Scala：向下转型抛出 java.lang.ClassCastException
从非 Java 背景转向 Scala 给我带来了很多困难，包括这个。
scala> class A
defined class A
scala> class B extends A
defined class B
scala> val a = new A
a: A = A@2e893a4a
scala> val b = new B
b: B = B@3a47c130
scala> a.asInstanceOf[B]
java.lang.ClassCastException: A cannot be cast to B
...
scala> b.asInstanceOf[A]
res1: A = B@3a47c130
我知道抛出 ClassCastException 是因为在运行时， a 看起来不像 B，但事实上，它是（据我所知）。这是怎么回事？有什么解决方法吗？谢谢。
编辑：JVM 如何理解 a 无法转换为 B？它是否在 a.getClass 和 B 之间执行一些浅层比较？
附注我试图将私有变量添加到库类中，并重写接受库中定义的类作为参数的类方法之一（我试图将字段添加到的类）。
Coming from a non-Java background to Scala has brought me a wide range of difficulties including this one.
scala> class A
defined class A
scala> class B extends A
defined class B
scala> val a = new A
a: A = A@2e893a4a
scala> val b = new B
b: B = B@3a47c130
scala> a.asInstanceOf[B]
java.lang.ClassCastException: A cannot be cast to B
...
scala> b.asInstanceOf[A]
res1: A = B@3a47c130
I understand that ClassCastException is thrown because at runtime, a doesn't seem like a B but in fact, it is (as far as I understand). What's going on here? Any workarounds? Thanks.
Edit: how does the JVM understand that a cannot be casted to B? Does it perform some shallow comparison between a.getClass and B?
ps. I'm trying to add a private variable to a library class, and override one of the class methods that accepts a class defined in the library as argument (the class I'm trying to add the field to).
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这是一个向下的层次结构。 A 类是一个超类，B 类从它扩展而来，因此它更具体。你无法笼统地提升层次结构。这是一种设计选择，与子类型相关。
有没有一种语言可以提升层次结构？因为你似乎在暗示存在这样一种语言。
It's a hierarchy that goes down. Class A is a super class and B extends from it so it's more specific. You cannot go up the hierarchy in generalization. It's a design choice and relates to subtyping.
Is there any language where you can go up the hierarchy? Cause it seems like you're implying that there is such a language.
您不能将
A转换为B，只能反过来，因为 B 比 A 更具体。You cant cast
AintoB, only the other way around, because B is more specific than A.