Variables (val, var) and null safety in Android Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the difference between val and var in Kotlin?

val declares a read-only variable (like a constant). You cannot change its value after assignment.<br>var declares a mutable variable, so you can change its value later.

beginner

What does null safety mean in Kotlin?

Null safety means Kotlin helps you avoid errors caused by null values by distinguishing nullable and non-nullable types at compile time.

beginner

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

Use 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?

Kotlin will show a compile-time error. Non-nullable variables cannot hold null values.

intermediate

How can you safely access a nullable variable's property or method?

Use the safe call operator ?.. It only calls the property or method if the variable is not null, otherwise returns null.

Which keyword declares a variable that cannot be reassigned in Kotlin?

A val

B var

C const

D let

val declares a read-only variable that cannot be reassigned.

How do you declare a nullable String variable named email?

A val email: String?

B var email: String

C val email: String

D var email: String?

Use String? to allow null values. var email: String? means email can be changed and can be null.

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

A It compiles and works fine

B Compile-time error

C It compiles but crashes at runtime

D Warning but compiles

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

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 does the !! operator do in Kotlin?

A Declares a variable as nullable

B Safely accesses a nullable variable

C Throws an exception if the value is null

D Assigns a default value

The !! operator asserts a value is not null and throws an exception if it is.

Explain the difference between val and var in Kotlin and when to use each.

Think about constants versus variables in everyday life.

Describe how Kotlin's null safety helps prevent app crashes and how you declare nullable variables.

Imagine avoiding a null pointer crash by checking if something is null first.