor interacts with the elements differently based on the element’s type and the visitor’s purpose. For instance, the way a refactoring tool changes a class structure is different from how it alters a method. This setup allows the IDE to easily extend its functionality by adding new visitors for different tasks without altering the core structure of the programming elements, demonstrating the flexibility and scalability offered by the Visitor pattern.

Structure of Visitor Pattern

Visitor Interface: Visitor interface or abstract class defines the interface to be implemented by concrete visitors, with each method corresponding to a specific element type.
Concrete Visitor: The Concrete Visitor, another key participant, implements the Visitor interface, specifying operations to be performed on the elements. Multiple concrete visitors can exist, each providing a distinct operation.
Element Interface: The Element Interface declares the accept method, enabling a visitor to visit the element by taking the visitor as an argument and invoking the appropriate method.
Concrete Element: The Concrete Element implements the Element interface, defines the accept method, and calls the corresponding visit method on the visitor to perform operations on the element.
Object: Finally, the Object Structure represents a collection or structure of elements accepting visitors and typically facilitates iteration over its elements.

Implementation of Visitor Pattern

Let’s recall the retail shopping example discussed earlier. In a retail shopping application, there are various types of products like Food, Clothing, and Electronics. The application frequently needs to apply different discount strategies to these products, particularly during different seasons or events. For instance, there may be a clearance sale during the end of the season or a holiday discount during Christmas.

Let’s look at the pseudocode of this example:

// Visitor Interface
interface DiscountVisitor {
    visitFood(Food food)
    visitClothing(Clothing clothing)
    visitElectronics(Electronics electronics)
}

// Concrete Visitors
class HolidayDiscountVisitor implements DiscountVisitor {
    visitFood(food) {
        // Apply holiday discount logic for food
    }
    visitClothing(clothing) {
        // Apply holiday discount logic for clothing
    }
    visitElectronics(electronics) {
        // Apply holiday discount logic for electronics
    }
}

class ClearanceDiscountVisitor implements DiscountVisitor {
    visitFood(food) {
        // Apply clearance discount logic for food
    }
    visitClothing(clothing) {
        // Apply clearance discount logic for clothing
    }
    visitElectronics(electronics) {
        // Apply clearance discount logic for electronics
    }
}

// Element Interface
interface Product {
    accept(DiscountVisitor visitor)
}

// Concrete Elements
class Food implements Product {
    accept(visitor) {
        visitor.visitFood(this)
    }
}

class Clothing implements Product {
    accept(visitor) {
        visitor.visitClothing(this)
    }
}

class Electronics implements Product {
    accept(visitor) {
        visitor.visitElectronics(this)
    }
}

// Usage
food = new Food()
clothing = new Clothing()
electronics = new Electronics()

holidayVisitor = new HolidayDiscountVisitor()
clearanceVisitor = new ClearanceDiscountVisitor()

// Applying different discounts
food.accept(holidayVisitor)
clothing.accept(clearanceVisitor)
electronics.accept(holidayVisitor)

Visitor Interface (DiscountVisitor): This defines a contract for the visitors, listing methods for each type of visitable element (in this case, different product types).
Concrete Visitors (HolidayDiscountVisitor, ClearanceDiscountVisitor): Implementations of the visitor interface, each encapsulating the logic for a specific type of discount (e.g., holiday, clearance). Each visitor knows how to apply its discount to different product types.
Element Interface (Product): Defines an interface for elements that can be visited. The key method here is accept, which takes a visitor as an argument.
Concrete Elements (Food, Clothing, Electronics): These are the actual objects to which we want to apply operations (discounts, in this case). Each element implements the accept method to allow a visitor to perform an operation on it.
Usage: To apply a discount, you create an instance of a visitor (e.g., HolidayDiscountVisitor) and pass it to the accept method of a product. The visitor then applies the appropriate logic based on the type of product.

Implementation

package com.ashok.designpatterns.visitor;
/**
 * 
 * @author ashok.mariyala
 *
 */
interface DiscountVisitor {
    void visitFood(Food food);
    void visitClothing(Clothing clothing);
    void visitElectronics(Electronics electronics);
}

interface Product {
    void accept(DiscountVisitor visitor);
}

class Food implements Product {
    public void accept(DiscountVisitor visitor) {
        visitor.visitFood(this);
    }
}

class Clothing implements Product {
    public void accept(DiscountVisitor visitor) {
        visitor.visitClothing(this);
    }
}

class Electronics implements Product {
    public void accept(DiscountVisitor visitor) {
        visitor.visitElectronics(this);
    }
}

class HolidayDiscountVisitor implements DiscountVisitor {
    public void visitFood(Food food) {
        System.out.println("Applying holiday discount to food.");
    }
    public void visitClothing(Clothing clothing) {
        System.out.println("Applying holiday discount to clothing.");
    }
    public void visitElectronics(Electronics electronics) {
        System.out.println("Applying holiday discount to electronics.");
    }
}

class ClearanceDiscountVisitor implements DiscountVisitor {
    public void visitFood(Food food) {
        System.out.println("Applying clearance discount to food.");
    }
    public void visitClothing(Clothing clothing) {
        System.out.println("Applying clearance discount to clothing.");
    }
    public void visitElectronics(Electronics electronics) {
        System.out.println("Applying clearance discount to electronics.");
    }
}

public class Solution {
    public static void main(String[] args) {
        Product food = new Food();
        Product clothing = new Clothing();
        Product electronics = new Electronics();

        DiscountVisitor holidayVisitor = new HolidayDiscountVisitor();
        DiscountVisitor clearanceVisitor = new ClearanceDiscountVisitor();

        food.accept(holidayVisitor);
        clothing.accept(clearanceVisitor);
        electronics.accept(holidayVisitor);
    }
}

Applications of Visitor Pattern

The Visitor Pattern is a behavioral design pattern that is widely used in various applications to separate algorithms from the objects on which they operate. Here are some common applications of the Visitor Pattern:

1. Document Object Model (DOM) Traversal

In web development, the Visitor Pattern can be applied to traverse and operate on nodes in a DOM tree. Different visitors can perform various operations on different types of nodes, providing a flexible way to process complex document structures.

2. Graphic Design Applications

In graphic design software, the Visitor Pattern can be employed to apply different filters or effects to various graphic elements. Visitors can encapsulate different image processing algorithms without modifying the classes of the graphic elements.

3. Database Query Optimization

Visitors can be utilized in database query optimization. A visitor may traverse a query execution plan and apply optimization techniques to different types of query nodes, such as join operations or filtering conditions.

4. Game Development

In game development, the Visitor Pattern can be applied to traverse and operate on the elements of a game world. Different visitors can handle tasks such as rendering, collision detection, or artificial intelligence for different types of game objects.

5. GUI Components

When working with graphical user interfaces, the Visitor Pattern can be used to perform operations on different types of GUI components. For instance, visitors can be employed to validate input fields, update the display of components, or handle user interactions.

6. Abstract Syntax Tree (AST) Transformations

In addition to compiler design, the Visitor Pattern is commonly employed in language processing to perform transformations on abstract syntax trees. Each visitor encapsulates a specific transformation, promoting modularity and extensibility.

These applications highlight the versatility of the Visitor Pattern in scenarios where there is a need to perform different operations on a set of related but distinct objects without modifying their class definitions

In conclusion, while the Visitor Pattern offers advantages in terms of separation of concerns, extensibility, and adherence to design principles, it also introduces complexities and potential challenges, especially in large and evolving systems. Careful consideration should be given to whether the benefits outweigh the drawbacks in a specific context.

The Visitor Pattern is a powerful tool for scenarios where an object structure needs to support many unrelated operations without cluttering the element classes with these operations.

That’s all about the Visitor Design Pattern. If you have any queries or feedback, please write us email at contact@waytoeasylearn.com. Enjoy learning, Enjoy Design patterns.!!

Visitor Pattern