Why Visitor pattern in LLD? - Purpose & Use Cases

The Big Idea

What if you could add new features without rewriting your whole code every time?

The Scenario

Imagine you have a collection of different shapes like circles, squares, and triangles. You want to perform various operations on them, such as calculating area, drawing, or exporting data. Doing this manually means writing separate code for each shape and each operation, mixing all logic together.

The Problem

This manual approach quickly becomes messy and hard to maintain. Every time you add a new operation, you must change all shape classes. This leads to duplicated code, errors, and lots of time spent updating multiple places.

The Solution

The Visitor pattern lets you separate operations from the objects they work on. You create visitor classes for each operation and let shapes accept visitors. This way, you add new operations without changing shape classes, keeping code clean and easy to extend.

Before vs After

✗ Before

class Circle { double area() { ... } void draw() { ... } }
class Square { double area() { ... } void draw() { ... } }

✓ After

interface Visitor { void visitCircle(Circle c); void visitSquare(Square s); }
class AreaVisitor implements Visitor { ... }
class DrawVisitor implements Visitor { ... }
class Circle { void accept(Visitor v) { v.visitCircle(this); } }
class Square { void accept(Visitor v) { v.visitSquare(this); } }

What It Enables

You can add new operations easily without touching existing shape code, making your system flexible and maintainable.

Real Life Example

In a graphic editor, you can add new features like exporting shapes to different formats or applying filters by just creating new visitors, without changing the shape classes.

Key Takeaways

Separates operations from object structures.

Makes adding new operations easy and safe.

Keeps code organized and maintainable.

Practice

1. What is the main purpose of the Visitor pattern in system design?

easy

A. To separate operations from the objects on which they operate

B. To create multiple instances of a class

C. To restrict access to certain parts of an object

D. To simplify database queries

Solution

1. Step 1: Understand the Visitor pattern concept

   The Visitor pattern allows defining new operations on objects without changing their classes.

2. Step 2: Identify the main goal

   Its main goal is to separate the operation logic from the object structure to keep code flexible.

3. Final Answer:

   To separate operations from the objects on which they operate -> Option A

4. Quick Check:

   Visitor pattern = separates operations [OK]

Hint: Visitor pattern separates operations from objects [OK]

Common Mistakes:

• Confusing Visitor with Singleton pattern
• Thinking it creates object instances
• Assuming it controls access permissions

2. Which of the following is the correct method signature for a visitor interface method visiting an element called ElementA?

easy

A. void accept(ElementA element);

B. void acceptVisitor(ElementA visitor);

C. void visitElementA();

D. void visit(ElementA element);

Solution

1. Step 1: Recall Visitor interface method naming

   The visitor interface defines methods named visit with the element type as parameter.

2. Step 2: Match correct signature

   The correct signature is void visit(ElementA element); to visit ElementA.

3. Final Answer:

   void visit(ElementA element); -> Option D

4. Quick Check:

   Visitor method = visit(Element) [OK]

Hint: Visitor methods are named visit(Element) [OK]

Common Mistakes:

• Confusing accept and visit method names
• Using no parameters in visit method
• Swapping visitor and element in parameters

3. Given the following code snippet, what will be the output?

class ElementA {
  accept(visitor) {
    visitor.visit(this);
  }
}

class PrintVisitor {
  visit(element) {
    console.log('Visited element');
  }
}

const element = new ElementA();
const visitor = new PrintVisitor();
element.accept(visitor);

medium

A. Visited ElementA

B. Error: visit is not a function

C. Visited element

D. No output

Solution

1. Step 1: Trace accept method call

   The accept method calls visitor.visit(this), passing the element instance.

2. Step 2: Check visit method behavior

   The visit method logs 'Visited element' to the console.

3. Final Answer:

   Visited element -> Option C

4. Quick Check:

   Visitor.visit logs message [OK]

Hint: Visitor.visit prints message when accept calls it [OK]

Common Mistakes:

• Expecting element type name in output
• Thinking visit method is missing
• Assuming no output without explicit return

4. Identify the error in this Visitor pattern implementation:

class ElementB {
  accept(visitor) {
    visitor.accept(this);
  }
}

class ConcreteVisitor {
  visit(element) {
    console.log('Visiting element');
  }
}

medium

A. ElementB calls visitor.accept instead of visitor.visit

B. ConcreteVisitor should not have a visit method

C. accept method should return a value

D. ElementB should not have an accept method

Solution

1. Step 1: Check accept method call

   The accept method calls visitor.accept(this), but visitor has no accept method.

2. Step 2: Identify correct visitor method

   The visitor interface defines visit methods, so accept should call visitor.visit(this).

3. Final Answer:

   ElementB calls visitor.accept instead of visitor.visit -> Option A

4. Quick Check:

   accept calls visit, not accept [OK]

Hint: accept calls visitor.visit, not visitor.accept [OK]

Common Mistakes:

• Confusing method names accept and visit
• Expecting accept to return a value
• Removing accept method from element

5. You have a system with multiple element types and want to add a new operation without modifying existing element classes. How does the Visitor pattern help in this scenario?

hard

A. By using inheritance to extend element classes with new operations

B. By creating a new visitor class implementing the operation for all element types

C. By adding new methods to each element class directly

D. By storing operations inside element objects as data

Solution

1. Step 1: Understand the problem of adding new operations

   Modifying existing element classes is risky and breaks encapsulation.

2. Step 2: Apply Visitor pattern solution

   Create a new visitor class that implements the new operation for all element types, keeping element classes unchanged.

3. Final Answer:

   By creating a new visitor class implementing the operation for all element types -> Option B

4. Quick Check:

   Visitor adds operations via new visitor classes [OK]

Hint: Add new visitor class for new operations [OK]

Common Mistakes:

• Modifying element classes directly
• Using inheritance to add operations
• Embedding operations as data in elements