Visitor Pattern

1. Definition

Separate the data structure from its operations, placing each in different classes.

2. UML

3. Java

3.1. Data Structure

• Course

public abstract class Course
{
    private String name;

    public String getName()
    {
        return name;
    }

    public void setName(String name)
    {
        this.name = name;
    }

    public abstract void accept(IVisitor visitor);
}

• CodingCourse

public class CodingCourse extends Course
{
    private int price;

    public int getPrice()
    {
        return price;
    }

    public void setPrice(int price)
    {
        this.price = price;
    }

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

• FreeCourse

public class FreeCourse extends Course
{
    @Override
    public void accept(IVisitor visitor)
    {
        visitor.visit(this);

    }
}

3.2. Operations

• IVisitor

public interface IVisitor
{
    void visit(FreeCourse course);
    void visit(CodingCourse course);

}

• Visitor

public class Visitor implements IVisitor
{
    @Override
    public void visit(FreeCourse course)
    {
        System.out.println("Free course: " + course.getName());
    }

    @Override
    public void visit(CodingCourse course)
    {
        System.out.println("Practical course: " + course.getName() + " price: " + course.getPrice());
    }
}

3.3. Client

public class Client
{
    public static void main(String[] args)
    {
        List<Course> courses = new ArrayList<>();

        FreeCourse freeCourse = new FreeCourse();
        freeCourse.setName("hahaha");


        CodingCourse codingCourse = new CodingCourse();
        codingCourse.setName("hahaha");
        codingCourse.setPrice(1111);

        courses.add(freeCourse);
        courses.add(codingCourse);


        for (Course cours : courses)
        {
            cours.accept(new Visitor());
        }

    }
}