java.lang.Integer 不能转换为 java.lang.Byte 错误与 Scala 中的任何类型

问题描述

我可以将 Int 数据转换为 Byte.

I can cast Int data to Byte.

scala> 10.asInstanceOf[Byte]
res8: Byte = 10

但是在 Any 类型中使用相同的值时，强制转换会引发错误.

However with the same value in Any type, the cast raises an error.

scala> val x : Any = 10
x: Any = 10

scala> x.asInstanceOf[Byte]
java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.Byte
    at scala.runtime.BoxesRunTime.unboxToByte(BoxesRunTime.java:98)
    at .<init>(<console>:10)

我可以施放两次.

scala> val y = x.asInstanceOf[Int]
y: Int = 10

scala> y.asInstanceOf[Byte]
res11: Byte = 10

还有比这更好的方法吗?

Are there better ways than this?

推荐答案

在 Scala 中，编译器试图隐藏原始类型和引用类型(盒装)之间的区别，默认为原始类型.有时，抽象泄漏，您会看到类似的问题.

In Scala, compiler tries to hide the distinction between primitive types and reference ones (boxed), defaulting to primitives. Sometimes, abstractions leak and you see that kind of problems.

在这里，您假装值是 Any，这需要编译器回退到装箱值:

Here, you're pretending that value is Any, which require compiler to fallback to boxed values:

override def set(value:Any) = {
    if (check(value.asInstanceOf[Byte])) {

在这里，您没有将值限制为引用，因此此类转换将在原始类型上完成:

And here, you're not limiting value to be referential, so such casting will be done on primitive types:

10.asInstanceOf[Byte]

换句话说:

scala> val x: Any = 10
x: Any = 10

scala> x.asInstanceOf[Byte]
java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.Byte
  at scala.runtime.BoxesRunTime.unboxToByte(BoxesRunTime.java:97)
  ... 32 elided

scala> val y: Int = 10
y: Int = 10

scala> y.asInstanceOf[Byte]
res4: Byte = 10

要克服这个问题，您可能需要进行额外的转换，例如字符串:

To overcome this problem, you probably have to go through an extra conversion to, say, String:

scala> x.toString.toInt
res6: Int = 10

scala> x.toString.toByte
res7: Byte = 10

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