Type checking with is operator in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the is operator do in Kotlin?

The is operator checks if a variable is of a specific type and returns true if it matches, otherwise false.

beginner

How do you check if a variable obj is a String in Kotlin?

Use if (obj is String) { ... }. This checks if obj is a String type.

intermediate

What happens to the variable inside the if block after checking with is?

Inside the if block, Kotlin smart casts the variable to the checked type automatically, so you can use it as that type without explicit casting.

beginner

How do you check if a variable is NOT a certain type using the is operator?

Use the negation operator: if (obj !is String) { ... } checks if obj is NOT a String.

intermediate

Can the is operator be used with nullable types?

Yes, but if the variable is null, is returns false because null is not an instance of any type.

What does obj is Int return if obj is 42?

A false

B true

C Throws an error

D Returns null

Since 42 is an Int, obj is Int returns true.

What is the result of obj !is String if obj is "hello"?

A false

B true

C Throws an error

D Returns null

Because obj is a String, obj !is String returns false.

Inside if (obj is String) { }, what type is obj treated as?

A String

B Nullable String

C Int

D Any

Kotlin smart casts obj to String inside the if block.

What does obj is String return if obj is null?

A Throws NullPointerException

B true

C false

D Returns null

Null is not an instance of any type, so is returns false.

Which operator checks if a variable is NOT a certain type?

A is

B not

C !==

D !is

The !is operator checks if a variable is NOT of a given type.

Explain how the is operator works in Kotlin and what happens to the variable inside the if block.

Think about how Kotlin lets you use the variable as the checked type without extra casting.

Describe how to check if a variable is NOT a certain type and what result you get if the variable is null.

Remember how null behaves with type checks and how to negate the check.