Smart casts in when and if in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a smart cast in Kotlin?

A smart cast is when Kotlin automatically converts a variable to a more specific type after checking its type with conditions like if or when, so you don't need to cast it manually.

beginner

How does Kotlin use smart casts inside an if statement?

Inside an if block, if you check a variable's type (e.g., if (x is String)), Kotlin automatically treats x as that type inside the block without needing a manual cast.

intermediate

Explain smart casts with a when expression.

When you use when to check a variable's type, Kotlin smart casts the variable to the matched type inside each branch, so you can use it directly as that type.

intermediate

Why can't Kotlin smart cast a variable if it is mutable (var) and can change?

If a variable is mutable (var) and can be changed elsewhere, Kotlin can't guarantee its type stays the same, so it won't smart cast it to avoid errors.

beginner

Show a simple Kotlin example using smart casts in an if statement.

fun printLength(obj: Any) { if (obj is String) { // obj is smart cast to String here println(obj.length) } else { println("Not a string") } }

What does Kotlin do after you check if (x is String) inside the if block?

A Throws an error if you use x as String

B Automatically treats x as String without manual cast

C Requires you to cast x manually

D Changes the type of x permanently

Kotlin smart casts x to String inside the if block after the type check.

Can Kotlin smart cast a var variable inside an if statement?

A Only if the variable is null

B Yes, always

C No, if the variable can change

D Only if the variable is a primitive type

Kotlin won't smart cast mutable variables if they can be changed elsewhere.

In a when expression, how does Kotlin handle smart casts?

A It smart casts the variable to the matched type in each branch

B It never smart casts in when expressions

C It requires manual casting in each branch

D It only smart casts if the variable is a String

Kotlin smart casts the variable to the matched type inside each when branch.

Which of these is a reason Kotlin might NOT smart cast a variable?

A The variable is immutable (val)

B The variable is a local variable

C The variable is checked with is operator

D The variable is mutable (var) and can be changed

If the variable is mutable and can change, Kotlin avoids smart casting to prevent errors.

What is the benefit of smart casts in Kotlin?

A You write less code by avoiding manual casts

B It makes the program slower

C It requires more manual type checks

D It disables type safety

Smart casts let you use variables as their checked type without extra casting code.

Explain how Kotlin smart casts work inside an if statement with a type check.

Think about how Kotlin treats the variable after confirming its type.

Describe why Kotlin might not smart cast a mutable variable and how this affects your code.

Consider what happens if the variable can change after the check.