Custom getters and setters in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin code with a custom getter?

Look at this Kotlin class with a custom getter. What will be printed when println(box.area) runs?

Kotlin

class Box(val width: Int, val height: Int) {
    val area: Int
        get() = width * height
}

fun main() {
    val box = Box(3, 4)
    println(box.area)
}

A 12

B 7

C 0

D Compilation error

💡 Hint

Remember the getter calculates the area by multiplying width and height.

The custom getter returns width * height. For width=3 and height=4, area is 12.

❓ Predict Output

intermediate

What happens when setting a property with a custom setter?

Consider this Kotlin class with a custom setter. What will be printed after person.age = 25 and println(person.age)?

Kotlin

class Person {
    var age: Int = 0
        set(value) {
            field = if (value >= 0) value else 0
        }
}

fun main() {
    val person = Person()
    person.age = 25
    println(person.age)
}

A -25

B 0

C Compilation error

D 25

💡 Hint

The setter sets the field only if the value is non-negative.

The setter assigns field = value if value is >= 0. Since 25 is positive, age becomes 25.

🔧 Debug

advanced

Why does this Kotlin code cause a stack overflow error?

Examine this Kotlin class. Why does setting counter.value = 5 cause a stack overflow?

Kotlin

class Counter {
    var value: Int = 0
        set(value) {
            field = value
        }
}

fun main() {
    val counter = Counter()
    counter.value = 5
    println(counter.value)
}

A The code has a syntax error due to missing 'field' keyword.

B The property 'value' is immutable and cannot be set.

C The setter recursively calls itself because it assigns to 'value' instead of 'field'.

D The getter is missing, causing infinite recursion.

💡 Hint

Check what happens when the setter assigns to 'value' instead of 'field'.

Inside the setter, assigning to 'value' calls the setter again, causing infinite recursion and stack overflow. The correct way is to assign to 'field'.

🧠 Conceptual

advanced

What is the purpose of backing fields in Kotlin custom setters/getters?

Why do Kotlin custom setters and getters use the field keyword inside their bodies?

A To call the superclass implementation of the property.

B To refer to the actual storage of the property and avoid recursive calls.

C To declare a new local variable inside the setter or getter.

D To make the property immutable.

💡 Hint

Think about what happens if you assign to the property name inside its setter.

The field keyword accesses the backing field that stores the property's value. Using it prevents recursive calls to the setter or getter.

❓ Predict Output

expert

What is the output of this Kotlin code with a custom setter modifying input?

Analyze this Kotlin code. What will be printed after account.balance = -100 and println(account.balance)?

Kotlin

class BankAccount {
    var balance: Int = 0
        set(value) {
            field = if (value < 0) 0 else value
        }
}

fun main() {
    val account = BankAccount()
    account.balance = -100
    println(account.balance)
}

A 0

B 100

C Compilation error

D -100

💡 Hint

The setter changes negative values to zero.

The setter checks if the value is negative. If yes, it sets balance to 0. Since -100 is negative, balance becomes 0.