设计模式---05.访问者模式

发表于 2020-03-22

访问者模式

访问者模式是一种对象和对象的访问者分开的一种设计模式，在一个对象稳定的情况下，使用访问者模式可以更好的扩展对数据的访问。相当于是我们在对象里面安插了一个“眼”，这个眼能够被外面实现，然后能拿到当前对象的各个属性。

举个例子

组装一台电脑，对于组装的对于组装者来说，只关心一个电脑零件构成（CPU,内存，鼠标键盘）。而对于电脑的消费者，关心的是电脑各个零件的价格以及总价。而对于有些人来说对于，可能只关心电脑各个零件的重要性以及核心程度。综上所述我们操作的对象是一个电脑，对象是不变的，电脑的访问者来说是不同的，他们从不同的维度来分析这个电脑的属性。下面我们就用类图来表示访问者模式的大概结构：

访问者模式

对于访问者模式来说有以下几个实现步骤:

定义一个对象（可以是一个基类，也可是其他）

public class ComputerPartBase   {
     private final String partName;
     private final int coreLevel;
     private final double price;
     
     public ComputerPartBase(String partName, int coreLevel, double price) {
          this.partName = partName;
          this.coreLevel = coreLevel;
          this.price = price;
     }
     
     public String getPartName() {
          return partName;
     }
     
     public int getCoreLevel() {
          return coreLevel;
     }
     
     public double getPrice() {
          return price;
     }
 
}

定义一个接口，实现”插眼”的约束

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 public interface ComputerVisitor {
public void accept(ComputerPartVisitor computerPartVisitor);
  }

对象继承”眼“，实现方法

  public class ComputerPartBase implements ComputerVisitor {
 
     //.....

@Override
public void accept(ComputerPartVisitor computerPartVisitor) {

   }
   }

各个零件的实现

 public class CPU  extends ComputerPartBase {
      public CPU() {
           super("CPU", 10,2000.00d);
      }
      
      @Override
      public void accept(ComputerPartVisitor computerPartVisitor) {
           computerPartVisitor.visit(this);
      }
 }

 public class Keyboard extends ComputerPartBase implements ComputerVisitor {
      public Keyboard() {
           super("keyboard", 1, 10);
      }
      
      @Override
      public void accept(ComputerPartVisitor computerPartVisitor) {
           computerPartVisitor.visit(this);
      }
 }

 public class Memery extends ComputerPartBase implements ComputerVisitor {
      public Memery() {
           super("memery", 8, 200d);
      }
      
      @Override
      public void accept(ComputerPartVisitor computerPartVisitor) {
           computerPartVisitor.visit(this);
      }
}

 public class Monitor extends ComputerPartBase implements ComputerVisitor {
      public Monitor() {
           super("monitor", 2, 800);
      }
      
      @Override
      public void accept(ComputerPartVisitor computerPartVisitor) {
           computerPartVisitor.visit(this);
      }
 }    

 public class Mouse extends ComputerPartBase implements ComputerVisitor {
      public Mouse() {
           super("mouse", 2, 100);
      }
      
      @Override
      public void accept(ComputerPartVisitor computerPartVisitor) {
           computerPartVisitor.visit(this);
      }
 }

真正的”眼“的接口


public interface ComputerPartVisitor {
     public void visit(CPU cpu);
     public void visit(Memery memery);
     public void visit(Mouse mouse);
     public void visit(Keyboard keyboard);
     public void visit(Monitor monitor);
}

访问者通过实现眼来获取数据

//电脑的组成
public class ComputerPartStrutchNameVisitor implements ComputerPartVisitor {

     private List<String> partSet=new ArrayList<>();
     @Override
     public void visit(CPU cpu) {
          partSet.add(cpu.getPartName());
     }
     
     @Override
     public void visit(Memery memery) {
          partSet.add(memery.getPartName());
          
     }
     
     @Override
     public void visit(Mouse mouse) {
          partSet.add(mouse.getPartName());
          
     }
     
     @Override
     public void visit(Keyboard keyboard) {
          partSet.add(keyboard.getPartName());
          
     }
     
     @Override
     public void visit(Monitor monitor) {
          partSet.add(monitor.getPartName());
          
     }
     
     public String getNeedPart(){
          return partSet.stream().collect(Collectors.joining(","));
     }
}


//电脑的组装价格
public class ComputerPartPriceVisitor implements ComputerPartVisitor {
          
          private double price = 0d;
          
          @Override
          public void visit(CPU cpu) {
               price += cpu.getPrice();
          }
          
          @Override
          public void visit(Memery memery) {
               price += memery.getPrice();
               
          }
          
          @Override
          public void visit(Mouse mouse) {
               price += mouse.getPrice();
          }
          
          @Override
          public void visit(Keyboard keyboard) {
               price += keyboard.getPrice();
               
          }
          
          @Override
          public void visit(Monitor monitor) {
               price += monitor.getPrice();
          }
          public double getPrice(){
               return price;
          }
}

//获得电脑的核心等级
public class ComputerPartCoreLevelVisitor implements ComputerPartVisitor {
	
     private List<String> levelList = new ArrayList<>();
     
     @Override
     public void visit(CPU cpu) {
          
          levelList.add("CPU:"+cpu.getCoreLevel());
     }
     
     @Override
     public void visit(Memery memery) {
          levelList.add("Memery:"+memery.getCoreLevel());
     }
     
     @Override
     public void visit(Mouse mouse) {
          levelList.add("Mouse:"+mouse.getCoreLevel());
     }
     
     @Override
     public void visit(Keyboard keyboard) {
          levelList.add("Keyboard:"+keyboard.getCoreLevel());
     }
     
     @Override
     public void visit(Monitor monitor) {
          levelList.add("Monitor:"+monitor.getCoreLevel());
     }
     public String getCoreLevel(){
          return levelList.stream().collect(Collectors.joining(","));
     }
}

调用的实现

public class AppMain {
     public static void main(String[] args) {
          Computer computer = new Computer();
          ComputerPartStrutchNameVisitor computerPartVisitor = new ComputerPartStrutchNameVisitor();
          computer.accept(computerPartVisitor);
          System.out.println("组装一个电脑需要零件：" + computerPartVisitor.getNeedPart());
          
          ComputerPartPriceVisitor computerPartPriceVisitor = new ComputerPartPriceVisitor();
          computer.accept(computerPartPriceVisitor);
          System.out.println("组装一个电脑需要Money:" + computerPartPriceVisitor.getPrice());
          
          ComputerPartCoreLevelVisitor computerPartCoreLevelVisitor = new ComputerPartCoreLevelVisitor();
          computer.accept(computerPartCoreLevelVisitor);
          System.out.println("电脑硬件的核心优先级为:" + computerPartCoreLevelVisitor.getCoreLevel());
     }
}

输出的结果为:

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组装一个电脑需要零件：CPU,memery,monitor,keyboard,mouse
组装一个电脑需要Money:3110.0
电脑硬件的核心优先级为:CPU:10,Memery:8,Monitor:2,Keyboard:1,Mouse:2