设计模式_訪问者模式

Visitor Pattern

Repressent an operation to be performed  on the elements of an object structure.Visitor lets
you define a new operation without changing the classees of the elements on which it operates.(封装一些作用于某种数据结构中的各种元素的操作，它能够在不改变数据结构的前提下定义作用于这些元素的新操作。)

直接上代码吧，心态不佳。

public abstract class Employee {
 public final static int MALE=0;
 public final static int FEMALE=1;

 private String name;
 private String salary;
 private int sex;

 public abstract void accept(IVisitor visitor); //allow a visitor

 public String getName() {
  return name;
 }
 public void setName(String name) {
  this.name = name;
 }
 public String getSalary() {
  return salary;
 }
 public void setSalary(String salary) {
  this.salary = salary;
 }
 public int getSex() {
  return sex;
 }
 public void setSex(int sex) {
  this.sex = sex;
 }
}

public class Manager extends Employee{
 private String performance;

 public String getPerformance() {
  return performance;
 }
 public void setPerformance(String performance) {
  this.performance = performance;
 }
 @Override
 public void accept(IVisitor visitor) {
  visitor.visit(this);
 }
}

public class CommonEmployee extends Employee{
 private String job;

 public String getJob() {
  return job;
 }
 public void setJob(String job) {
  this.job = job;
 }
 @Override
 public void accept(IVisitor visitor) {
  visitor.visit(this);
 }
}

public interface IVisitor {
 public void visit(CommonEmployee commonEmployee);
 public void visit(Manager manager);
}

真正的处理类在这里。

public class Visitor implements IVisitor {
 public void visit(CommonEmployee commonEmployee) {
  System.out.println(this.getCommonEmployee(commonEmployee));
 }
 public void visit(Manager manager) {
  System.out.println(this.getManagerInfo(manager));
 }
 private String getBasicInfo(Employee employee) {
  String info = "name:" + employee.getName() + "\t sex:"
    + (employee.getSex() == Employee.FEMALE ?

"man" : "women")
    + "\t salary:" + employee.getSalary();
  return info;
 }

 private String getManagerInfo(Manager manager) {
  String basic = this.getBasicInfo(manager);
  String otherInfo = "\t performance" + manager.getPerformance();
  return basic + otherInfo;
 }

 private String getCommonEmployee(CommonEmployee commonEmployee){
  String basic=this.getBasicInfo(commonEmployee)+"\t";
  String other="job:"+commonEmployee.getJob();
  return basic+other;
 }
}

測试

public class Client {
 public static void main(String[] args) {
  for(Employee e:mockEmployee()){
   e.accept(new Visitor());
  }
 }

 public static List<Employee> mockEmployee(){
  List<Employee> list=new ArrayList<Employee>();

  CommonEmployee zhangsan=new CommonEmployee();
  zhangsan.setJob("coding everyday!");
  zhangsan.setName("zhangsan");
  zhangsan.setSalary("1800");
  zhangsan.setSex(Employee.MALE);

  CommonEmployee lirong=new CommonEmployee();
  lirong.setJob("page coding,It‘s not fansion");
  lirong.setName("lirong");
  lirong.setSalary("1900");
  lirong.setSex(Employee.FEMALE);

  Manager wang=new Manager();
  wang.setName("wamg");
  wang.setPerformance("It‘s unpay basically,but he is a horser!");
  wang.setSalary("10221");
  wang.setSex(Employee.MALE);

  list.add(zhangsan);
  list.add(lirong);
  list.add(wang);
  return list;
 }
}

长处

● 符合单一职责原则

● 优秀的扩展性

● 灵活性很高

缺点

● 详细元素对訪问者发布细节

● 详细元素变更比較困难

● 违背了依赖倒置转原则

使用场景

● 一个对象结构包括非常多类对象，它们有不同的接口，而你想对这些对象实施一些依赖 于其详细类的操作。也就说是用迭代器模式已经不能胜任的情景。

● 须要对一个对象结构中的对象进行非常多不同而且不相关的操作，而你想避免让这些操 作“污染”这些对象的类。

扩展实例

1.统计功能

统计总工资total仅仅须要在IVisitor抽象中加入一个public int getTotalSalary();然后在详细訪问者中实现就可以。

public class Visitor implements IVisitor {
//部门经理的工资系数是5
private final static int MANAGER_COEFFICIENT = 5;
//员工的工资系数是2
private final static int COMMONEMPLOYEE_COEFFICIENT = 2;
//普通员工的工资总和
private int commonTotalSalary = 0;
//部门经理的工资总和
private int managerTotalSalary =0;
//计算部门经理的工资总和
private void calManagerSalary(int salary){
this.managerTotalSalary = this.managerTotalSalary + salary
*MANAGER_COEFFICIENT ;
}
//计算普通员工的工资总和
private void calCommonSlary(int salary){
this.commonTotalSalary = this.commonTotalSalary +
salary*COMMONEMPLOYEE_COEFFICIENT;
}
//获得全部员工的工资总和
public int getTotalSalary(){
return this.commonTotalSalary + this.managerTotalSalary;
}
}

2.多个訪问者

由展示->汇总

IShowVisitor extends IVisitor

ITotalVisitor  extends IVisitor

双分派

java的静态绑定和动态绑定。多态，重写，重载。

你调用我的，我调用你的。谁是主谁是次，真正在哪里处理问题。

我是菜鸟，我在路上。