Pebellslearn by building
0%
RoadmapsMatrixCoursesSandboxSign in
Variables and Data Types  ›  Capstone Project

Type Checks and Casts

Type Checks & Casts 15 minThe is operator checks a value's type and smart-casts it inside the checked branch, as forces a cast and throws if wrong, and as? returns null instead of throwing.

Type Checks and Casts

Sometimes a value arrives whose exact type you do not know: a receipt line that might be a drink name or a quantity, stored as Any. Kotlin lets you ask "what are you?" and then, in the same breath, use the value as that type.

The rule

is asks and smart-casts; as insists and may crash; as? shrugs null.

is and !is check a type and give a Boolean. The magic is smart casting: once Kotlin has seen the check succeed, it treats the value as that type automatically.

val item: Any = "latte"

if (item is String) {
    println(item.uppercase())   // LATTE — no manual cast needed
}
if (item !is Int) {
    println("not a quantity")
}

Smart casts work when the compiler can be sure the value did not change between the check and the use, for example a local val.

as is an unsafe cast. It tells the compiler "trust me". If you are wrong, the program throws ClassCastException:

val qty: Any = 3
val name = qty as String        // crashes: an Int is not a String

as? is the safe cast. It gives the value on success and null on failure, so the result type is nullable:

val name: String? = qty as? String   // null, no crash

A cast never converts a value. It only changes how the compiler views an object that already is that type. val n: Any = 3; n as Long throws, because 3 is an Int. To change a number's type, use toLong().

The shortcut

Pick the tool by asking "What should happen if I am wrong?"

  • Branch to other code → is with smart cast.
  • Get null and handle it → as?.
  • Crash, because being wrong is a bug → as.
Printing a receipt line of unknown typeStep 1 of 4

  1. Ask the type

    if (item is String) — the check succeeds for "latte".

The trap is reaching for as out of habit from other languages. In Kotlin, an is check usually makes the cast unnecessary.

Drills

Answer fast — trust the rule.

quick drill

val x: Any = "mocha". Inside if (x is String) { … }, what can you call?

quick drill

val x: Any = 5. What does x as? String produce?

quick drill

val x: Any = 5. What does x as String do?

quick drill

val n: Any = 3. How do you get a Long with value 3?

Remember

  • is checks a type and smart-casts inside the branch.
  • as crashes on a wrong guess; as? returns null instead.
  • Casts change the view, not the value: convert numbers with to…().

Practice — level up

0/1 solved

Write your answer, then compare it with a rubric and a model answer.

ExplainHandling mixed receipt data

A receipt line arrives as val item: Any, which may be a String (a drink name) or an Int (a quantity). Explain how you would print the drink name in upper case or double the quantity, and compare using is, as and as? for this job.