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问访客模式使用
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Stack Overflow用户

提问于 2012-02-11 13:22:22

回答 4查看 1.2K关注 0票数 1

https://stackoverflow.com/questions/9239445/sample-of-using-visitor-patternbefore-and-after

我是否正确理解访客模式的主要目的？据我所知：

在1之前

public class Main {

    public static void main(String[] args) {
        List<CompanyItem> items = new ArrayList<CompanyItem>();
        items.add(new Employee(10));
        items.add(new Employee(10.6));
        items.add(new Employee(15.9));
        items.add(new Manager(20.1));
        items.add(new Boss(30));

        double totalSalary = 0;
        for(CompanyItem i:items){
            if (i instanceof Employee) {
                totalSalary += ((Employee) i).getSalary();
            } else if (i instanceof Manager) {
                totalSalary += ((Manager) i).getSalary();
                totalSalary += ((Manager) i).getBonusses();
            }else if (i instanceof Boss) {
                totalSalary += ((Boss) i).getSalary();
                totalSalary += ((Boss) i).getAdditionalSalary();
            }
        }
        System.out.println(totalSalary);
    }

    interface CompanyItem {
    }

    static class Employee implements CompanyItem {
        double salary;

        public Employee(double salary) {
            this.salary = salary;
        }

        public double getSalary() {
            return salary;
        }
    }

    static class Manager implements CompanyItem {
        double salary, bonusses;

        public Manager(double salary) {
            this.salary = salary;
            this.bonusses = 1.5 * salary;
        }

        public double getSalary() {
            return  salary;
        }

        public double getBonusses() {
            return bonusses;
        }
    }

    static class Boss implements CompanyItem {
        double salary, addSalary;

        public Boss(double salary) {
            this.salary = salary;
            this.addSalary = 3 * salary;
        }

        public double getSalary() {
            return salary;
        }

        public double getAdditionalSalary() {
            return addSalary;
        }
    }
}

在2之前

public class Main3 {

    public static void main(String[] args) {
        List<CompanyItem> items = new ArrayList<CompanyItem>();
        items.add(new Employee(10));
        items.add(new Employee(10.6));
        items.add(new Employee(15.9));
        items.add(new Manager(20.1));
        items.add(new Boss(30));

        double totalSalary = 0;
        for(CompanyItem i:items){
            totalSalary+=i.getSalary();
            totalSalary+=i.getBonusses();
            totalSalary+=i.getAdditionalSalary();
        }
        System.out.println(totalSalary);
    }

    interface CompanyItem {
        public double getSalary();
        public double getBonusses();
        public double getAdditionalSalary();
    }

    static class Employee implements CompanyItem {
        double salary;

        public Employee(double salary) {
            this.salary = salary;
        }

        public double getSalary() {
            return salary;
        }

        @Override
        public double getBonusses() {
            return 0;
        }

        @Override
        public double getAdditionalSalary() {
            return 0;
        }
    }

    static class Manager implements CompanyItem {
        double salary, bonusses;

        public Manager(double salary) {
            this.salary = salary;
            this.bonusses = 1.5 * salary;
        }

        public double getSalary() {
            return  salary;
        }

        public double getBonusses() {
            return bonusses;
        }

        @Override
        public double getAdditionalSalary() {
            return 0;
        }
    }

    static class Boss implements CompanyItem {
        double salary, addSalary;

        public Boss(double salary) {
            this.salary = salary;
            this.addSalary = 3 * salary;
        }

        public double getSalary() {
            return salary;
        }

        public double getAdditionalSalary() {
            return addSalary;
        }

        @Override
        public double getBonusses() {
            return 0;
        }
    }
}

后(使用访问者模式？)

public class Main1 {

    public static void main(String[] args) {
        List<CompanyItem> items = new ArrayList<CompanyItem>();
        items.add(new Employee(10));
        items.add(new Employee(10.6));
        items.add(new Employee(15.9));
        items.add(new Manager(20.1));
        items.add(new Boss(30));

        SalaryVisitor visitor = new SalaryVisitor();
        for(CompanyItem i:items){
            i.accept(visitor);
        }
        System.out.println(visitor.getTotalSalary());
    }

     interface CompanyItem {
        public void accept(Visitor v);
    }

    static class Employee implements CompanyItem {
        double salary;

        public Employee(double salary) {
            this.salary = salary;
        }

        public double getSalary() {
            return salary;
        }

        @Override
        public void accept(Visitor v) {
            v.visit(this);
        }
    }

    static class Manager implements CompanyItem {
        double salary,bonusses;

        public Manager(double salary) {
            this.salary = salary;
            this.bonusses = 1.5 * salary;
        }

        public double getSalary() {
            return  salary;
        }

        public double getBonusses(){
            return bonusses;
        }

        @Override
        public void accept(Visitor v) {
            v.visit(this);
        }
    }

    static class Boss implements CompanyItem {
        double salary, addSalary;

        public Boss(double salary) {
            this.salary = salary;
            this.addSalary = 3 * salary;
        }

        public double getSalary() {
            return  salary;
        }
        public double getAdditionalSalary(){
            return addSalary;
        }

        @Override
        public void accept(Visitor v) {
            v.visit(this);
        }
    }

    interface Visitor {
        public void visit(Employee e);
        public void visit(Manager m);
        public void visit(Boss b);
    }

    static class SalaryVisitor implements Visitor {
        double totalSalary;

        public SalaryVisitor() {
            totalSalary = 0;
        }

        public double getTotalSalary(){
            return totalSalary;
        }

        @Override
        public void visit(Employee e) {
            totalSalary += e.getSalary();           
        }

        @Override
        public void visit(Manager m) {
            totalSalary += (m.getSalary()+m.getBonusses()); 
        }

        @Override
        public void visit(Boss b) {
            totalSalary += (b.getSalary()+b.getAdditionalSalary()); 
        }
    }
}

我说的对吗？

design-patterns

java

回答 4

Stack Overflow用户

回答已采纳

发布于 2012-02-11 14:25:23

从技术上讲，该示例实现了访问者模式。但是这个例子并没有促进游客的优势。要点是:如果您期望几个独立的算法在相同的数据结构上工作-而不更改数据的结构，则实现访问者模式开销。

为了加强您的示例，我建议进行以下更改:将简单的奖金系统替换为固定奖金(例如，本年度的100 k美元)在所有经理之间按照每个经理的一些奖金点数分配。如果有两位经理，一位有140分，另一位有60分，那么第一位得到70k美元，第二位得到30k美元。

这允许您有几个访问者：

One总结所有经理的所有奖金点，
One在经理之间分配奖金(100 K美元)，使用上一步的总和。将计算出来的个人奖金设置到Manager
A第三访问者(PaydayVisitor)的字段中，打印出员工、老板和经理的支票，并返回所有已支付款项的总和。

在代码中编辑(只为简洁起见省略了getter/setter)：

import java.util.ArrayList;
import java.util.List;

public class VisitorExample {
    public static void main(String[] args) {
        List<CompanyItem> items = new ArrayList<CompanyItem>();
        items.add(new Employee(10));
        items.add(new Employee(10.6));
        items.add(new Employee(15.9));
        items.add(new Manager(20.1, 140));
        items.add(new Manager(42.1, 70));
        items.add(new Boss(30, 10));

        // sum up all bonus points of all Managers
        BonusPointVisitor bonusPointVisitor = new BonusPointVisitor();
        for(CompanyItem i: items)
            i.accept(bonusPointVisitor);

        // distribute given bonus sum among the managers
        BonusDistributorVisitor bonusDistributorVisitor = 
            new BonusDistributorVisitor(bonusPointVisitor.totalBonusPoints, 100.0);
        for(CompanyItem i: items)
            i.accept(bonusDistributorVisitor);

        // PayDay - print all checks
        PrintCheckVisitor printCheckVisitor = new PrintCheckVisitor();
        for(CompanyItem i: items)
            i.accept(printCheckVisitor);
        System.out.println("total money spent this month: "+printCheckVisitor.totalPayments);
    }

    interface CompanyItem {
        public void accept(Visitor v);
    }

    interface Visitor {
        public void visit(Employee e);
        public void visit(Manager m);
        public void visit(Boss b);
    }

    static class Employee implements CompanyItem {
        double salary;

        public Employee(double salary) {
            this.salary = salary;
        }

        @Override
        public void accept(Visitor v) {
            v.visit(this);
        }
    }

    static class Manager implements CompanyItem {
        double salary, bonusPoints, bonus;

        public Manager(double salary, double bonusPoints) {
            this.salary = salary;
            this.bonusPoints = bonusPoints;
            this.bonus = 0;
        }

        @Override
        public void accept(Visitor v) {
            v.visit(this);
        }
    }

    static class Boss implements CompanyItem {
        double salary, addSalary;

        public Boss(double salary, double addSalary) {
            this.salary = salary;
            this.addSalary = addSalary;
        }

        @Override
        public void accept(Visitor v) {
            v.visit(this);
        }
    }

    static class BonusPointVisitor implements Visitor {
        double totalBonusPoints = 0d;

        @Override
        public void visit(Employee e) {
        }

        @Override
        public void visit(Manager m) {
            totalBonusPoints += m.bonusPoints;
        }

        @Override
        public void visit(Boss b) {
        }
    }


    static class BonusDistributorVisitor  implements Visitor {
        double totalBonusPoints, totalBonus;

        public BonusDistributorVisitor(double totalBonusPoints, double totalBonus) {
            this.totalBonusPoints = totalBonusPoints;
            this.totalBonus = totalBonus;
        }

        @Override
        public void visit(Employee e) {
        }

        @Override
        public void visit(Manager m) {
            m.bonus = (m.bonusPoints / totalBonusPoints) * totalBonus;
        }

        @Override
        public void visit(Boss b) {
        }
    }

    static class PrintCheckVisitor implements Visitor {
        double totalPayments = 0;

        @Override
        public void visit(Employee e) {
            advisePayment(e.salary);
        }

        @Override
        public void visit(Manager m) {
            advisePayment(m.salary + m.bonus);
        }

        @Override
        public void visit(Boss b) {
            advisePayment(b.salary + b.addSalary);
        }

        private void advisePayment(double amount){
            System.out.println("pay "+amount+" credits");
            totalPayments += amount;
        }
    }
}

剩下要做的是:给每个项目一些可打印的名称，以便在advisePayment中使用。

票数 8

Stack Overflow用户

发布于 2012-02-11 13:39:14

看起来挺好的。但是，在这种情况下，我只使用多态性而不使用任何模式。只要让CompanyItem有一个TotalSalary函数，就可以使用它。如果你有不同的访客，访客会很有用。

票数 1

Stack Overflow用户

发布于 2012-02-11 13:39:29

是的，它是访问者模式的正确用法，因为它为每个类做了不同的事情。但是请记住，当您使用Visitor模式时，您应该涵盖实现相应接口的所有类。

票数 1

页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持

原文链接：

https://stackoverflow.com/questions/9240551

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