Computer Science Design Patterns/Visitor

Example in Java

The following example is in the Java programming language:

interface CarElementVisitor {
    void visit(Wheel wheel);
    void visit(Engine engine);
    void visit(Body body);
    void visitCar(Car car);
}
interface CarElement {
    void accept(CarElementVisitor visitor); // CarElements have to provide accept().
}
class Wheel implements CarElement {
    private String name;
    Wheel(String name) {
        this.name = name;
    }
    String getName() {
        return this.name;
    }
    public void accept(CarElementVisitor visitor) {
        visitor.visit(this);
    }
}
 
class Engine implements CarElement {
    public void accept(CarElementVisitor visitor) {
        visitor.visit(this);
    }
}
 
class Body implements CarElement {
    public void accept(CarElementVisitor visitor) {
        visitor.visit(this);
    }
}
 
class Car  {
    CarElement[] elements;
    public CarElement[] getElements() {
        return elements.clone(); // Return a copy of the array of references.
    }
    public Car() {
        this.elements = new CarElement[]
          { new Wheel("front left"), new Wheel("front right"),
            new Wheel("back left") , new Wheel("back right"),
            new Body(), new Engine() };
    }
}
 
class CarElementPrintVisitor implements CarElementVisitor {
    public void visit(Wheel wheel) {      
        System.out.println("Visiting "+ wheel.getName()
                            + " wheel");
    }
    public void visit(Engine engine) {
        System.out.println("Visiting engine");
    }
    public void visit(Body body) {
        System.out.println("Visiting body");
    }
 
    public void visitCar(Car car) {
        System.out.println("\nVisiting car");
        for(CarElement element : car.getElements()) {
            element.accept(this);
        }
        System.out.println("Visited car");
    }
}
 
class CarElementDoVisitor implements CarElementVisitor {
    public void visit(Wheel wheel) {
        System.out.println("Kicking my "+ wheel.getName());
    }
    public void visit(Engine engine) {
        System.out.println("Starting my engine");
    }
    public void visit(Body body) {
        System.out.println("Moving my body");
    }
    public void visitCar(Car car) {
        System.out.println("\nStarting my car");
        for(CarElement carElement : car.getElements()) {
            carElement.accept(this);
        }
        System.out.println("Started car");
    }
 
}
 
public class VisitorDemo {
    static public void main(String[] args){
        Car car = new Car();
        CarElementVisitor printVisitor = new CarElementPrintVisitor();
        CarElementVisitor doVisitor = new CarElementDoVisitor();
        printVisitor.visitCar(car);
        doVisitor.visitCar(car);
    }
}

Example in D

The following example is in the D programming language:

import std.stdio;
import std.string;
 
interface CarElementVisitor {
    void visit(Wheel wheel);
    void visit(Engine engine);
    void visit(Body bod);
    void visitCar(Car car);
}
 
interface CarElement{
    void accept(CarElementVisitor visitor);
}
 
class Wheel : CarElement {
    private string name;
    this(string name) {
        this.name = name;
    }
    string getName() {
        return name;
    }
    public void accept(CarElementVisitor visitor) {
        visitor.visit(this);
    }
}
 
class Engine : CarElement {
    public void accept(CarElementVisitor visitor) {
        visitor.visit(this);
    }
}
 
class Body : CarElement {
    public void accept(CarElementVisitor visitor) {
        visitor.visit(this);
    }
}
 
class Car {
    CarElement[] elements;
    public CarElement[] getElements(){
        return elements;
    }
    public this() {
        elements =
        [
            cast(CarElement) new Wheel("front left"),
            cast(CarElement) new Wheel("front right"),
            cast(CarElement) new Wheel("back left"),
            cast(CarElement) new Wheel("back right"),
            cast(CarElement) new Body(),
            cast(CarElement) new Engine()
        ];
    }
}
 
class CarElementPrintVisitor : CarElementVisitor {
    public void visit(Wheel wheel) {
        writefln("Visiting "~ wheel.getName() ~ " wheel");
    }
    public void visit(Engine engine) {
        writefln("Visiting engine");
    }
    public void visit(Body bod) {
        writefln("Visiting body");
    }
    public void visitCar(Car car) {
        writefln("\nVisiting car");
        foreach(CarElement element ; car.elements) {
            element.accept(this);
        }
        writefln("Visited car");
    }
}
 
class CarElementDoVisitor : CarElementVisitor {
    public void visit(Wheel wheel) {
        writefln("Kicking my "~ wheel.name);
    }
    public void visit(Engine engine) {
        writefln("Starting my engine");
    }
    public void visit(Body bod) {
        writefln("Moving my body");
    }
    public void visitCar(Car car) {
        writefln("\nStarting my car");
        foreach(CarElement carElement ; car.getElements()) {
            carElement.accept(this);
        }
        writefln("Started car");
    }
}
 
void main(){
    Car car = new Car;
    CarElementVisitor printVisitor = new CarElementPrintVisitor;
    CarElementVisitor doVisitor = new CarElementDoVisitor;
    printVisitor.visitCar(car);
    doVisitor.visitCar(car);
}

Example in C#

The following example is an example in the C# programming language:

using System;
 
namespace VisitorPattern
{
    class Program
    {
        static void Main(string[] args)
        {
            var car = new Car();
            CarElementVisitor printVisitor = new CarElementPrintVisitor();
            CarElementVisitor doVisitor = new CarElementDoVisitor();
            printVisitor.visitCar(car);
            doVisitor.visitCar(car);
        }
    }
 
    public interface CarElementVisitor
    {
        void visit(Wheel wheel);
        void visit(Engine engine);
        void visit(Body body);
        void visitCar(Car car);
    }
    public interface CarElement
    {
        void accept(CarElementVisitor visitor); // CarElements have to provide accept().
    }
    public class Wheel : CarElement
    {
 
        public String name { get; set; }
 
        public void accept(CarElementVisitor visitor)
        {
            visitor.visit(this);
        }
    }
 
    public class Engine : CarElement
    {
        public void accept(CarElementVisitor visitor)
        {
            visitor.visit(this);
        }
    }
 
    public class Body : CarElement
    {
        public void accept(CarElementVisitor visitor)
        {
            visitor.visit(this);
        }
    }
 
    public class Car
    {
        public CarElement[] elements { get; private set; }
 
        public Car()
        {
            elements = new CarElement[]
          { new Wheel{name = "front left"}, new Wheel{name = "front right"},
            new Wheel{name = "back left"} , new Wheel{name="back right"},
            new Body(), new Engine() };
        }
    }
 
    public class CarElementPrintVisitor : CarElementVisitor
    {
        public void visit(Wheel wheel)
        {
            Console.WriteLine("Visiting " + wheel.name + " wheel");
        }
        public void visit(Engine engine)
        {
            Console.WriteLine("Visiting engine");
        }
        public void visit(Body body)
        {
            Console.WriteLine("Visiting body");
        }
 
        public void visitCar(Car car)
        {
            Console.WriteLine("\nVisiting car");
            foreach (var element in car.elements)
            {
                element.accept(this);
            }
            Console.WriteLine("Visited car");
        }
    }
 
    public class CarElementDoVisitor : CarElementVisitor
    {
        public void visit(Wheel wheel)
        {
            Console.WriteLine("Kicking my " + wheel.name);
        }
        public void visit(Engine engine)
        {
            Console.WriteLine("Starting my engine");
        }
        public void visit(Body body)
        {
            Console.WriteLine("Moving my body");
        }
        public void visitCar(Car car)
        {
            Console.WriteLine("\nStarting my car");
            foreach (var element in car.elements)
            {
                element.accept(this);
            }
        }
 
    }
}