Visitor Pattern (Visitor设计模式)

//java code:



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());

    }

}