Smart Casts Workbook

Practice problems for Kotlin Smart Casts. Each takes a minute or two. Write your own answer first, then click Show answer — nothing here is a trick question, just direct practice of the syntax from the lesson.

the basic cast

1. Check, then use

Given x: Any, print its length when it’s a String — without writing a cast.

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if (x is String) {
    println(x.length)   // x is smart-cast to String here
}

2. Negate and return early

Given x: Any, return early when it is not a String; afterward, use it as a String.

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if (x !is String) return
println(x.length)       // x is String from here on

3. Null check narrows

Given name: String?, print its length only when it isn’t null — no safe call, no !!.

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if (name != null) {
    println(name.length)   // name is smart-cast to String
}

in when and conditions

4. Smart cast inside when

Given x: Any, return x + 1 if it’s an Int, else 0. Use the smart cast.

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when (x) {
    is Int -> x + 1
    else -> 0
}

5. Combined condition

Given x: Any, return true only when x is a non-empty String.

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x is String && x.isNotEmpty()

After the is check on the left, x is smart-cast for the right-hand side.

6. Elvis with an early exit

Given value: Any, bind a String named s or return early if value isn’t one. Use a safe cast.

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val s = value as? String ?: return
println(s.uppercase())

the edges

7. Why won’t this smart-cast?

This fails to compile: a nullable var property name accessed as name.length after a null check. Why, and how do you fix it?

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A var property could change between the check and the use, so the compiler won’t smart-cast it. Copy it into a local val first:

val n = name
if (n != null) println(n.length)

8. After a throw

Given config: Config?, throw IllegalStateException when it’s null; afterward use it as non-null.

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if (config == null) throw IllegalStateException("missing config")
config.load()           // config is non-null here

9. When smart casts can’t help

You only have a val x: Any, and you’re certain it’s a String, but there’s no check in the code path. Write the explicit cast to call .length.

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(x as String).length

A plain as throws if you’re wrong; prefer a checked is or a safe as? when you aren’t certain.

10. Combine type and null

Given x: Any?, return its length when it is a non-null String, else -1.

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if (x is String) x.length else -1

A smart cast to String also rules out null, since String is non-nullable.

Back to the lesson, Kotlin Smart Casts, or on to the next one: null safety.

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