Map-backed delegated properties in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of map-backed delegated properties with default values

What is the output of this Kotlin code using map-backed delegated properties?

Kotlin

class User(val map: Map<String, Any?>) {
    val name: String by map
    val age: Int by map
}

fun main() {
    val user = User(mapOf("name" to "Alice", "age" to 30))
    println("Name: ${user.name}, Age: ${user.age}")
}

A Compilation error due to missing property

B Name: Alice, Age: 30

C Name: null, Age: 30

D Name: Alice, Age: null

💡 Hint

The delegated properties read values from the map by their keys.

The properties 'name' and 'age' are delegated to the map. The map contains both keys with correct types, so the values are retrieved and printed.

❓ Predict Output

intermediate

Behavior when map key is missing in map-backed delegated properties

What happens when a delegated property tries to access a missing key in the map?

Kotlin

class User(val map: Map<String, Any?>) {
    val name: String by map
    val age: Int by map
}

fun main() {
    val user = User(mapOf("name" to "Bob"))
    println("Name: ${user.name}, Age: ${user.age}")
}

A Throws NoSuchElementException at runtime

B Name: Bob, Age: null

C Compilation error due to missing key

D Name: Bob, Age: 0

💡 Hint

Accessing a missing key in the map throws an exception when using map delegation.

The delegated property 'age' tries to get a value from the map but the key 'age' is missing, causing a NoSuchElementException at runtime.

🔧 Debug

advanced

Identify the runtime error in map-backed delegated properties

What runtime error occurs when running this Kotlin code?

Kotlin

class User(val map: Map<String, Any?>) {
    val name: String by map
    val age: Int by map
}

fun main() {
    val user = User(mapOf("name" to "Charlie", "age" to "thirty"))
    println("Name: ${user.name}, Age: ${user.age}")
}

A NoSuchElementException due to missing key

B NullPointerException due to null value

C ClassCastException due to wrong type for 'age'

D Compilation error due to type mismatch

💡 Hint

The map contains a String for 'age' but the property expects an Int.

The property 'age' expects an Int but the map provides a String, causing a ClassCastException at runtime.

📝 Syntax

advanced

Correct syntax for mutable map-backed delegated properties

Which option correctly declares mutable map-backed delegated properties in Kotlin?

Kotlin

class User(val map: MutableMap<String, Any?>) {
    var name: String by map
    var age: Int by map
}

A

val name: String by mutableMapOf()
val age: Int by mutableMapOf()

B

val name: String by map
val age: Int by map

C

var name: String by map.toMap()
var age: Int by map.toMap()

D

var name: String by map
var age: Int by map

💡 Hint

Mutable properties require var and a mutable map for delegation.

Using 'var' with a MutableMap allows the delegated properties to be mutable and backed by the map. Using 'val' or immutable maps does not allow mutation.

🚀 Application

expert

How many entries are in the map after property mutation?

Given this Kotlin code, how many entries does the map contain after the mutation?

Kotlin

fun main() {
    val data = mutableMapOf<String, Any?>("name" to "Dana", "age" to 25)
    class User(val map: MutableMap<String, Any?>) {
        var name: String by map
        var age: Int by map
    }
    val user = User(data)
    user.age = 26
    user.name = "Dana S."
    println(data.size)
}

A 2

B 3

C 1

D 0

💡 Hint

Mutating delegated properties updates existing map entries, not adding new ones.

The map initially has 2 entries. Changing values via delegated properties updates those entries without adding or removing keys, so size remains 2.