Non-nullable types by default in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean that Kotlin has non-nullable types by default?

In Kotlin, variables cannot hold null unless explicitly declared. This helps prevent errors caused by null values, making programs safer.

beginner

How do you declare a variable that can hold null in Kotlin?

You add a question mark ? after the type. For example, var name: String? means name can be a string or null.

beginner

What happens if you try to assign null to a non-nullable variable in Kotlin?

The code will not compile. Kotlin forces you to handle nulls explicitly to avoid runtime errors.

beginner

Explain the difference between String and String? in Kotlin.

String cannot be null. String? can be null. You must check for null before using String? safely.

intermediate

What Kotlin feature helps you safely use nullable variables?

The safe call operator ?. lets you access properties or methods only if the variable is not null. Otherwise, it returns null.

In Kotlin, what does var age: Int mean?

A age cannot be null

B age can be null

C age is a nullable integer

D age is a string

Without a ?, the type is non-nullable, so age cannot be null.

How do you declare a nullable string in Kotlin?

A var name: ?String

B var name: String

C var name: String?

D var name: Nullable String

Adding ? after the type makes it nullable.

What operator helps safely access a property of a nullable variable?

A ?.

B !!

C ::

D ->

The safe call operator ?. returns null if the variable is null, avoiding errors.

What happens if you assign null to a non-nullable variable?

A Variable becomes null

B Variable is ignored

C Runtime exception

D Compilation error

Kotlin prevents this at compile time to avoid null pointer errors.

Which of these is a non-nullable type in Kotlin?

A Int?

B Int

C String?

D null

Int is non-nullable, while Int? can hold null.

Explain why Kotlin uses non-nullable types by default and how it helps programmers.

Think about how null values cause errors and how Kotlin stops that.

Describe how to safely work with a nullable variable in Kotlin.

Remember the special operator that helps avoid null problems.