Null Object 模式

NULL OBJECT看着这个名字应该有一个笼统的印象了。翻译成中文就是“空对象”。提供一个对象给指定的类型，用以代替这个对象为空的情况。 Null Object提供了“什么也不做”的行为,隐藏来自它的合作者的细节

那在FrameWork中有这样的设计模式吗？答案是肯定的，比如：String.Empty，Decimal.Zero等 以下是简单例子：

public interface Employee{



public void pay(); 



public boolean isTimeToPay(DateTime payDate);



public static final Employee NULL = new Employee(){



public boolean isTimeToPay(DateTime payDate){

return false;

}



public void pay(){

}

};



}



 



public class DB{



private static Map<String,Employee> EmployMap= new HashMap<String,Employee>();



static {



EmployMap.put("Wilson", new EmployImp());



}

    public static Employee getEmployee(String name){  



    Employee e = EmployMap.get(name);



        if(e == null)  



            return Employee.NULL;  



        return e;  



    }  



}  



 

 

public class EmployImp implements Employee {

 



public Override void pay() {

           DebugLog.log("payed");

}

 

public Override boolean isTimeToPay(Date payDate) {

return true;

}

 

}

 

 

public class Test {

 

public static void main(String[] args) {

Employee e = DB.getEmployee("xxxx");

Date today =new Date();

if(e.isTimeToPay(today))  

   e.pay();  

}

   

public void testPay()

{

Employee e = DB.getEmployee("Bob");

Date today =new Date();

if(e != null && e.isTimeToPay(today))  

   e.pay();  

}

}