Design Patterns

Visitor Design pattern

the visitor design pattern is a way of separating an algorithm from an object structure it operates on. A practical result of this separation is the ability to add new operations to existing object structures without modifying those structures. It is one way to easily follow the open/closed principle.

the visitor allows one to add new virtual functions to a family of classes without modifying the classes themselves; instead, one creates a visitor class that implements all of the appropriate specializations of the virtual function. The visitor takes the instance reference as input, and implements the goal

interface CarElementVisitor {
    void visit(Wheel wheel);
    void visit(Engine engine);
    void visit(Body body);
    void visit(Car car);
}

interface CarElement {
    void accept(CarElementVisitor visitor); // CarElements have to provide accept().
}

class Wheel implements CarElement {
    private String name;

    public Wheel(String name) {
        this.name = name;
    }

    public 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 implements CarElement{
    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() };
    }

    public void accept(CarElementVisitor visitor) {
        for(CarElement element : this.getElements()) {
            element.accept(visitor);
        }
        visitor.visit(this);
    }
}

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 visit(Car car) {
        System.out.println("Visiting car");
    }
}

class CarElementDoVisitor implements CarElementVisitor {
    public void visit(Wheel wheel) {
        System.out.println("Kicking my "+ wheel.getName() + " wheel");
    }

    public void visit(Engine engine) {
        System.out.println("Starting my engine");
    }

    public void visit(Body body) {
        System.out.println("Moving my body");
    }

    public void visit(Car car) {
        System.out.println("Starting my car");
    }
}

public class VisitorDemo {
    static public void main(String[] args){
        Car car = new Car();
        car.accept(new CarElementPrintVisitor());
        car.accept(new CarElementDoVisitor());
    }
}
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