java.lang.Integer不能使用Scala中的任何类型转换为java.lang.Byte错误

Question

I can cast Int data to Byte.

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

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)

I can cast twice.

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

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

Are there better ways than this?

Answer 1

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.

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])) {

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

10.asInstanceOf[Byte]

In other words:

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

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

Answer 2

Try converting to int and then to Byte:

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

scala> x.asInstanceOf[Int].asInstanceOf[Byte]
res1: Byte = 10

Or as Ionuț G. Stan suggested:

scala> x.asInstanceOf[Int].toByte
res4: Byte = 10

Although I cannot explain why this work.

Answer 3

An integer is 32 bits in Java, while a byte is obviously 8 bits. The problem is what bits do you truncate to make an integer a byte? The least significant 24 bits or the most significant 24 bits? The correct answer is in the context of your problem.