访问者模式

1.访问者模式简介

在访问者模式（Visitor Pattern）中，我们使用了一个访问者类，它改变了元素类的执行算法。通过这种方式，元素的执行算法可以随着访问
者改变而改变。这种类型的设计模式属于行为型模式。根据模式，元素对象已接受访问者对象，这样访问者对象就可以处理元素对象上的操作。

关键代码：在数据基础类里面有一个方法接受访问者，将自身引用传入访问者。

应用实例：老师家访，父亲Tom接待。

2.示例Demo

interface FamilyMember {
    void accept(Visitor visitor);
}

class Wife implements FamilyMember {
    public void accept(Visitor visitor) {
        visitor.sayHello(this);
    }
}

class Daughter implements FamilyMember {
    public void accept(Visitor visitor) {
        visitor.sayHello(this);
    }
}

class Son implements FamilyMember {
    public void accept(Visitor visitor) {
        visitor.sayHello(this);
    }
}


class Reception implements FamilyMember {
    private String name;
    private FamilyMember members[];
    public Reception(String name) {
        this.name = name;
        members = new FamilyMember[] {new Wife(), new Daughter(), new Son()}; //Attention here
    }

    public String getName() {
        return name;
    }

    public void accept(Visitor visitor) {
        visitor.sayHello(this);
        for (FamilyMember member : members) {
            member.accept(visitor);
        }
    }
}

/*对每一个家庭成员的访问方法可能由于访问者的不同而不同*/
interface Visitor {
    void sayHello(Reception reception);
    void sayHello(Wife wife);
    void sayHello(Daughter daughter);
    void sayHello(Son son);
}


class Teacher implements Visitor {
    private String receptionName;

    public void sayHello(Reception reception) {
        receptionName = reception.getName();
        System.out.println("Hello " + receptionName);
    }
    public void sayHello(Wife wife) {
        System.out.println("Hello " + receptionName + "'s wife");
    }
    public void sayHello(Daughter daughter) {
        System.out.println("Hello " + receptionName + "'s daughter");
    }
    public void sayHello(Son son) {
        System.out.println("Hello " + receptionName + "'s son");
    }
}


public class VisitorPatternDemo {
    public static void main(String args[]) {
        Visitor visitor = new Teacher();
        FamilyMember reception = new Reception("Tom");
        reception.accept(visitor);
    }
}

/*
# java VisitorPatternDemo
Hello Tom
Hello Tom's wife
Hello Tom's daughter
Hello Tom's son
*/

参考：http://www.runoob.com/design-pattern/visitor-pattern.html