Variables (val, var) and null safety in Android Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

📝 Syntax

intermediate

Understanding val and var declaration

What will be the output of this Kotlin code snippet?

val x = 10
var y = 5
y = 7
println("x = $x, y = $y")

Android Kotlin

val x = 10
var y = 5
y = 7
println("x = $x, y = $y")

A x = 10, y = 7

B x = 10, y = 5

C x = 7, y = 7

D Compilation error

💡 Hint

Remember val means constant, var means variable.

val declares a read-only variable, so x cannot be changed. var declares a mutable variable, so y can be reassigned.

❓ ui_behavior

intermediate

Null safety with nullable types

What will happen when this Kotlin code runs?

var name: String? = null
println(name?.length)

Android Kotlin

var name: String? = null
println(name?.length)

A Throws NullPointerException

B Prints 0

C Compilation error

D Prints null

💡 Hint

The ?. operator safely accesses properties of nullable variables.

The safe call operator ?. returns null if the variable is null, so name?.length prints null safely.

❓ lifecycle

advanced

Lateinit var usage and initialization

What error will this Kotlin code cause when run?

lateinit var message: String
println(message)

Android Kotlin

lateinit var message: String
println(message)

A Prints null

B Prints empty string

C Throws UninitializedPropertyAccessException

D Compilation error

💡 Hint

Lateinit variables must be initialized before use.

Accessing a lateinit var before initialization throws UninitializedPropertyAccessException at runtime.

🔧 Debug

advanced

Null safety and !! operator behavior

What happens when this Kotlin code runs?

var text: String? = null
println(text!!.length)

Android Kotlin

var text: String? = null
println(text!!.length)

A Throws KotlinNullPointerException

B Prints 0

C Prints null

D Compilation error

💡 Hint

The !! operator asserts non-null and throws if null.

Using !! on a null variable throws KotlinNullPointerException at runtime.

🧠 Conceptual

expert

Choosing val or var for thread safety

In a multi-threaded Android app, which variable declaration is safer to avoid unexpected changes?

val data = mutableListOf()
var data = mutableListOf()

A var because it allows changing the list reference safely

B val because it prevents reassignment but list contents can still change

C val because it makes the list immutable

D var because it prevents concurrent modification exceptions

💡 Hint

val prevents changing the reference but does not make the object immutable.

val prevents the variable from pointing to a different list, but the list itself can still be modified, so thread safety depends on synchronization, not val/var alone.