CODEWITHSUNDEEP
scalacastingtype-conversion
elm
elm

Reputation: 20415

Scala Boolean to String conversions

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

Upvotes: 6

Views: 12907

Answers (2)

Electric Coffee
Electric Coffee

Reputation: 12104

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.

Upvotes: 6

grotrianster
grotrianster

Reputation: 2468

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.

Upvotes: 0

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