Secondary constructors in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of secondary constructor call

What is the output of this Kotlin code using a secondary constructor?

Kotlin

class Person(val name: String) {
    var age: Int = 0

    constructor(name: String, age: Int) : this(name) {
        this.age = age
    }

    fun info() = "$name is $age years old"
}

fun main() {
    val p = Person("Alice", 30)
    println(p.info())
}

A Alice is 30 years old

B Alice is 0 years old

C Runtime error: Uninitialized property

D Compilation error due to secondary constructor

💡 Hint

Remember that the secondary constructor calls the primary constructor first.

The secondary constructor calls the primary constructor with 'this(name)', initializing 'name'. Then it sets 'age' to 30. So info() returns 'Alice is 30 years old'.

❓ Predict Output

intermediate

Value of property after secondary constructor

What is the value of 'age' after creating an instance with the secondary constructor?

Kotlin

class Animal(val species: String) {
    var age: Int = 5

    constructor(species: String, age: Int) : this(species) {
        this.age = age
    }
}

fun main() {
    val a = Animal("Dog", 7)
    println(a.age)
}

A 7

B 5

C 0

D Compilation error

💡 Hint

Check how the secondary constructor sets the 'age' property.

The primary constructor sets age to 5, but the secondary constructor overrides it to 7.

🔧 Debug

advanced

Identify the error in secondary constructor usage

What error does this Kotlin code produce?

Kotlin

class Car(val brand: String) {
    var year: Int

    constructor(brand: String, year: Int) {
        this.brand = brand
        this.year = year
    }
}

fun main() {
    val c = Car("Toyota", 2020)
    println(c.brand + " " + c.year)
}

A Compilation error: Val cannot be reassigned

B Runtime error: Uninitialized property 'year'

C Compilation error: Secondary constructor must delegate to primary

D No error, prints 'Toyota 2020'

💡 Hint

Check how the secondary constructor calls the primary constructor.

In Kotlin, a secondary constructor must delegate to the primary constructor using 'this(...)'. This code does not, causing a compilation error.

📝 Syntax

advanced

Correct syntax for secondary constructor delegation

Which option shows the correct syntax for a secondary constructor delegating to the primary constructor?

Kotlin

class Book(val title: String) {
    var pages: Int = 0

    // Secondary constructor here
}

A constructor(title: String, pages: Int) { this.pages = pages }

B constructor(title: String, pages: Int) : this(title) { this.pages = pages }

C constructor(title: String, pages: Int) -> this(title) { this.pages = pages }

D constructor(title: String, pages: Int) : super(title) { this.pages = pages }

💡 Hint

Look for the correct delegation syntax using ': this(...)'.

Option B correctly delegates to the primary constructor using ': this(title)'. Option B misses delegation, C uses invalid arrow syntax, D incorrectly uses 'super'.

🚀 Application

expert

Number of instances created with secondary constructor

Given this Kotlin code, how many times is the primary constructor called when creating an instance with the secondary constructor?

Kotlin

class User(val username: String) {
    init {
        println("Primary constructor called for $username")
    }

    constructor(username: String, email: String) : this(username) {
        println("Secondary constructor called for $username with email $email")
    }
}

fun main() {
    val u = User("john_doe", "[e-mail]")
}

A Primary constructor is called after secondary constructor

B Primary constructor is called twice

C Primary constructor is not called

D Primary constructor is called once

💡 Hint

Remember how secondary constructors delegate to primary constructors in Kotlin.

The secondary constructor delegates to the primary constructor exactly once before executing its own body.