从不同角度看同一数据结构-访问者模式

访问者模式是一种复杂的设计模式，使用频率不高，下面举一个公司对员工业绩考核的例子，CEO和CTO对员工的考核标准是不一样的

员工抽象类

public abstract class Staff {
    public String name;
    public int kpi;
    public Staff(String name) {
        this.name = name;
        kpi = new Random().nextInt(10);
    }

    public abstract void accept(Visitor visitor);
}

基类中只有员工的公共属性，这里是姓名和kpi

员工具体类

1. 工程师类

   public class Engineer extends Staff {
       public Engineer(String name) {
           super(name);
       }
   
       @Override
       public void accept(Visitor visitor) {
           visitor.visit(this);
       }
   
       public int getCodeLines() {
           return new Random().nextInt(10*10000);
       }
   }

2. 产品经理类

public class Manager extends Staff {
    public Manager(String name) {
        super(name);
    }

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

    public int getProducts() {
        return new Random().nextInt(10);
    }
}

在具体类中加入独特的属性，这样访问者访问时就可以区别对待

抽象访问者类

public interface Visitor {
    void visit(Engineer engineer);
    void visit(Manager manager);
}

具体访问类

1. CEO：只关心员工的姓名和KPI

   public class CEO implements Visitor {
       @Override
       public void visit(Engineer engineer) {
           System.out.println("Engineer："+ engineer.name + ", kpi: " +engineer.kpi);
       }
   
       @Override
       public void visit(Manager manager) {
           System.out.println("Manager: "+ manager.name + ", kpi: "+manager.kpi);
       }
   }

2. CTO：根据岗位的不同关心不同的技术指标

public class CTO implements Visitor {
    @Override
    public void visit(Engineer engineer) {
        System.out.println("Engineer: " + engineer.name +", code: " + engineer.getCodeLines());
    }

    @Override
    public void visit(Manager manager) {
        System.out.println("Manager: " + manager.name + ", product: "+ manager.getProducts());
    }
}

具体示例如下

public class Main {
    public static void main(String[] args) {
        BusinessReport businessReport = new BusinessReport();
        System.out.println("======CEO=====");
        businessReport.showReport(new CEO());
        System.out.println("\n=====CTO=====");
        businessReport.showReport(new CTO());
    }
}

结果

访问者模式可以很方便地添加访问者，方便地从多种角度观察同一种数据结构