Visitor pattern in LLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of the Visitor pattern?

The Visitor pattern lets you add new operations to objects without changing their classes. It separates an algorithm from the objects it works on.

beginner

In the Visitor pattern, what roles do 'Element' and 'Visitor' play?

Elements are objects that accept visitors. Visitors define operations to perform on elements. Elements have an 'accept' method that takes a visitor.

intermediate

Why is the Visitor pattern useful when you want to add new operations frequently?

Because you can add new visitor classes with new operations without changing the element classes, avoiding code changes and recompilation of elements.

intermediate

What is a common drawback of the Visitor pattern?

It can be hard to add new element types because you must update all visitor classes to handle the new element.

advanced

How does the Visitor pattern relate to the Open/Closed Principle?

It helps keep element classes closed for modification but open for extension by adding new visitors for new operations.

What method must an element implement to work with a visitor?

A accept(visitor)

B visit(element)

C execute(visitor)

D handle(visitor)

Elements implement an accept(visitor) method to allow visitors to perform operations on them.

Which of these is a benefit of using the Visitor pattern?

A Simplify element class hierarchy

B Easily add new element types without changing visitors

C Add new operations without changing element classes

D Avoid double dispatch

Visitor pattern allows adding new operations by creating new visitors, without modifying element classes.

What is a typical downside of the Visitor pattern?

A Hard to add new operations

B Visitors cannot access element data

C Elements must know visitor internals

D Hard to add new element types

Adding new element types requires updating all visitor classes, which can be cumbersome.

Which principle does the Visitor pattern help to follow?

A Single Responsibility Principle

B Open/Closed Principle

C Liskov Substitution Principle

D Dependency Inversion Principle

Visitor pattern keeps element classes closed for modification but open for extension via new visitors.

In the Visitor pattern, what is double dispatch?

A Selecting a method based on both visitor and element types

B Calling two methods in sequence

C Calling visitor twice on the same element

D Using two visitors on one element

Double dispatch means the operation depends on both the visitor and element types, enabling correct method selection.

Explain how the Visitor pattern separates operations from object structures and why this is useful.

Think about how you can add new features without touching existing code.

Describe a scenario where using the Visitor pattern would be beneficial and one where it might cause problems.

Consider how often operations or element types change.

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