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();
}
}