模式的定义

空对象模式(Null Object Pattern)定义如下：
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类图

20161117124923108.png

模式的简单实现：

先定义一个AbstractObject类：

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

再定义一个实现类：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;
    }
}

我们再定义一个工厂方法类:ObjectFactory

/**
 * 生成对象
 */
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;
    }
}

客户端如下调用即可：

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("----------------------------");
    }
}

而在没有采用此方式的情况下，我们需要在每次创建的时候做判空处理。