Nullable types with ? suffix in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the ? suffix mean in Kotlin types?

It means the variable can hold either a value of that type or null. For example, String? means a variable can be a string or null.

beginner

How do you declare a nullable integer variable in Kotlin?

You add a ? after the type. For example: var number: Int? = null means number can be an integer or null.

intermediate

Why does Kotlin use nullable types instead of allowing null everywhere?

To prevent errors caused by null values, called NullPointerExceptions. Nullable types force you to check for null before using the value.

beginner

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

The code will not compile. Kotlin requires you to explicitly mark a type as nullable with ? to allow null values.

intermediate

How do you safely access a property or method of a nullable variable in Kotlin?

Use the safe call operator ?.. For example, name?.length returns the length if name is not null, otherwise it returns null.

What does String? mean in Kotlin?

A A non-nullable string

B A string that cannot be null

C A nullable integer

D A string that can also be null

String? means the variable can hold a string or null.

Which operator safely accesses a property of a nullable variable?

A ?.

B !!

C :

D ?:

The safe call operator ?. accesses properties only if the variable is not null.

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

A Code compiles and runs

B Code compiles but throws error at runtime

C Code does not compile

D Code ignores the null

Kotlin prevents assigning null to non-nullable types at compile time.

How do you declare a nullable Boolean variable named flag?

A var flag: Boolean = null

B var flag: Boolean? = null

C var flag: Boolean?

D var flag: Boolean

You declare it as var flag: Boolean? = null to allow null values.

Why does Kotlin use nullable types?

A To prevent NullPointerExceptions by forcing null checks

B To allow any variable to be null without checks

C To make code longer

D To allow nulls everywhere

Nullable types help avoid errors by requiring you to handle null values explicitly.

Explain what nullable types are in Kotlin and how the ? suffix works.

Think about how Kotlin prevents null errors.

Describe how to safely use a nullable variable's property or method in Kotlin.

How do you avoid crashes when a variable might be null?