Null safety in collections in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does null safety mean in Kotlin collections?

Null safety means Kotlin collections can be designed to either allow or disallow null values, helping prevent errors caused by unexpected nulls.

beginner

How do you declare a list that cannot contain null values in Kotlin?

Use List<Type> without a question mark. For example, val list: List<String> means no nulls allowed in the list.

beginner

How do you declare a list that can contain null values in Kotlin?

Use a nullable type inside the list, like List<String?>. This means the list can hold strings or nulls.

intermediate

What happens if you try to add a null to a non-nullable collection in Kotlin?

The code will not compile because Kotlin enforces null safety at compile time, preventing nulls in non-nullable collections.

intermediate

Explain the difference between MutableList<String> and MutableList<String?>.

MutableList<String> can only hold non-null strings, while MutableList<String?> can hold strings and null values.

Which Kotlin collection declaration allows null values inside the collection?

A MutableList<String>

B List<String>

C List<String?>

D Set<String>

Only List<String?> allows null values because the type inside the list is nullable.

What will happen if you try to add null to a MutableList<String>?

A Compile-time error

B Null is added successfully

C Runtime exception

D The list ignores the null

Kotlin prevents adding null to non-nullable collections at compile time.

How do you declare a map in Kotlin that can have null values but non-null keys?

A Map<String, String>

B Map<String?, String>

C Map<String?, String?>

D Map<String, String?>

Keys are non-null String, values are nullable String?.

Which of these is a nullable collection type in Kotlin?

A List<String?>

B List<String>

C MutableList<Int>

D Set<Double>

Only List<String?> allows null elements.

Why is null safety important in Kotlin collections?

A To allow any type of data

B To avoid null pointer exceptions

C To make collections slower

D To allow duplicates

Null safety helps prevent null pointer exceptions by controlling nulls in collections.

Describe how Kotlin handles null safety in collections and why it is useful.

Think about how Kotlin uses question marks to allow or disallow nulls.

Explain the difference between List<String> and List<String?> in Kotlin.

Focus on the question mark after the type inside the list.