Custom delegated properties in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple custom delegate

What is the output of this Kotlin code using a custom delegated property?

Kotlin

import kotlin.reflect.KProperty

class Delegate {
    operator fun getValue(thisRef: Any?, property: KProperty<*>): String {
        return "Hello from delegate"
    }

    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: String) {
        println("Setting value to '$value'")
    }
}

class Example {
    var p: String by Delegate()
}

fun main() {
    val e = Example()
    println(e.p)
    e.p = "New Value"
}

A

Hello from delegate
Setting value to 'New Value'

B

Hello from delegate
New Value

C

Setting value to 'New Value'
Hello from delegate

D

Hello from delegate
Setting value to 'p'

💡 Hint

Remember that the setValue prints a message, and getValue returns a fixed string.

The getValue returns the string "Hello from delegate" which is printed first. Then setting the property triggers setValue which prints the message about setting the value.

❓ Predict Output

intermediate

Behavior of a delegate with backing field

What will be printed when running this Kotlin code with a custom delegate that stores a value internally?

Kotlin

import kotlin.reflect.KProperty

class StorageDelegate {
    private var storedValue: Int = 0

    operator fun getValue(thisRef: Any?, property: KProperty<*>): Int {
        return storedValue
    }

    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: Int) {
        storedValue = value
    }
}

class Counter {
    var count: Int by StorageDelegate()
}

fun main() {
    val c = Counter()
    c.count = 10
    println(c.count)
    c.count = 20
    println(c.count)
}

A

10
10

B

0
0

C

10
20

D

20
20

💡 Hint

The delegate stores the value internally and returns it on get.

The delegate's storedValue is updated on setValue and returned on getValue, so the prints show the assigned values.

🔧 Debug

advanced

Identify the error in this custom delegate code

This Kotlin code tries to create a custom delegate but fails. What error does it produce when compiled?

Kotlin

class BadDelegate {
    operator fun getValue(thisRef: Any, property: kotlin.reflect.KProperty<*>): String {
        return "value"
    }

    operator fun setValue(thisRef: Any, property: kotlin.reflect.KProperty<*>, value: Int) {
        // do nothing
    }
}

class Test {
    var prop: String by BadDelegate()
}

fun main() {
    val t = Test()
    println(t.prop)
    t.prop = "new"
}

A Unresolved reference: BadDelegate

B Type mismatch: setValue expects Int but assigned String

C No error, prints 'value' and ignores assignment

D Property delegate must be immutable

💡 Hint

Check the types used in setValue compared to the property type.

The setValue method expects an Int value but the property is String, so assigning a String causes a type mismatch error.

🧠 Conceptual

advanced

Understanding lazy initialization with custom delegate

Consider this Kotlin custom delegate for lazy initialization. What will be the output when main runs?

Kotlin

class LazyDelegate<T>(val initializer: () -> T) {
    private var value: T? = null

    operator fun getValue(thisRef: Any?, property: kotlin.reflect.KProperty<*>): T {
        if (value == null) {
            value = initializer()
            println("Initialized")
        }
        return value!!
    }
}

class Example {
    val lazyValue: String by LazyDelegate { "Hello" }
}

fun main() {
    val e = Example()
    println(e.lazyValue)
    println(e.lazyValue)
}

A

Initialized
Initialized
Hello
Hello

B

Hello
Hello

C

Hello
Initialized
Hello

D

Initialized
Hello
Hello

💡 Hint

The initializer runs only once when value is null.

The first access triggers initialization and prints "Initialized" then returns "Hello". The second access returns cached value without printing "Initialized" again.

❓ Predict Output

expert

Custom delegate with observable behavior

What is the output of this Kotlin program using a custom delegate that observes changes?

Kotlin

import kotlin.reflect.KProperty

class ObservableDelegate<T>(var value: T, val onChange: (old: T, new: T) -> Unit) {
    operator fun getValue(thisRef: Any?, property: KProperty<*>): T = value

    operator fun setValue(thisRef: Any?, property: KProperty<*>, newValue: T) {
        val oldValue = value
        value = newValue
        if (oldValue != newValue) {
            onChange(oldValue, newValue)
        }
    }
}

class Person {
    var name: String by ObservableDelegate("Unknown") { old, new ->
        println("Name changed from $old to $new")
    }
}

fun main() {
    val p = Person()
    println(p.name)
    p.name = "Alice"
    p.name = "Alice"
    p.name = "Bob"
}

A

Unknown
Name changed from Unknown to Alice
Name changed from Alice to Bob

B

Unknown
Name changed from Unknown to Alice
Name changed from Alice to Alice
Name changed from Alice to Bob

C

Name changed from Unknown to Alice
Name changed from Alice to Bob
Unknown

D

Unknown
Name changed from Unknown to Alice
Name changed from Alice to Bob
Bob

💡 Hint

The onChange callback triggers only when the new value differs from the old.

The initial print shows "Unknown". Setting to "Alice" triggers onChange. Setting again to "Alice" does not trigger onChange because value is unchanged. Setting to "Bob" triggers onChange again.