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Visitor Design Pattern in Java

Published on August 3, 2022
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By Pankaj
Developer and author at DigitalOcean.
Visitor Design Pattern in Java

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Visitor Design Pattern is one of the behavioral design pattern.

Visitor Design Pattern

Visitor pattern is used when we have to perform an operation on a group of similar kind of Objects. With the help of visitor pattern, we can move the operational logic from the objects to another class. For example, think of a Shopping cart where we can add different type of items (Elements). When we click on checkout button, it calculates the total amount to be paid. Now we can have the calculation logic in item classes or we can move out this logic to another class using visitor pattern. Let’s implement this in our example of visitor pattern.

Visitor Design Pattern Java Example

To implement visitor pattern, first of all we will create different type of items (Elements) to be used in shopping cart. ItemElement.java

package com.journaldev.design.visitor;

public interface ItemElement {

	public int accept(ShoppingCartVisitor visitor);
}

Notice that accept method takes Visitor argument. We can have some other methods also specific for items but for simplicity I am not going into that much detail and focusing on visitor pattern only. Let’s create some concrete classes for different types of items. Book.java

package com.journaldev.design.visitor;

public class Book implements ItemElement {

	private int price;
	private String isbnNumber;
	
	public Book(int cost, String isbn){
		this.price=cost;
		this.isbnNumber=isbn;
	}
	
	public int getPrice() {
		return price;
	}

	public String getIsbnNumber() {
		return isbnNumber;
	}

	@Override
	public int accept(ShoppingCartVisitor visitor) {
		return visitor.visit(this);
	}

}

Fruit.java

package com.journaldev.design.visitor;

public class Fruit implements ItemElement {
	
	private int pricePerKg;
	private int weight;
	private String name;
	
	public Fruit(int priceKg, int wt, String nm){
		this.pricePerKg=priceKg;
		this.weight=wt;
		this.name = nm;
	}
	
	public int getPricePerKg() {
		return pricePerKg;
	}


	public int getWeight() {
		return weight;
	}

	public String getName(){
		return this.name;
	}
	
	@Override
	public int accept(ShoppingCartVisitor visitor) {
		return visitor.visit(this);
	}

}

Notice the implementation of accept() method in concrete classes, its calling visit() method of Visitor and passing itself as argument. We have visit() method for different type of items in Visitor interface that will be implemented by concrete visitor class. ShoppingCartVisitor.java

package com.journaldev.design.visitor;

public interface ShoppingCartVisitor {

	int visit(Book book);
	int visit(Fruit fruit);
}

Now we will implement visitor interface and every item will have it’s own logic to calculate the cost. ShoppingCartVisitorImpl.java

package com.journaldev.design.visitor;

public class ShoppingCartVisitorImpl implements ShoppingCartVisitor {

	@Override
	public int visit(Book book) {
		int cost=0;
		//apply 5$ discount if book price is greater than 50
		if(book.getPrice() > 50){
			cost = book.getPrice()-5;
		}else cost = book.getPrice();
		System.out.println("Book ISBN::"+book.getIsbnNumber() + " cost ="+cost);
		return cost;
	}

	@Override
	public int visit(Fruit fruit) {
		int cost = fruit.getPricePerKg()*fruit.getWeight();
		System.out.println(fruit.getName() + " cost = "+cost);
		return cost;
	}

}

Lets see how we can use visitor pattern example in client applications. ShoppingCartClient.java

package com.journaldev.design.visitor;

public class ShoppingCartClient {

	public static void main(String[] args) {
		ItemElement[] items = new ItemElement[]{new Book(20, "1234"),new Book(100, "5678"),
				new Fruit(10, 2, "Banana"), new Fruit(5, 5, "Apple")};
		
		int total = calculatePrice(items);
		System.out.println("Total Cost = "+total);
	}

	private static int calculatePrice(ItemElement[] items) {
		ShoppingCartVisitor visitor = new ShoppingCartVisitorImpl();
		int sum=0;
		for(ItemElement item : items){
			sum = sum + item.accept(visitor);
		}
		return sum;
	}

}

When we run above visitor pattern client program, we get following output.

Book ISBN::1234 cost =20
Book ISBN::5678 cost =95
Banana cost = 20
Apple cost = 25
Total Cost = 160

Notice that implementation if accept() method in all the items are same but it can be different, for example there can be logic to check if item is free then don’t call the visit() method at all.

Visitor Design Pattern Class Diagram

Class diagram for our visitor design pattern implementation is: visitor design pattern, visitor pattern class diagram

Visitor Pattern Benefits

The benefit of this pattern is that if the logic of operation changes, then we need to make change only in the visitor implementation rather than doing it in all the item classes. Another benefit is that adding a new item to the system is easy, it will require change only in visitor interface and implementation and existing item classes will not be affected.

Visitor Pattern Limitations

The drawback of visitor pattern is that we should know the return type of visit() methods at the time of designing otherwise we will have to change the interface and all of its implementations. Another drawback is that if there are too many implementations of visitor interface, it makes it hard to extend. Thats all for visitor design pattern, let me know if I have missed anything. Please share it with others also if you liked it.

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About the authors
Default avatar
Pankaj

author

Developer and author at DigitalOcean.

Still looking for an answer?

Was this helpful?

Using java 8 the calculate method could be rewritten into this: private static int calculatePrice(ItemElement[] items) { ShoppingCartVisitor visitor = new ShoppingCartVisitorImplementation(); return of(items).mapToInt(itemElement -> itemElement.accept(visitor)).sum(); }

- Leandro Maro

    One can write a generic visit() method …that way you don;t have to change the interface each time a new ItemElement is created. @Override public int visit(ItemElement item) { if (item instanceof Book) { Book book = (Book) item; int cost = 0; // apply 5$ discount if book price is greater than 50 if (book.getPrice() > 50) { cost = book.getPrice() - 5; } else cost = book.getPrice(); System.out.println(“Book ISBN::” + book.getIsbnNumber() + " cost = " + cost); return cost; } else if (item instanceof Fruit) { Fruit fruit = (Fruit) item; int cost = fruit.getPricePerKg() * fruit.getWeight(); System.out.println(fruit.getName() + " cost = " + cost); return cost; } // default return nothing return 0; }

    - shashi

      It’s a nice example of Visitor Design Pattern, I hava some personal opinions about “Visitor Pattern Benefits”. In visitor design pattern, adding a new operation is easy, not adding an item. If we want add a operation such as get summary weight( if possible ), we just need to add an ShoppingCartVisitor’s implementation. However, adding an item will add a file and modify two files, I think it’s not easy . :)

      - robothy

        i’m not sure how : ``` return visitor.visit(this); ``` gives us ShoppingCartVisitorImpl method. Does it automaticly find implementation of interface and uses it ?

        - yami

          Thanks… you explained with great details

          - Parmod

            For me, just do simply like this public interface ItemElement { public int prix (); } public static class Fruit implements ItemElement { … @Override public int prix() { int cost = getPricePerKg()*getWeight(); System.out.println(getName() + " cost = “+cost); return cost; } } public static class Book implements ItemElement { … @Override public int prix() { int cost=0; //apply 5$ discount if book price is greater than 50 if(getPrice() > 50){ cost = getPrice()-5; }else cost = getPrice(); System.out.println(“Book ISBN::”+getIsbnNumber() + " cost =”+cost); return cost; } } The question is what is the advantage of using VisitorPattern ???

            - vqa nguyen

              Hi, I’ve a doubt why the accept methods from Book and Fruit need to have a ShoppingCartVisitor instead of a Visitor one? A ShoppingCartVisitor needs to implements the Visitor interface hence we can allow the polymorphism do its job. If we insist to only use the ShoppingCartVisitor we lose the hability to accept Visitor for more than one Class. Am I right? BR

              - MaikoID

                I would like to visit() each of the below comments (in a loop) and accept() what they have said and collate it for you!!! Thanks.

                - Jay

                  The best explanation of Visitor Pattern I found in net. Great Work

                  - Vijay

                    Thank you for this simple but all explaining example of a fairly neglected Java pattern. Good job!

                    - Maurice Kingsley