Visitor Design pattern - Implementation [Kids]

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The Scenario: A Doctor Visiting Different Patient Categories

Consider a healthcare system where various types of patients—such as children, adults, and senior citizens—need medical assessments. Each category of patient requires distinct checkup protocols, diagnostic tests, and dosage considerations.

If you hardcode checkup procedures directly into the patient classes, you quickly run into architectural bottlenecks. Whenever medical regulations update, or a new specialist (e.g., a dentist or cardiologist) visits the patient, you would be forced to open and modify every patient class, directly violating the Open/Closed Principle.

The Solution: Applying the Visitor Pattern

The Visitor Design Pattern separates medical assessment logic from the patient data models by utilizing Double Dispatch:

• Visitable Element (Patient): Each patient class defines an accept(Visitor visitor) method, passing itself to the visiting medical practitioner.
• Visitor Interface (Doctor): Declares specialized visit operations for each concrete patient type (e.g., visit(ChildPatient patient), visit(SeniorPatient patient)).
• Concrete Visitor (GeneralPhysician / Specialist): Implements the specific medical assessment or treatment routines tailored to the specific patient type without altering the patient's internal structure.

Architectural Advantages in Practice

Using the Visitor pattern for domain modeling offers key architectural benefits:

• Open/Closed Principle: Introduce new doctors, health inspectors, or billing visitors without modifying existing patient classes.
• Single Responsibility Principle: Gathers related diagnostic and medical checkup operations into dedicated visitor classes instead of bloating data models.
• Extensible Operations: Easy to introduce new operations (like health index calculations or vaccination audits) across an entire group of patient objects.

Related Tutorials from Ram N Java

• 👉 Visitor Design Pattern Implementation: Products Example
• 👉 Visitor Design Pattern: Introduction and Basics
• 👉 Visitor Design Pattern: Class Diagram Explained

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Visitor Design pattern - Implementation [Kids]

Visitor Design pattern - Implementation [Kids] - Class Diagram

Visitable.java

public interface Visitable

{

public void accept(Visitor visitor);

}

Child.java

public class Child implements Visitable
{

private String name;

private String healthStatus;

public Child( String name )
{
super();
this.name = name;
}

public String getName()
{
return name;
}

public String getHealthStatus()
{
return healthStatus;
}

public void setHealthStatus( String healthStatus )
{
this.healthStatus = healthStatus;
}

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

}

Visitor.java

public interface Visitor

{

public void visit(Visitable visitable);

}

ChildSpecialistDoctor.java

public class ChildSpecialistDoctor implements Visitor

{

private String name;

public ChildSpecialistDoctor( String name )

{

super();

this.name = name;

}

public String getName()

{

return name;

}

@Override

public void visit( Visitable visitable )

{

Child child = (Child) visitable;

child.setHealthStatus("Bad");

System.out.println("ChildSpecialistDoctor: '" + this.getName() + "' did the checkup of the child: '"

               + child.getName()

               + "' and found health is not good so updated the health status as 'Bad'\n");

}

}

School.java

import java.util.ArrayList;

public class School

{

static ArrayList<Child> childList;

static

{

childList = new ArrayList<Child>();

Child john = new Child("John");

Child steve = new Child("Steve");

Child rohit = new Child("Rohit");

Child sonu = new Child("Sonu");

childList.add(john);

childList.add(steve);

childList.add(rohit);

childList.add(sonu);

}

public static void doHealthCheckup()

{

ChildSpecialistDoctor childSpecialistDoctor = new ChildSpecialistDoctor("Dr.Smith");

for( Child child : childList )

                {

               child.accept(childSpecialistDoctor);

                }

}

}

Client.java

public class Client

{

public static void main( String[] args )

{

School.doHealthCheckup();

}

}

Output

ChildSpecialistDoctor: 'Dr.Smith' did the checkup of the child: 'John' and found health is not good so updated the health status as 'Bad'

ChildSpecialistDoctor: 'Dr.Smith' did the checkup of the child: 'Steve' and found health is not good so updated the health status as 'Bad'

ChildSpecialistDoctor: 'Dr.Smith' did the checkup of the child: 'Rohit' and found health is not good so updated the health status as 'Bad'

ChildSpecialistDoctor: 'Dr.Smith' did the checkup of the child: 'Sonu' and found health is not good so updated the health status as 'Bad'

See also:

Visitor Design pattern - Introduction

Visitor Design pattern - Real time Example [Kids]

Visitor Design pattern - Real time Example [Products]

Visitor Design pattern - Class Diagram

Visitor Design pattern - Implementation [Products]

Visitor Design pattern - KeyPoints

All Design Patterns Links