访问者模式

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。

示例演示：

1. 定义一个people接口和三个动作的实现类

   package test.visitor;
   
   public interface People {
       void action(PeopleVisitor peopleVisitor);
   }

   package test.visitor;
   
   public class PeopleAwake implements People{
       @Override
       public void action(PeopleVisitor peopleVisitor) {
           peopleVisitor.visit(this);
       }
   }

   package test.visitor;
   
   public class PeopleEat implements People{
       @Override
       public void action(PeopleVisitor peopleVisitor) {
           peopleVisitor.visit(this);
       }
   }

   package test.visitor;
   
   public class PeopleSleep implements People{
       @Override
       public void action(PeopleVisitor peopleVisitor) {
           peopleVisitor.visit(this);
       }
   }

2. 定义一个访问者接口

   package test.visitor;
   
   public interface PeopleVisitor {
   
       void visit(PeopleAwake peopleAwake);
   
       void visit(PeopleEat peopleEat);
   
       void visit(PeopleSleep peopleSleep);
   }

3. 新建一个实现了访问者接口的访问者实体类

   package test.visitor;
   
   public class PeopleDisplayVisitor implements PeopleVisitor {
       @Override
       public void visit(PeopleAwake peopleAwake) {
           System.out.println("peopleAwake...");
       }
   
       @Override
       public void visit(PeopleEat peopleEat) {
           System.out.println("peopleEat...");
       }
   
       @Override
       public void visit(PeopleSleep peopleSleep) {
           System.out.println("peopleSleep...");
       }
   }

4. 完成上述步骤，我们就可以开始演示了

   package test.visitor;
   
   import java.util.ArrayList;
   import java.util.List;
   
   public class VisitorDemo {
       private static List<People> peoples = new ArrayList<>();
   
       static void action(PeopleVisitor peopleVisitor) {
           peoples.add(new PeopleAwake());
           peoples.add(new PeopleEat());
           peoples.add(new PeopleSleep());
           for (int i = 0; i < peoples.size(); i++) {
               peoples.get(i).action(peopleVisitor);
           }
       }
   
       public static void main(String[] args) {
           action(new PeopleDisplayVisitor());
       }
   }

   运行结果：

   peopleAwake...
   peopleEat...
   peopleSleep...

说明：

优点： 1、符合单一职责原则。 2、优秀的扩展性。 3、灵活性。

缺点： 1、具体元素对访问者公布细节，违反了迪米特原则。 2、具体元素变更比较困难。 3、违反了依赖倒置原则，依赖了具体类，没有依赖抽象。