visitor

Visitor Pattern

The Visitor Pattern is a behavioral design pattern that allows you to add new operations to objects without changing their structure. It separates an algorithm from the objects it operates on.

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📖 What is the Visitor Pattern?

The Visitor Pattern involves two key elements:

1. Visitor: Defines a set of operations to be performed on elements of an object structure.
2. Element: Defines an accept() method that allows the visitor to access its data.

Key features:

1. Open/Closed Principle: Allows adding new operations without modifying existing classes.
2. Double Dispatch: Executes the operation on the visitor while providing access to the element's state.
3. Extensibility: Supports adding new operations to existing structures.

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🤔 Why Use the Visitor Pattern?

1. Extending Behavior: Add new operations without changing the classes of the elements.
2. Separation of Concerns: Keeps the logic for specific operations separate from the elements.
3. Consistency: Ensures operations are applied consistently across similar objects.

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🔧 Implementation

The implementation of the Visitor Pattern can be found in:

• Figure.java: Defines the base interface for figures.
• Circle.java, Triangle.java, Square.java: Concrete elements implementing the accept() method.
• FigureVisitor.java: Visitor interface defining operations for all figure types.
• HeightVisitor.java, AreaVisitor.java: Concrete visitors implementing specific behaviors.
• ObjectStructure.java: Manages the collection of figures.
• Main.java: Demonstrates the usage of the Visitor Pattern.

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🛠️ Example Usage

To see the Visitor Pattern in action, refer to the Main.java file. It demonstrates how a collection of figures can be traversed, applying different visitor behaviors.

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🌐 Real-World Examples

• Compilers:
  • Applying different operations like code generation or optimization to syntax trees.
• Graphic Editors:
  • Rendering, exporting, or analyzing shapes in an editor.
• File Systems:
  • Performing operations like compression or indexing on files and folders.

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📊 UML Diagram

classDiagram
direction LR
    class Client {
    }

    class ObjectStructure {
    }

    class Element {
        +Accept(Visitor)
    }

    class ElementA {
        +Accept(v: Visitor)
    }

    class ElementB {
        +Accept(v: Visitor)
    }

    class Visitor {
        +VisitElementA(ElementA)
        +VisitElementB(ElementB)
    }

    class ConcreteVisitor1 {
        +VisitElementA(ElementA)
        +VisitElementB(ElementB)
    }

    class ConcreteVisitor2 {
        +VisitElementA(ElementA)
        +VisitElementB(ElementB)
    }

    Client --> ObjectStructure
    Client --> Visitor
    ObjectStructure --> Element
    Element <|-- ElementA
    Element <|-- ElementB
    Element --> Visitor
    Visitor <|-- ConcreteVisitor1
    Visitor <|-- ConcreteVisitor2

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Note

If the UML above is not rendering correctly, you can view the diagram from the visitor_uml.png file.

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📝 Key Takeaways

• The Visitor Pattern separates algorithms from the objects they operate on, adhering to the Open/Closed Principle.
• It’s ideal for applying operations to a collection of objects with varying types.
• Use it when you need to perform multiple unrelated operations on objects without modifying their structure.

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