Class declaration syntax in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin class instantiation?

Consider this Kotlin code snippet. What will be printed when the main function runs?

Kotlin

class Person(val name: String, val age: Int) {
    fun greet() = "Hello, my name is $name and I am $age years old."
}

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

A Hello, my name is Alice and I am 30 years old.

B Hello, my name is name and I am age years old.

C Compilation error due to missing constructor

D Runtime error: Uninitialized property

💡 Hint

Look at how the primary constructor parameters are declared and used.

The class Person has a primary constructor with parameters name and age declared as val, so they become properties. The greet() function uses these properties to return the greeting string. Instantiating Person with "Alice" and 30 sets these properties correctly.

❓ Predict Output

intermediate

What does this Kotlin class print?

Analyze this Kotlin code and determine the output when main() is executed.

Kotlin

class Rectangle(val width: Int, val height: Int) {
    val area = width * height
}

fun main() {
    val rect = Rectangle(5, 4)
    println(rect.area)
}

A 20

B Compilation error: val area must be initialized in constructor

C 9

D Runtime error: Uninitialized property area

💡 Hint

Check how the property 'area' is initialized using constructor parameters.

The property 'area' is initialized by multiplying width and height, which are constructor parameters. For width=5 and height=4, area is 20.

🔧 Debug

advanced

Identify the error in this Kotlin class declaration

This Kotlin code does not compile. What is the cause of the error?

Kotlin

class Car(model: String, year: Int) {
    fun info() = "Model: $model, Year: $year"
}

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

A No error, code runs and prints the info

B Error because info() function is missing return type

C Error because model and year are not declared as properties with val or var

D Error because class Car lacks a primary constructor

💡 Hint

Check if constructor parameters are accessible inside class methods.

Constructor parameters without val or var are not properties and cannot be accessed in other class members. The info() function tries to use model and year which are not properties, causing a compilation error.

📝 Syntax

advanced

Which Kotlin class declaration is syntactically correct?

Choose the option that correctly declares a Kotlin class with a primary constructor and a property.

A class Book(title: String) val title: String

B class Book(title: String) { val title = title }

C class Book(var title String) {}

D class Book(val title: String)

💡 Hint

Remember the syntax for declaring properties in the primary constructor.

Option D correctly declares a class with a primary constructor that has a property 'title' using val. Option D tries to assign constructor parameter to property without val/var, which is allowed but redundant and less idiomatic. Option D misses colon after var. Option D is invalid syntax.

🚀 Application

expert

How many instances are created and what is printed?

Given this Kotlin code, how many instances of class User are created and what is the output?

Kotlin

class User(val name: String) {
    init {
        println("User created: $name")
    }
}

fun main() {
    val users = listOf(User("Anna"), User("Bob"), User("Cara"))
    println("Total users: ${users.size}")
}

A 1 instance created; prints User created: Anna, Total users: 1

B 3 instances created; prints User created: Anna, User created: Bob, User created: Cara, Total users: 3

C 3 instances created; prints Total users: 3 only

D Compilation error due to missing constructor

💡 Hint

Each User() call creates an instance and runs init block.

Each User("...") creates an instance and runs the init block printing the creation message. Then the list size is printed as 3.