Extension properties in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of extension property with backing field

What is the output of this Kotlin code using an extension property?

Kotlin

var String.lastChar: Char
    get() = this[length - 1]
    set(value) { println("Setter called with: $value") }

fun main() {
    val word = "hello"
    println(word.lastChar)
    word.lastChar = 'a'
}

A Compilation error: Extension properties cannot have backing fields

B

o
Setter called with: a

C

Setter called with: o
Setter called with: a

D

o
a

💡 Hint

Extension properties cannot store data, but can define custom getters and setters.

The getter returns the last character 'o'. The setter only prints a message; it does not store the value. So output is 'o' then 'Setter called with: a'.

❓ Predict Output

intermediate

Extension property on nullable type

What is the output of this Kotlin code with an extension property on a nullable String?

Kotlin

val String?.isNullOrEmpty: Boolean
    get() = this == null || this.isEmpty()

fun main() {
    val a: String? = null
    val b: String? = ""
    val c: String? = "abc"
    println(a.isNullOrEmpty)
    println(b.isNullOrEmpty)
    println(c.isNullOrEmpty)
}

A

true
true
false

B

false
true
false

C Compilation error: Extension properties cannot be declared on nullable types

D

true
false
false

💡 Hint

Extension properties can be declared on nullable types and can check for null.

The property returns true if the string is null or empty. So null and empty string print true, non-empty prints false.

🔧 Debug

advanced

Why does this extension property cause a compilation error?

This Kotlin code tries to declare an extension property with a backing field. Why does it fail?

Kotlin

var String.extensionProp: Int = 0
    get() = field
    set(value) { field = value }

fun main() {
    println("test".extensionProp)
}

A Extension properties cannot have backing fields, so 'field' is not allowed.

B The property must be declared as 'val' not 'var' for extensions.

C Extension properties must be top-level functions, not inside main.

D The getter and setter must be omitted for extension properties.

💡 Hint

Remember how extension properties work internally in Kotlin.

Extension properties do not actually insert fields into the class, so backing fields like 'field' are not allowed and cause compilation errors.

❓ Predict Output

advanced

Extension property with custom getter logic

What is the output of this Kotlin code using an extension property with a custom getter?

Kotlin

val List<Int>.sumOfSquares: Int
    get() = this.sumOf { it * it }

fun main() {
    val numbers = listOf(1, 2, 3)
    println(numbers.sumOfSquares)
}

A 6

B Runtime error: Unresolved reference sumOf

C Compilation error: sumOf requires Kotlin 1.4+

D 14

💡 Hint

The property calculates the sum of squares of the list elements.

1*1 + 2*2 + 3*3 = 1 + 4 + 9 = 14, so the output is 14.

❓ Predict Output

expert

Extension property shadowing and receiver resolution

What is the output of this Kotlin code with extension properties and shadowing?

Kotlin

open class A {
    open val String.prop: String
        get() = "A"

    fun printProp(s: String) {
        println(s.prop)
    }
}

class B : A() {
    override val String.prop: String
        get() = "B"
}

fun main() {
    val b = B()
    b.printProp("hello")
}

A A

B Compilation error: Cannot override extension property

C B

D Runtime error: NoSuchMethodError

💡 Hint

Member extension properties declared in classes can be overridden in subclasses.

Member extension properties can be overridden. When calling b.printProp (where b is B), the dispatch receiver is B, so the overridden getter from B is used, printing 'B'.