设计模式——访问者模式

访问者模式：对于一组对象，在不改变数据结构的前提下，增加作用于这些结构元素新的功能。

适用于数据结构相对稳定，它把数据结构和作用于其上的操作解耦，使得操作集合可以相对自由地演化。
优点：
符合单一职责原则
扩展性良好
有益于系统的管理和维护
缺点：
增加新的元素类变得很困难
破坏封装性
注意事项：
系统有比较稳定的数据结构
与迭代器的关系
适用场合：
如果一个系统有比较稳定的数据结构，又有经常变化的功能需求，那么访问者模式就是比较合适的

Visitor .class

public interface Visitor {    abstract public void Visit( Element element );}

CompensationVisitor.class

public class CompensationVisitor implements Visitor {    @Override    public void Visit(Element element) {        // TODO Auto-generated method stub        Employee employee = ((Employee) element);        System.out.println(employee.getName() + "'s Compensation is "                + (employee.getDegree() * employee.getVacationDays() * 10));    }}

Employee .class

public class Employee extends Element {    private String name;    private float income;    private int vacationDays;    private int degree;    public Employee(String name, float income, int vacationDays, int degree) {        this.name = name;        this.income = income;        this.vacationDays = vacationDays;        this.degree = degree;    }    public void setName(String name) {        this.name = name;    }    public String getName() {        return name;    }    public void setIncome(float income) {        this.income = income;    }    public float getIncome() {        return income;    }    public void setVacationDays(int vacationDays) {        this.vacationDays = vacationDays;    }    public int getVacationDays() {        return vacationDays;    }    public void setDegree(int degree) {        this.degree = degree;    }    public int getDegree() {        return degree;    }    @Override    public void Accept(Visitor visitor) {        // TODO Auto-generated method stub        visitor.Visit(this);    }}

Element .class

public abstract class Element {    abstract public void Accept(Visitor visitor);}

Employees .class

mport java.util.ArrayList;import java.util.HashMap;public class Employees {    private HashMap<String, Employee> employees;    public Employees() {        employees = new HashMap();    }    public void Attach(Employee employee) {        employees.put(employee.getName(), employee);    }    public void Detach(Employee employee) {        employees.remove(employee);    }    public Employee getEmployee(String name) {        return employees.get(name);    }    public void Accept(Visitor visitor) {        for (Employee e : employees.values()) {            e.Accept(visitor);        }    }}

MainTest .class

import java.util.Random;public class MainTest {    public static void main(String[] args) {        Employees mEmployees = new Employees();        mEmployees.Attach(new Employee("Tom", 4500, 8, 1));        mEmployees.Attach(new Employee("Jerry", 6500, 10, 2));        mEmployees.Attach(new Employee("Jack", 9600, 12, 3));        mEmployees.Accept(new CompensationVisitor());    }}