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.
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.

Practice

1. Which keyword should you use in Kotlin for a variable whose value will never change after assignment?

easy

A. let

B. var

C. val

D. const

Solution

1. Step 1: Understand Kotlin variable keywords

   val is used for read-only variables, meaning their value cannot change once assigned.

2. Step 2: Compare with other keywords

   var allows changing the value; const is not a Kotlin keyword for variables; let is from other languages like Swift or JavaScript.

3. Final Answer:

   val -> Option C

4. Quick Check:

   Immutable variable = val [OK]

Hint: Use val for fixed values, var for changeable ones [OK]

Common Mistakes:

• Confusing val and var
• Using const which is not Kotlin variable keyword
• Thinking let is Kotlin keyword

2. Which of the following is the correct way to declare a nullable String variable in Kotlin?

easy

A. var name: String? = null

B. var name: String?

C. var name: String = null

D. var name: String = "null"

Solution

1. Step 1: Understand nullable type syntax

   In Kotlin, adding ? after the type means the variable can hold null.

2. Step 2: Check initialization

   Nullable variables should be initialized with null if no value is assigned yet.

3. Final Answer:

   var name: String? = null -> Option A

4. Quick Check:

   Nullable variable syntax = var name: String? = null [OK]

Hint: Use ? after type and assign null to allow null values [OK]

Common Mistakes:

• Assigning null to non-nullable type
• Forgetting ? for nullable types
• Using string "null" instead of null

3. What will be the output of this Kotlin code?

var count: Int? = 5
count = null
println(count)

medium

A. null

B. 0

C. 5

D. Compilation error

Solution

1. Step 1: Analyze variable declaration

   count is declared as nullable Int (Int?) and initially set to 5.

2. Step 2: Assign null and print

   Then count is assigned null, which is allowed for nullable types, so printing it outputs null.

3. Final Answer:

   null -> Option A

4. Quick Check:

   Nullable variable assigned null prints null [OK]

Hint: Nullable variables can hold null and print it directly [OK]

Common Mistakes:

• Expecting 5 after reassignment
• Thinking null prints 0
• Assuming compilation error for null assignment

4. Identify the error in this Kotlin code snippet:

val message: String = null
println(message)

medium

A. No error, code runs fine

B. val cannot be used with String type

C. println cannot print val variables

D. Cannot assign null to non-nullable type String

Solution

1. Step 1: Check variable type and assignment

   message is declared as non-nullable String but assigned null, which is not allowed.

2. Step 2: Understand Kotlin null safety rules

   Non-nullable types cannot hold null; to allow null, type must be String?.

3. Final Answer:

   Cannot assign null to non-nullable type String -> Option D

4. Quick Check:

   Non-nullable String cannot be null [OK]

Hint: Non-nullable types cannot hold null values [OK]

Common Mistakes:

• Thinking val disallows null but var allows
• Assuming println can't print val
• Ignoring Kotlin's null safety rules

5. You want to create a variable that can change its value but sometimes hold no value at all. Which declaration is correct in Kotlin?

hard

A. val data: Int = null

B. var data: Int? = null

C. val data: Int? = 0

D. var data: Int = null

Solution

1. Step 1: Understand mutability and nullability

   The variable must be changeable, so var is needed, and it must allow null, so type must be nullable with ?.

2. Step 2: Check each option

   val data: Int = null uses val (immutable) and assigns null to non-nullable type (error). var data: Int? = null uses var and nullable type with null initial value (correct). val data: Int? = 0 uses val (immutable) but nullable type with 0 initial value (not changeable). var data: Int = null uses var but non-nullable type assigned null (error).

3. Final Answer:

   var data: Int? = null -> Option B

4. Quick Check:

   Mutable and nullable = var with ? [OK]

Hint: Use var for changeable and ? for nullable types [OK]

Common Mistakes:

• Using val when variable should change
• Assigning null to non-nullable type
• Confusing val and var usage