不能在运行时在java中向下转发 [英] Can't not downcast in java at runtime

问题描述

我有一个类Animal和一个类Dog如下：

I have a class Animal and a class Dog as following:

  class Animal{}
  class Dog extend Animal{}

主要类别：

   class Test{
       public static void main(String[] args){
           Animal a= new Animal();
           Dog dog = (Dog)a;
       }
   }

错误显示：

Exception in thread "main" java.lang.ClassCastException: com.example.Animal cannot be cast to com.example.Dog

推荐答案

动物不能是一只狗可以是一只猫或其他像你的情况动物

An animal could not be a dog can be a cat or something else like in your case an Animal

Animal a= new Animal(); // a points in heap to Animal object
Dog dog = (Dog)a; // A dog is an animal but not all animals are  dog

对于向下转动你必须这样做

For downcasting you have to do this

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

顺便说一下，向下转发很危险你可以拥有这个 RuntimeException ，如果它是用于培训目的，那就没关系。

By the way, downcasting is dangerous you can have this RuntimeException , if it's for training purpose it's ok.

如果你想避免运行时异常，你可以做这个检查，但它会慢一点。

If you want to avoid runtime exceptions you can do this check but it'll be a little more slower.

 Animal a = new Dog();
 Dog dog = null;
  if(a instanceof Dog){
    dog = (Dog)a;
  }

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