Data class copy and destructuring in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin code using data class copy?

Consider the following Kotlin data class and code snippet. What will be printed?

Kotlin

data class Person(val name: String, val age: Int)

fun main() {
    val original = Person("Alice", 30)
    val copy = original.copy(age = 35)
    println(copy)
}

A Person(name=Alice, age=35)

B Person(name=Alice, age=30)

C Person(name=Alice, age=0)

D Person(name=, age=35)

💡 Hint

Remember that copy() creates a new object with the same properties unless you override them.

The copy() function creates a new Person object with the same name but the age is overridden to 35. So the output shows Person(name=Alice, age=35).

❓ Predict Output

intermediate

What is the output when destructuring a Kotlin data class?

Given the Kotlin data class and code below, what will be printed?

Kotlin

data class Point(val x: Int, val y: Int)

fun main() {
    val point = Point(10, 20)
    val (a, b) = point
    println("a = $a, b = $b")
}

A a = 20, b = 10

B a = 0, b = 0

C a = Point(x=10, y=20), b = null

D a = 10, b = 20

💡 Hint

Destructuring extracts properties in the order they are declared in the data class.

The destructuring declaration val (a, b) = point assigns a to x (10) and b to y (20).

🔧 Debug

advanced

What error does this Kotlin code raise when copying a data class?

Examine the Kotlin code below. What error will it produce when run?

Kotlin

data class User(val username: String, val email: String)

fun main() {
    val user = User("bob", "[e-mail]")
    val newUser = user.copy(username = null)
    println(newUser)
}

A SyntaxError: unexpected null

B Type mismatch: null cannot be a value of a non-null type String

C NullPointerException at runtime

D No error, prints User(username=null, email=[e-mail])

💡 Hint

Check the type of the username property and what null means in Kotlin.

The username property is non-nullable String. Passing null to it causes a compile-time type mismatch error.

❓ Predict Output

advanced

What is the output of destructuring with default values in Kotlin?

Consider this Kotlin data class and code. What will be printed?

Kotlin

data class Rectangle(val width: Int = 5, val height: Int = 10)

fun main() {
    val rect = Rectangle(height = 20)
    val (w, h) = rect
    println("Width: $w, Height: $h")
}

A Width: 5, Height: 20

B Width: 0, Height: 20

C Width: 5, Height: 10

D Width: 20, Height: 20

💡 Hint

Default values are used when arguments are not provided during object creation.

The width uses the default value 5, while height is set to 20 explicitly. Destructuring assigns these values accordingly.

🧠 Conceptual

expert

How many items are in the resulting map after copying and destructuring?

Given the Kotlin code below, how many entries does the resultMap contain?

Kotlin

data class Employee(val id: Int, val name: String, val department: String)

fun main() {
    val emp1 = Employee(1, "John", "Sales")
    val emp2 = emp1.copy(name = "Jane")
    val (id, name, dept) = emp2
    val resultMap = mapOf("id" to id, "name" to name, "department" to dept)
    println(resultMap.size)
}

A 1

B 2

C 3

D 0

💡 Hint

Count the number of key-value pairs added to the map.

The map contains three entries: "id", "name", and "department". So the size is 3.