空物件模式--- Pattern Null Object

摘要： 模式的定義 空物件模式(Null Object Pattern)定義如下： Provide an object as a surrogate for the lack of an object of a given type. The Null Object provides inte...

模式的定義

空物件模式(Null Object Pattern)定義如下：

Provide an object as a surrogate for the lack of an object of a given type. The Null Object provides intelligent do nothing behavior, hiding the details from its collaborators.

UML類圖

模式的簡單實現：

先定義一個AbstractObject類：

/**
 * 抽象類
 */
public abstract class AbstractObject {
public int id;
public String name;
//抽象方法
public abstract void doAction();
public abstract boolean isNull();
}

再定義一個實現類：ConcreteObject

public class ConcreteObject extends AbstractObject {
public ConcreteObject(int id,String name) {
super.id = id;
super.name = name;
}

@Override
public void doAction() {
System.out.println("id:"+id+"--name:"+name+"---doAction()");
}

@Override
public boolean isNull() {
return false;
}
}

我們再定義一個工廠方法類:ObjectFactory

/**
 * 生成物件
 */
public class ObjectFactory {
public static AbstractObject creator(int id) {
AbstractObject result = null;
switch (id) {
case 1:
result = new ConcreteObject(1, "hello");
break;
case 2:
result = new ConcreteObject(2, "world");
break;
default:
result = new NullObject();
break;
}
return result;
}
}

客戶端如下呼叫即可：

public class Main {
public static void main(String[] args) {
AbstractObject aObject = ObjectFactory.creator(1);
aObject.doAction();
System.out.println("----------------------------");
aObject = ObjectFactory.creator(2);
aObject.doAction();
System.out.println("----------------------------");
aObject = ObjectFactory.creator(-1);
System.out.println("is null"+aObject.isNull());
aObject.doAction();
System.out.println("----------------------------");
}
}

而在沒有采用此方式的情況下，我們需要在每次建立的時候做判空處理。