Primary constructor and init blocks in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of init block with primary constructor

What is the output of this Kotlin code?

Kotlin

class Person(val name: String) {
    init {
        println("Hello, $name")
    }
}

fun main() {
    val p = Person("Anna")
}

A Hello, name

B Compilation error

C No output

D Hello, Anna

💡 Hint

The init block runs right after the primary constructor.

The init block runs immediately after the primary constructor. It prints "Hello, Anna" because the name property is initialized with "Anna".

❓ Predict Output

intermediate

Value of property after init block

What is the value of 'age' after creating the object?

Kotlin

class User(val name: String, val birthYear: Int) {
    val age: Int
    init {
        age = 2024 - birthYear
    }
}

fun main() {
    val u = User("Tom", 1990)
    println(u.age)
}

A Compilation error

B 0

C 34

D 1990

💡 Hint

Age is calculated in the init block using birthYear.

The init block calculates age as 2024 - 1990 = 34, so u.age is 34.

❓ Predict Output

advanced

Multiple init blocks execution order

What is the output of this Kotlin code?

Kotlin

class Box(val width: Int) {
    init {
        println("First init: width = $width")
    }
    init {
        println("Second init: width * 2 = ${width * 2}")
    }
}

fun main() {
    val b = Box(5)
}

A

First init: width = 5
Second init: width * 2 = 10

B

Second init: width * 2 = 10
First init: width = 5

C First init: width = 5

D Compilation error

💡 Hint

Init blocks run in the order they appear in the class.

Init blocks run top to bottom. First prints width, then prints width * 2.

❓ Predict Output

advanced

Property initialization with primary constructor and init

What is the output of this Kotlin program?

Kotlin

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

fun main() {
    val r = Rectangle(3, 4)
    println(r.area)
}

A 7

B 12

C 0

D Compilation error

💡 Hint

Area is calculated in the init block from height and width.

The init block sets area to 3 * 4 = 12, so printing r.area outputs 12.

❓ Predict Output

expert

Effect of property shadowing in init block

What is the output of this Kotlin code?

Kotlin

class Sample(x: Int) {
    val x: Int = x + 1
    init {
        println(this.x)
    }
}

fun main() {
    val s = Sample(5)
}

A 6

B 5

C Compilation error

D Runtime error

💡 Hint

Class property shadows constructor parameter; init uses property.

The property 'x' is initialized to 5 + 1 = 6. The init block prints the property value 6.