Parent obj = new Child();有道理? [英] Does Parent obj = new Child(); make sense?

问题描述

在Java中:

Parent obj = new Child();

我创建了一个类型为Parent的对象.我假设我只能调用Parent类中定义的方法.因此，我无法调用Child中定义的其他"方法或访问Child中定义的变量. 因此，我认为这是浪费内存，因为我新建了一个Child实例，该实例比Parent占用更多的内存空间.至此，我认为这没有任何意义. 任何人都可以请问一些有意义的理由/例子吗?

I created an object of type Parent. I assume that I can only invoke the methods defined in Parent class. Thus, I can't invoke the "additional" methods defined in Child or access variables defined in Child. So I think it's a waste of memory since I have newed a instance of Child which takes additional memory space than Parent. To this point I think it does not make sense. Can anyone please give some reason/example where this make sense?

推荐答案

是的，确实有道理.

如果要使用Child，可以这样做，但是将与它的交互限制为Parent方法.

You would do this if you want to use a Child, but constrain your interaction with it to Parent methods.

这种习语在Java集合中尤为常见:

This idiom is particularly common with Java collections:

List<String> myList = new ArrayList<String>();

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