Scala Boolean to String conversions

Question

One way to convert true: Boolean onto a String is

scala> true.toString
res: String = true

However,

scala> true.asInstanceOf[String]
java.lang.ClassCastException: java.lang.Boolean cannot be cast to java.lang.String

Why the latter attempt fails ?

Many Thanks

The first way isn't a cast, it's a method defined on the `Boolean` type. — Michael Zajac, Jul 29 '14 at 18:52
@LimbSoup ok, thanks, updated question formulation accordingly :) — elm, Jul 29 '14 at 18:54
Possible duplication of this question: http://stackoverflow.com/questions/6686992/scala-asinstanceof-with-parameterized-types — Chris Stewart, Jul 29 '14 at 19:13

score 6 · Accepted Answer · answered Jul 29 '14 at 19:27

This is because Boolean isn't a subtype of String or type compatible with String

scala> val a: Any = "Hello"
a: Any = Hello

scala> val b = a.asInstanceOf[String]
b: String = Hello

scala> val c: Char = 123
c: Char = {

scala> val d = c.asInstanceOf[Int]
d: Int = 123

scala> (123).asInstanceOf[Double]
res0: Double = 123.0

scala> (1.3).asInstanceOf[Int]
res1: Int = 1

As you can see, I can easily cast any type compatible types back and forth, in this case numbers (including chars) are cross compatible. All types are compatible with Any, because Any just holds any arbitrary object reference, so as long as you "unbox" using the right types, you're golden.

scala> val arr = Array("Hello", 245, true)
arr: Array[Any] = Array(Hello, 245, true)

scala> val first = arr(0).asInstanceOf[String]
first: String = Hello

scala> val second = arr(1).asInstanceOf[Int]
second: Int = 245

scala> val third = arr(2).asInstanceOf[Boolean]
third: Boolean = true

This works fine, and as long as you're careful with how you unbox, you won't get any errors. This is how you'd do it in Java before generics were introduced to the language. That said, if you cast any one of the array elements to the wrong type, you'll get the cast exception.

The .toString method is different from a type cast, in that it's not a cast at all, it exists to serve as a concise way of converting any object, to a string representation of itself.

scala> val a = List(1,2,3,4)
a: List[Int] = List(1, 2, 3, 4)

scala> a.toString
res0: String = List(1, 2, 3, 4)

This is done so that any type can be printed if necessary, and has nothing to do with casting.

score 0 · Answer 2 · answered Jul 29 '14 at 19:25

When invoking toString on primitives, scala compiler automatically performing boxing (putting primitive to object wrapper) and then performing toString method on Boolean object:

public String toString() {
        return value ? "true" : "false";
}

while invoking asInstanceOf means casting from Boolean to String, these are different types, so CCE is thrown.

Scala Boolean to String conversions

2 Answers2