检查子类的属性 [英] Check attribute of a subclass

问题描述

我偶然发现了这种情况，但我不知道如何以正确的方式处理它：

I stumbled across that situation but I don't know how to handle it the right way:

class Myclass { }
class MyclassWithAwesomeStuff extends Myclass {
    public boolean awesomeStuff;
}

我将Myclass对象保存在数组列表中，然后通过此列表进行迭代：

I'm saving the Myclass objects in an arraylist and iterate later through this list:

for(Myclass m : list) {
    //here I want to check if awesomeStuff is true, in case it is a MyclasswithAwesomeStuff
}

这里的问题是：父类Myclass doesn' t知道awesomeStuff属性（它不应该，因为这是一个功能，只有与派生类）。但我该如何管理呢？问题是，数组列表包含Myclass和MyclassWithAwesomeStuff元素，foreach循环将它们总是转换为Myclass。

The problem here is: the parent class Myclass doesn't know the awesomeStuff attribute (and it shouldn't because that is a feature that comes only with the derived class). But how can I manage this? The problem is, that the arraylist contains Myclass and MyclassWithAwesomeStuff elements and the foreach loop casts them always to Myclass.

我想知道这是否是设计失败？

I wonder if this is a design failure?

//编辑：

好吧，为了让我的问题更有形，这里有更多的信息。我试图建立一个小咖啡店：

Okay, to make my question a little bit more tangible, here more infos. I'm trying to build a small coffee shop:

class Coffee { }
class CoffeeMix extends Coffee {
    public boolean shaken;
}

我将数组列表中的咖啡项目保存：

I'm saving the coffee items in an array list:

ArrayList<Coffee> coffees = new ArrayList<Coffee>();

因此在这个数组列表中存在普通的咖啡对象和咖啡混合对象。现在我想显示所有的咖啡混合对象，摇动：

So in this array list exist normal coffee objects and coffee mix objects. Now I want to display all coffee mix objects, that are shaken:

for(Coffee c : coffees) {
    //here is the same problem as above
}

/评论：instanceof似乎是一个坏主意，因为它螺丝的想法背后oo和一个接口，两个类实现是一个坏主意，因为一个正常的咖啡不能动摇。那么如何处理呢？

As I can see from the answers/comments: instanceof seems to be a bad idea, because it screws the idea behind oo up and an interface that both classes implement is a bad idea too, because a normal coffee can not be shaken. So how to handle this?

推荐答案

测试 m code> 运算符

Test if m is a MyclassWithAwesomeStuff with the instanceof operator.

if (m instanceof MyclassWithAwesomeStuff)
{
    MyclassWithAwesomeStuff mwas = (MyclassWithAwesomeStuff) m;
    // Now you can access "awesomeStuff" with "mwas"
}

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