Type checking with is operator in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin code using 'is' operator?

Consider the following Kotlin code snippet. What will it print?

Kotlin

fun main() {
    val obj: Any = "Hello"
    if (obj is String) {
        println(obj.length)
    } else {
        println("Not a string")
    }
}

A 5

B Not a string

C Compilation error

D Runtime exception

💡 Hint

Remember that 'is' checks if the object is of a certain type and smart casts it.

The variable 'obj' is of type Any but holds a String. The 'is String' check succeeds, so inside the if block, 'obj' is smart cast to String and 'obj.length' prints 5.

❓ Predict Output

intermediate

What does this Kotlin code print when using 'is' with nullable type?

Analyze the output of this Kotlin code snippet:

Kotlin

fun main() {
    val obj: Any? = null
    if (obj is String) {
        println("String with length ${obj.length}")
    } else {
        println("Not a string or null")
    }
}

A NullPointerException at runtime

B String with length 0

C Compilation error due to null check

D Not a string or null

💡 Hint

Check how 'is' operator behaves with null values.

The variable 'obj' is null. The 'is String' check returns false for null, so the else block runs printing 'Not a string or null'.

🔧 Debug

advanced

Why does this Kotlin code cause a compilation error?

Examine the code below and identify why it does not compile:

Kotlin

fun main() {
    val obj: Any = 123
    if (obj is String) {
        println(obj.length)
    } else {
        println(obj.length)
    }
}

A The variable 'obj' is immutable and cannot be checked

B Smart cast does not work inside if blocks

C The else block tries to access 'length' on 'Any' which is not guaranteed to have it

D The 'is' operator cannot be used with 'Any' type

💡 Hint

Consider what type 'obj' has in the else block and if 'length' is accessible.

In the else block, 'obj' is still of type 'Any' which does not have a 'length' property, causing a compilation error.

🧠 Conceptual

advanced

What is the behavior of 'is' operator with inheritance in Kotlin?

Given the classes below, what will the output be?

Kotlin

open class Animal
class Dog : Animal()

fun main() {
    val pet: Animal = Dog()
    if (pet is Dog) {
        println("It's a dog")
    } else {
        println("It's not a dog")
    }
}

A It's a dog

B It's not a dog

C Compilation error due to inheritance

D Runtime exception

💡 Hint

The 'is' operator checks the actual object type, not the declared type.

The variable 'pet' is declared as Animal but holds a Dog instance. The 'is Dog' check returns true, so it prints 'It's a dog'.

📝 Syntax

expert

Which option causes a syntax error in Kotlin when using 'is' operator?

Identify which code snippet below will cause a syntax error:

A if (obj is String) println(obj.length)

B if obj is String { println(obj.length) }

C if (obj is String) { println(obj.length) }

D if (obj is String) println(obj.length) else println("Not a string")

💡 Hint

Check the syntax of the if statement in Kotlin.

Option B misses parentheses around the condition, which is required in Kotlin if statements, causing a syntax error.