设计模式（23）——Visitor（访问者）

来源：互联网发布：天心软件怎么用编辑：程序博客网时间：2024/06/03 15:22

访问者模式是什么？

访问者模式表示一个作用于某对象结构中的每个元素的操作，而不需要关注每个元素的类型。

访问者模式的适用范围：

想定义一个新的操作，而不想改变被操作对象的类型。

访问者模式的示例：

下面用在汽车的部件上添加打印操作的示例来说明访问者模式：

interface ICarElementVisitor {    void visit(Wheel wheel);    void visit(Engine engine);    void visit(Body body);    void visit(Car car);} interface ICarElement {    void accept(ICarElementVisitor visitor); // CarElements have to provide accept().} class Wheel implements ICarElement {    private String name;     public Wheel(String name) {        this.name = name;    }     public String getName() {        return this.name;    }     public void accept(ICarElementVisitor visitor) {        /*         * accept(ICarElementVisitor) in Wheel implements         * accept(ICarElementVisitor) in ICarElement, so the call         * to accept is bound at run time. This can be considered         * the first dispatch. However, the decision to call         * visit(Wheel) (as opposed to visit(Engine) etc.) can be         * made during compile time since 'this' is known at compile         * time to be a Wheel. Moreover, each implementation of         * ICarElementVisitor implements the visit(Wheel), which is         * another decision that is made at run time. This can be         * considered the second dispatch.         */        visitor.visit(this);    }} class Engine implements ICarElement {    public void accept(ICarElementVisitor visitor) {        visitor.visit(this);    }} class Body implements ICarElement {    public void accept(ICarElementVisitor visitor) {        visitor.visit(this);    }} class Car implements ICarElement {    ICarElement[] elements;     public Car() {        //create new Array of elements        this.elements = new ICarElement[] { new Wheel("front left"),            new Wheel("front right"), new Wheel("back left") ,            new Wheel("back right"), new Body(), new Engine() };    }     public void accept(ICarElementVisitor visitor) {            for(ICarElement elem : elements) {            elem.accept(visitor);        }        visitor.visit(this);        }} class CarElementPrintVisitor implements ICarElementVisitor {    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 ICarElementVisitor {    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 {    public static void main(String[] args) {        ICarElement car = new Car();        car.accept(new CarElementPrintVisitor());        car.accept(new CarElementDoVisitor());    }}

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