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Questions and Answers
What allows the visitor class to execute the proper method on an object without using conditionals?
What allows the visitor class to execute the proper method on an object without using conditionals?
- Single Dispatch
- Double Dispatch (correct)
- Method Overriding
- Method Overloading
What is the main advantage of using the Visitor pattern?
What is the main advantage of using the Visitor pattern?
- It makes the code more complex
- It reduces code duplication
- It improves the performance of the application
- It allows adding new behaviors to nodes without changing the code (correct)
What do nodes do when they 'accept' a visitor?
What do nodes do when they 'accept' a visitor?
- They refuse the visitor's method
- They execute the visitor's method
- They tell the visitor what method to execute (correct)
- They ignore the visitor's method
What is a common interface extracted for all visitors?
What is a common interface extracted for all visitors?
Why can't method overloading be used to solve the problem of calling the correct visitor method?
Why can't method overloading be used to solve the problem of calling the correct visitor method?
What pattern is related to the idea of adding new behaviors to objects without changing their code?
What pattern is related to the idea of adding new behaviors to objects without changing their code?
What is the main difference between the Visitor pattern and the Composite pattern?
What is the main difference between the Visitor pattern and the Composite pattern?
What is the main disadvantage of using the Visitor pattern?
What is the main disadvantage of using the Visitor pattern?
What is the main concern of the system architect when it comes to altering existing node classes?
What is the main concern of the system architect when it comes to altering existing node classes?
What design pattern is suggested as a solution to the problem of adding XML export functionality to the node classes?
What design pattern is suggested as a solution to the problem of adding XML export functionality to the node classes?
What is the primary job of the node classes in the given system?
What is the primary job of the node classes in the given system?
Why was the initial solution of adding an export method to each node class not ideal?
Why was the initial solution of adding an export method to each node class not ideal?
What is the main advantage of using polymorphism in the initial solution?
What is the main advantage of using polymorphism in the initial solution?
What is the relationship between the nodes in the graph?
What is the relationship between the nodes in the graph?
Which design pattern is most similar to the Visitor pattern in terms of decoupling behavior from classes?
Which design pattern is most similar to the Visitor pattern in terms of decoupling behavior from classes?
What is the main difference between the Visitor pattern and the Proxy pattern?
What is the main difference between the Visitor pattern and the Proxy pattern?
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Study Notes
The Problem with Adding XML Export to Node Classes
- The system architect refused to allow changes to existing node classes because they were already in production and he didn't want to risk breaking them.
- The proposed solution to add an export method to each node class and use recursion to export the graph into XML format was rejected.
- The primary job of node classes was to work with geodata, and adding XML export behavior would be unrelated to their main purpose.
The Visitor Pattern Solution
- The Visitor pattern suggests placing new behavior into a separate class called visitor, instead of integrating it into existing classes.
- The visitor class defines a set of methods, each of which can take arguments of different types, allowing for different implementation for various node classes.
- The visitor pattern addresses the problem of calling methods with different signatures by using a technique called Double Dispatch.
Double Dispatch
- Double Dispatch is a technique that helps execute the proper method on an object without cumbersome conditionals.
- It delegates the choice of method to the objects being passed as an argument, allowing them to pick a proper method on the visitor.
- Objects "accept" a visitor and tell it what visiting method should be executed.
Benefits of the Visitor Pattern
- The change to the node classes is trivial, and it allows for adding further behaviors without altering the code again.
- Extracting a common interface for all visitors allows all existing nodes to work with any visitor.
- Introducing a new behavior related to nodes only requires implementing a new visitor class.
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