Object expressions (anonymous objects) in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of an anonymous object with overridden method

What is the output of this Kotlin code using an anonymous object?

Kotlin

val obj = object {
    fun greet() = "Hello from anonymous object"
}
println(obj.greet())

A Hello from anonymous object

B Compilation error: anonymous object cannot have functions

C null

D Runtime error: Unresolved reference greet

💡 Hint

Anonymous objects can have functions and properties just like regular classes.

The anonymous object defines a function greet() which returns the string. Calling greet() prints that string.

🧠 Conceptual

intermediate

Type of anonymous object when assigned to a variable

Consider this Kotlin code snippet: val obj = object { val x = 10 } What is the static type of 'obj'?

A Any

B Int

C Unit

D The exact anonymous object type with property 'x'

💡 Hint

When assigned to a val, the compiler infers the full anonymous object type.

When an anonymous object is assigned to a variable, its full type including properties is preserved.

❓ Predict Output

advanced

Output when anonymous object is returned from a function

What is the output of this Kotlin code?

Kotlin

fun create() = object {
    val name = "Kotlin"
    fun greet() = "Hello, $name!"
}

val obj = create()
println(obj.greet())

A Compilation error: Cannot access greet() on anonymous object returned from function

B Hello, $name!

C Hello, Kotlin!

D Runtime error: Unresolved reference greet

💡 Hint

Functions returning anonymous objects have their type erased to Any unless explicitly typed.

When a function returns an anonymous object without an explicit type, the caller sees it as Any, so greet() is not accessible.

🔧 Debug

advanced

Why does this anonymous object code cause a compilation error?

Identify the cause of the compilation error in this Kotlin code: val obj: Any = object { fun sayHi() = "Hi" } println(obj.sayHi())

Kotlin

val obj: Any = object {
    fun sayHi() = "Hi"
}
println(obj.sayHi())

A Anonymous objects cannot have functions

B The variable obj is typed as Any, which does not have sayHi() method

C Missing parentheses when calling sayHi

D Cannot assign anonymous object to Any type

💡 Hint

Check the declared type of obj and what methods it exposes.

The variable obj is declared as type Any, which does not have sayHi(), so calling obj.sayHi() causes a compilation error.

🚀 Application

expert

Using anonymous objects to implement an interface

What is the output of this Kotlin code?

Kotlin

interface Speaker {
    fun speak(): String
}

val speaker = object : Speaker {
    override fun speak() = "I am anonymous"
}

println(speaker.speak())

A Runtime error: speak() not implemented

B Compilation error: anonymous object must implement all interface methods

C I am anonymous

D I am Speaker

💡 Hint

Anonymous objects can implement interfaces and override their methods.

The anonymous object implements Speaker and overrides speak(), so calling speak() prints the returned string.