如何在 Scala 中正确使用 asInstanceOf [英] how to use asInstanceOf properly in Scala
问题描述
我一直在使用基本的 Scala 数据类型.我注意到 scala.Any 类定义了来自 这里 API 可以将接收器对象转换为 T0 类型".使用这种方法作为起点，我想研究 Scala 中的转换.另外，我在 stackoverflow 上查找了关于这个主题的其他问题，我想出了 这个有了这些信息，我编写了一个愚蠢的程序.
I have been playing with basic Scala data types. I noticed that the scala.Any class defines the method asInstanceOf[T0]: T0 from here The API has it that it can "Cast the receiver object to be of type T0". Using this method as a starting point, I wanted to investigate casting in Scala. Also, I looked up stackoverflow for other questions on this topic and I came up with this
With this information in hand, I wrote up a silly program.
package com.att.scala
import com.att.scala.Sheltie
object Casting {
//def foo(x: String){
def foo(x: Int) {
println("x is " + x)
//if(x.isInstanceOf[String])
if(x.isInstanceOf[Int])
println("Int x is " + x)
//println("String x is " + x)
}
def entry() {
//val double: Any = 123.123
val double: Double = 123.23
val int = double.asInstanceOf[Int] //exception expected here
//val str: String = "123"
foo(int)
}
}
<小时>
我的目标是了解在以下情况下会发生什么(以及为什么):1) 从 Any 类型转换为 Int.2) 从 Double 类型转换为 Int3) 从 String 到 Int 的转换
My goal is to understand what happens (and why) in the following cases: 1) casting from an Any type to an Int. 2) casting from Double type to an Int 3) casting from String to Int
在第一种情况下，当我将程序运行为 com.att.scala.Casting.entry 时，我得到了一个运行时 ClasscastException，如下所示.异常如下所示:
In the first case, I got a run time ClasscastException as below, when I ran the program as - com.att.scala.Casting.entry. The exception is shown below:
java.lang.ClassCastException: java.lang.Double 不能转换为 java.lang.Integer在 scala.runtime.BoxesRunTime.unboxToInt(来源不明)在 com.att.scala.Casting$.entry(Casting.scala:17)
在第二种情况下，我得到以下结果:整数是 123x 是 123整数 x 是 123
In the second case, I get the following result: int is 123 x is 123 Int x is 123
在这种情况下，代码应该产生 ClasscastException，但它没有.这就是我的担心.
In this case, the code is supposed to produce a ClasscastException, but it does not. That is my worry.
- 在第三种情况下，我得到一个 classcastexception:
java.lang.ClassCastException: java.lang.String 不能转换为 java.lang.Integer在 scala.runtime.BoxesRunTime.unboxToInt(来源不明)在 com.att.scala.Casting$.entry(Casting.scala:20)
通过此示例，我的目标是了解 Scala 中强制转换的基础知识.我知道这个例子无论如何都不是真实世界的例子，但我试图让我的头脑围绕基础知识.
With this example,my goal is to get to the very basics of casting in Scala. I know that this example is not a real world example by any means, but I was trying to get my head wrapped around the basics.
推荐答案
Java(和 Scala)允许您将原始 double 转换为 int(在 Scala 的情况下，Double 到 Int).另一方面，您不能将 java.lang.Double 转换为 java.lang.Int.
Java (and Scala) allows you to cast a primitive double to int (in Scala's case, Double to Int). On the other hand, you cannot cast java.lang.Double to java.lang.Int.
当您将 Double 声明为 Any 时，您明确要求编译器忘记您给它一个 Double.因此，为了支持 Any 接口，编译器将值存储为装箱双精度值(即 java.lang.Double).
When you declare the Double as an Any, you're explicitly asking the compiler to forget that you gave it a Double. Therefore, in order to support the Any interface, the compiler is storing the value as a boxed double (i.e., java.lang.Double).
这种行为看起来确实令人困惑，但这不是错误.根据 Scala 语言规范的 §12.1:
The behavior does seem confusing, but it's not a bug. According §12.1 of the Scala Language Spec:
如果 T 是数值，则测试 x.asInstanceOf[T] 被特殊对待类型(第 12.2 节).在这种情况下，演员表将被翻译成一个应用程序转换方法 x.toT(第 12.2.1 节).
The test x.asInstanceOf[T] is treated specially if T is a numeric value type (§12.2). In this case the cast will be translated to an application of a conversion method x.toT (§12.2.1).
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