Object declaration syntax in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin object declaration?

Consider the following Kotlin code. What will be printed when Main.main() is run?

Kotlin

object Counter {
    var count = 0
    fun increment() {
        count++
    }
}

fun main() {
    Counter.increment()
    Counter.increment()
    println(Counter.count)
}

A 2

B 0

C Compilation error

D 1

💡 Hint

Remember that object declarations create a singleton instance.

The object Counter is a singleton. Calling increment() twice increases count from 0 to 2. So, printing Counter.count outputs 2.

❓ Predict Output

intermediate

What error does this Kotlin object declaration cause?

What error will this Kotlin code produce when compiled?

Kotlin

object Sample {
    val x: Int
}

fun main() {
    println(Sample.x)
}

A Compilation error: Property must be initialized or be abstract

B Runtime error: UninitializedPropertyAccessException

C Prints 0

D No error, prints null

💡 Hint

Check if properties in objects need initialization.

In Kotlin, properties in objects must be initialized or declared abstract. Here, x is neither initialized nor abstract, causing a compilation error.

🔧 Debug

advanced

Why does this Kotlin object declaration cause a runtime error?

This Kotlin code compiles but throws an exception at runtime. What causes the error?

Kotlin

object Config {
    val data = loadData()

    fun loadData(): String {
        throw IllegalStateException("Failed to load")
    }
}

fun main() {
    println(Config.data)
}

A Null pointer exception because data is null

B Exception thrown during object initialization blocks main thread

C Compilation error due to missing return type

D No error, prints "Failed to load"

💡 Hint

Object initialization runs when first accessed.

The data property is initialized by calling loadData(), which throws an exception. This happens during object initialization, causing a runtime exception before main proceeds.

📝 Syntax

advanced

Which Kotlin object declaration syntax is correct?

Which of the following Kotlin object declarations is syntactically correct?

A

object Logger {
    fun log(message: String)
        println(message)
}

B

object Logger {
    fun log(message: String) = {
        println(message)
    }
}

C

object Logger {
    fun log(message: String) {
        println(message)
    }
}

D

}
}    
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{ reggoL tcejbo

💡 Hint

Check for proper function body braces and object closing brace.

Option C correctly defines a function with braces and closes the object declaration properly. Option C misses braces for function body, C uses invalid lambda syntax for function body, and D misses the closing brace for the object.

🚀 Application

expert

How many instances of this Kotlin object are created?

Given this Kotlin code, how many instances of Database are created during program execution?

Kotlin

object Database {
    init {
        println("Database initialized")
    }

    fun query() = "data"
}

fun main() {
    println(Database.query())
    println(Database.query())
}

A Multiple instances, one per println

B Two instances, one per query call

C No instances, object is static

D One instance, initialized once

💡 Hint

Objects in Kotlin are singletons by design.

Kotlin's object declarations create a single instance (singleton). The Database object is initialized once when first accessed, so only one instance exists.