Any differences between asInstanceOf[X] and toX for value types?

问题

I used IntelliJ's ability to convert Java code to Scala code which generally works quite well.

It seems that IntelliJ replaced all casts with calls to asInstanceOf.

Is there any valid usage of asInstanceOf[Int], asInstanceOf[Long] etc. for value types which can't be replaced by toInt, toLong, ...?

回答1:

I do not know of any such cases. You can check yourself that the emitted bytecode is the same by compiling a class like

class Conv {
  def b(i: Int) = i.toByte
  def B(i: Int) = i.asInstanceOf[Byte]
  def s(i: Int) = i.toShort
  def S(i: Int) = i.asInstanceOf[Short]
  def f(i: Int) = i.toFloat 
  def F(i: Int) = i.asInstanceOf[Float]  
  def d(i: Int) = i.toDouble
  def D(i: Int) = i.asInstanceOf[Double]
}

and using javap -c Conv to get

public byte b(int);
  Code:
   0:   iload_1
   1:   i2b
   2:   ireturn

public byte B(int);
  Code:
   0:   iload_1
   1:   i2b
   2:   ireturn

...

where you can see that the exact same bytecode is emitted in each case.

回答2:

Well, toInt and toLong are not casts. The correct conversion of type casting is asInstanceOf indeed. For example:

scala> val x: Any = 5
x: Any = 5

scala> if (x.isInstanceOf[Int]) x.asInstanceOf[Int] + 1
res6: AnyVal = 6

scala> if (x.isInstanceOf[Int]) x.toInt + 1
<console>:8: error: value toInt is not a member of Any
       if (x.isInstanceOf[Int]) x.toInt + 1
                                  ^

来源：https://stackoverflow.com/questions/4528960/any-differences-between-asinstanceofx-and-tox-for-value-types