Observable property delegation in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin code using observable delegation?

Consider the following Kotlin code that uses observable property delegation. What will be printed when the code runs?

Kotlin

import kotlin.properties.Delegates

class User {
    var name: String by Delegates.observable("Unknown") { prop, old, new ->
        println("Property '${prop.name}' changed from '$old' to '$new'")
    }
}

fun main() {
    val user = User()
    user.name = "Alice"
    user.name = "Bob"
}

A Property 'name' changed from 'Unknown' to 'Bob'

B Property 'name' changed from 'Alice' to 'Bob'

C

Property 'name' changed from 'Unknown' to 'Alice'
Property 'name' changed from 'Alice' to 'Bob'

D No output

💡 Hint

Think about how observable delegation calls the callback on every property change.

The observable delegate calls the lambda every time the property changes. Initially, the property is 'Unknown'. When set to 'Alice', the lambda prints the change from 'Unknown' to 'Alice'. Then when set to 'Bob', it prints the change from 'Alice' to 'Bob'.

🧠 Conceptual

intermediate

What is the main purpose of observable property delegation in Kotlin?

Why would a Kotlin developer use observable property delegation?

A To automatically run code whenever a property value changes

B To make a property immutable

C To delay the initialization of a property until first use

D To create a property that cannot be accessed outside the class

💡 Hint

Think about what happens when a property changes.

Observable property delegation allows running custom code whenever the property's value changes, such as logging or validation.

🔧 Debug

advanced

Why does this Kotlin code using observable delegation cause a stack overflow?

Examine the code below. Why does it cause a stack overflow error when run?

Kotlin

import kotlin.properties.Delegates

class Counter {
    var count: Int by Delegates.observable(0) { _, _, new ->
        if (new < 10) {
            count = new + 1
        }
    }
}

fun main() {
    val counter = Counter()
    counter.count = 1
    println(counter.count)
}

A Because the println statement is inside the observable callback

B Because the initial value 0 is invalid for observable properties

C Because the property count is immutable and cannot be changed

D Because the observable callback modifies the property it observes, causing infinite recursion

💡 Hint

Think about what happens when the property is changed inside its own observer.

The observable callback sets the property 'count' again, which triggers the callback again, causing infinite recursion and stack overflow.

📝 Syntax

advanced

Which option correctly declares an observable property in Kotlin?

Select the correct Kotlin syntax to declare an observable property 'score' initialized to 0 that prints changes.

A var score: Int = Delegates.observable(0) { prop, old, new -> println("Score changed from ${old} to ${new}") }

B var score: Int by Delegates.observable(0) { prop, old, new -> println("Score changed from ${old} to ${new}") }

C val score: Int by Delegates.observable(0) { prop, old, new -> println("Score changed from ${old} to ${new}") }

D var score: Int by observable(0) { prop, old, new -> println("Score changed from ${old} to ${new}") }

💡 Hint

Remember the correct syntax for property delegation uses 'by' and the delegate object.

Option B correctly uses 'var' with 'by Delegates.observable' and a lambda for the callback. Option B assigns the delegate directly to the property which is invalid. Option B uses 'val' which cannot be delegated with observable since it must be mutable. Option B uses 'observable' without 'Delegates.' which is undefined.

🚀 Application

expert

How many times will the observable callback run in this Kotlin code?

Given the code below, how many times will the observable callback print a message?

Kotlin

import kotlin.properties.Delegates

class Temperature {
    var celsius: Int by Delegates.observable(20) { _, old, new ->
        println("Temperature changed from ${old} to ${new}")
    }
}

fun main() {
    val temp = Temperature()
    temp.celsius = 25
    temp.celsius = 25
    temp.celsius = 30
}

A 2

B 1

C 0

D 3

💡 Hint

Consider if setting the same value triggers the observable callback.

The observable callback runs only when the property value actually changes. Setting the same value (25 to 25) does not trigger it. So it runs when changing from 20 to 25 and from 25 to 30, total 2 times.

Practice

1. What does observable property delegation in Kotlin do?

easy

A. It delays the initialization of a property until first use.

B. It makes a property immutable and unchangeable.

C. It automatically converts a property to a string.

D. It watches a property and runs code when the property value changes.

Solution

1. Step 1: Understand observable delegation purpose

   Observable delegation lets you watch a property for changes and react when it changes.

2. Step 2: Compare options with definition

   Only It watches a property and runs code when the property value changes. describes this behavior correctly; others describe different Kotlin features.

3. Final Answer:

   It watches a property and runs code when the property value changes. -> Option D

4. Quick Check:

   Observable delegation = watch property changes [OK]

Hint: Observable runs code on property change, not on initialization [OK]

Common Mistakes:

• Confusing observable with lazy initialization
• Thinking observable makes property immutable
• Assuming observable converts types automatically

2. Which of the following is the correct syntax to declare an observable property in Kotlin?

easy

A. var name: String by observable("") { prop, old, new -> println("Changed") }

B. var name: String = observable("") { prop, old, new -> println("Changed") }

C. val name: String by observable("") { prop, old, new -> println("Changed") }

D. var name: String observable("") { prop, old, new -> println("Changed") }

Solution

1. Step 1: Recall observable property syntax

   Observable properties use by observable(initialValue) { prop, old, new -> ... } with var.

2. Step 2: Check each option

   var name: String by observable("") { prop, old, new -> println("Changed") } matches correct syntax. var name: String = observable("") { prop, old, new -> println("Changed") } uses wrong assignment. val name: String by observable("") { prop, old, new -> println("Changed") } uses val which is immutable. var name: String observable("") { prop, old, new -> println("Changed") } misses by.

3. Final Answer:

   var name: String by observable("") { prop, old, new -> println("Changed") } -> Option A

4. Quick Check:

   Use 'var' and 'by observable' syntax [OK]

Hint: Use 'var' with 'by observable' and initial value [OK]

Common Mistakes:

• Using val instead of var for observable
• Omitting 'by' keyword
• Assigning observable without 'by'

3. What will be the output of this Kotlin code?

import kotlin.properties.Delegates

class User {
    var name: String by Delegates.observable("Unknown") { prop, old, new ->
        println("Name changed from $old to $new")
    }
}

fun main() {
    val user = User()
    user.name = "Alice"
    user.name = "Bob"
}

medium

A. Name changed from Unknown to Alice

B. Name changed from Alice to Bob

C. Name changed from Unknown to Alice
Name changed from Alice to Bob

D. No output

Solution

1. Step 1: Understand observable callback calls

   The observable lambda runs every time the property changes, printing old and new values.

2. Step 2: Trace property changes in main()

   Initially name is "Unknown". First change to "Alice" triggers print. Second change to "Bob" triggers print again.

3. Final Answer:

   Name changed from Unknown to Alice Name changed from Alice to Bob -> Option C

4. Quick Check:

   Observable prints on each change [OK]

Hint: Observable prints on every property change [OK]

Common Mistakes:

• Thinking only last change triggers print
• Ignoring initial value in output
• Expecting output without property change

4. Identify the error in this Kotlin code using observable delegation:

import kotlin.properties.Delegates

class Counter {
    var count: Int by Delegates.observable(0) { prop, old, new ->
        if (new < old) {
            println("Count cannot decrease")
        }
    }
}

fun main() {
    val c = Counter()
    c.count = 5
    c.count = 3
}

medium

A. Logical error: observable does not prevent count from decreasing.

B. Syntax error: '<' should be '<' without escaping.

C. Runtime error: observable lambda cannot have if statements.

D. No error; code works as expected printing a warning when count decreases.

Solution

1. Step 1: Check syntax and runtime correctness

   The code syntax is correct; escaped '<' is valid in HTML but in code it should be '<'. Assuming code runs, no syntax error.

2. Step 2: Analyze observable behavior

   The observable lambda prints a message if count decreases but does not stop the change. So count can decrease despite warning.

3. Final Answer:

   Logical error: observable does not prevent count from decreasing. -> Option A

4. Quick Check:

   Observable reacts but does not block changes [OK]

Hint: Observable warns but doesn't stop changes automatically [OK]

Common Mistakes:

• Expecting observable to block property changes
• Confusing HTML escape with Kotlin syntax
• Thinking if statement causes runtime error

5. You want to create a Kotlin class Temperature with an observable property celsius. When celsius changes, it should update another property fahrenheit automatically. Which code correctly implements this behavior?

hard

A. class Temperature {
var celsius: Double = 0.0
var fahrenheit: Double by Delegates.observable(32.0) { _, _, new ->
celsius = (new - 32) * 5 / 9
}
}

B. class Temperature {
var celsius: Double by Delegates.observable(0.0) { _, _, new ->
fahrenheit = new * 9 / 5 + 32
}
var fahrenheit: Double = 32.0
}

C. class Temperature {
var celsius: Double by Delegates.observable(0.0) { _, _, new ->
fahrenheit = new + 32
}
var fahrenheit: Double = 32.0
}

D. class Temperature {
var celsius: Double by Delegates.observable(0.0) { _, _, new ->
fahrenheit = new * 5 / 9 + 32
}
var fahrenheit: Double = 32.0
}

Solution

1. Step 1: Understand temperature conversion formulas

   Fahrenheit = Celsius * 9/5 + 32. class Temperature { var celsius: Double by Delegates.observable(0.0) { _, _, new -> fahrenheit = new * 9 / 5 + 32 } var fahrenheit: Double = 32.0 } uses this formula correctly.

2. Step 2: Check observable updates fahrenheit on celsius change

   class Temperature { var celsius: Double by Delegates.observable(0.0) { _, _, new -> fahrenheit = new * 9 / 5 + 32 } var fahrenheit: Double = 32.0 } updates fahrenheit inside observable lambda of celsius, which is correct behavior.

3. Final Answer:

   class Temperature { var celsius: Double by Delegates.observable(0.0) { _, _, new -> fahrenheit = new * 9 / 5 + 32 } var fahrenheit: Double = 32.0 } -> Option B

4. Quick Check:

   Use correct formula and update fahrenheit in celsius observable [OK]

Hint: Use correct formula and update dependent property inside observable [OK]

Common Mistakes:

• Using wrong temperature conversion formula
• Updating celsius inside fahrenheit observable causing infinite loop
• Adding instead of multiplying in formula