Behavioral_Visitor

✅ Visitor Pattern

• add new operations to existing object structures
• without modifying those classes

• 👉🏻 separate algorithms(operations) from the objects on which they operate

• 👀 Shapes such as circle and rectangle has calculatePermiteter() method
• I want to add calculateArea() without chaning circle and rectangle

✅ Diagram

👎🏻 before

• circle and rectangle has their own operations

class Circle{
  calculatePermiteter();
}

class Rectangle{
  calculatePermiteter();
}

• 👎🏻 If I want to add calculateArea(), I need to change the code for both classes
• 👎🏻 violate open/closed principle

👍🏻 after

• calculatePermiteter() is not inside Circle nor Rectangle
• objects only accept visitors
• and the visitors will perform the operation

✔️ Element

• now elements only accept visitor

interface Shape {
    void accept(Visitor visitor);
}

✔️ Concrete Element

• circle and rectangle do NOT how to calculateParameter() ❌
• only knows how to accept visitor

class Circle implements Shape {

    double radius;

    @Override
    public void accept(Visitor visitor) {
        visitor.visit(this);
    }
}

class Rectangle implements Shape {
    double width, height;

    @Override
    public void accept(Visitor visitor) {
        visitor.visit(this);
    }
}

✔️ Visitor

// Visitor interface
interface Visitor {
    void visit(Circle circle);
    void visit(Rectangle rectangle);
}

✔️ Concrete visitor

• visitor will do all the work of calculateParameter()
• as PerimeterVisitor

class PerimeterVisitor implements Visitor {
    @Override
    public void visit(Circle circle) {
        //define how to get perimeter for circle
    }

    @Override
    public void visit(Rectangle rectangle) {
        //define how to get perimeter for rectangle
    }
}

💡 Add more functions

• now if I want to add operation calculateArea()
• I can just create a new visitor
• 👍🏻 do not have to change circle code

class AreaVisitor implements Visitor {
    @Override
    public void visit(Circle circle) {
        //calculate circle area
    }

    @Override
    public void visit(Rectangle rectangle) {
        //calculate rectangle area
    }
}

💡 Add more shapes

• now if I want to add Element like triangle,
• create triangle class and make it accept() visitor
• and create visitor for triangle

class Triangle implements Shape {
    @Override
    public void accept(Visitor visitor) {
        visitor.visit(this);
    }
}

class PerimeterVisitor implements Visitor {
    @Override
    public void visit(Circle circle) {}
    @Override
    public void visit(Rectangle rectangle) {}
    @Override
    public void visit(Triangle triangle) {
      //define how to calculate perimeter for triangle
    }
}

✔️ Client

public class Main {
    public static void main(String[] args) {
        Shape circle = new Circle(5);
        Shape rectangle = new Rectangle(4, 6);

        Visitor areaVisitor = new AreaVisitor();
        Visitor perimeterVisitor = new PerimeterVisitor();

        circle.accept(areaVisitor); //calculate circle area
        rectangle.accept(areaVisitor);

        circle.accept(perimeterVisitor); //calculate circle perimeter
        rectangle.accept(perimeterVisitor);
    }
}

🛠️

• class structure은 stable하게 잡혔는데
• 계속 새로운 operation을 추가하고 싶을 때
• used for file system
• for calculating file size, compression