将对象从父类转换为子类 [英] Typecasting an object from parent class to child

问题描述

我对用Java语言进行类型转换有误解.问题是 ClassCastException .例如，在此代码中，假设Animal是Dog类的父类，

I have a misunderstanding about typecasting in Java language. The problem is ClassCastException. For example, in this code, assuming Animal is the parent class of the Dog class,

Animal animal = new Animal();
Dog dog = (Dog) animal;

在执行后引发 ClassCastException . 但是，在研究android程序包时，考虑到该Java示例，我发现了一个有关类型转换的示例，该示例应抛出 ClassCastException .

throws ClassCastException after execution. However, while studying android packages, I found an example about typecasting which should throw a ClassCastException, considering that java example.

EditText editText = (EditText) findViewById(R.id.edit_message);

在此代码中， findViewById 方法返回一个 View 类对象，该对象是 EditText 类的超类之一. > android.view.View 到 android.widget.EditText )，代码运行正常.谁能解释我是否犯了错或如何发生?

In this code, findViewById method returns a View class object, which is one of the superclasses of EditText class.(from android.view.View to android.widget.EditText) The code runs fine. Could anyone explain if I made a mistake or how this happens?

提前谢谢.

推荐答案

创建对象后，就无法更改其类型.这就是为什么您不能将动物投向狗的原因.

Once you create an object, you can't change its type. That's why you can't cast an Animal to a Dog.

但是，如果创建子类的对象，则可以在超类类型的变量中保留对它的引用，以后再将其强制转换为子类类型.

However, if you create an object of a sub-class, you can keep a reference to it in a variable of the super-class type, and later you can cast it to the sub-class type.

这将起作用:

Animal a = new Dog ();
Dog d = (Dog) a;

在Android示例中，您具有一个如下所示的布局资源:

In the Android example, you have a layout resource that looks like this :

<EditText
    android:id="@+id/edit_message"
 ..."/>

此定义将导致Android创建EditText的实例，因此您可以将findViewById返回的视图强制转换为EditText.您不能将其强制转换为不是EditText超级类型的其他任何对象.

This definition will cause Android to create an instance of EditText, and therefore you can cast the view returned by findViewById to EditText. You can't cast it to anything else that isn't a super-type of EditText.

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