Properties with val and var in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a read-only property.

Kotlin

class Person {
    val name: String = [1]
}

Drag options to blanks, or click blank then click option'

Aval

Bvar

Cfun

D"Alice"

Attempts:

3 left

💡 Hint

Common Mistakes

Using var instead of a value for assignment.

Forgetting to use quotes for string values.

The val keyword declares a read-only property. Here, name is assigned the value "Alice".

2fill in blank

medium

Complete the code to declare a mutable property.

Kotlin

class Car {
    var speed: Int = [1]
}

Drag options to blanks, or click blank then click option'

A0

B"fast"

Cval

Dfun

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning a string to an integer property.

Using val instead of var for mutable properties.

The var keyword declares a mutable property. Here, speed is initialized to 0 and can be changed later.

3fill in blank

hard

Fix the error in the property declaration to make it mutable.

Kotlin

class Book {
    [1] title: String = "Kotlin Guide"
}

Drag options to blanks, or click blank then click option'

Afun

Bvar

Cval

Dconst

Attempts:

3 left

💡 Hint

Common Mistakes

Using val which makes the property read-only.

Using fun which is for functions, not properties.

Using var makes the property mutable, allowing its value to be changed after initialization.

4fill in blank

hard

Fill both blanks to declare a read-only property with a custom getter.

Kotlin

class Circle(val radius: Double) {
    val area: Double
        get() = [1] * radius [2] radius
}

Drag options to blanks, or click blank then click option'

AMath.PI

B*

C+

Dval

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition instead of multiplication for squaring.

Using val inside the getter instead of a value.

The area of a circle is calculated as π times radius squared. Here, Math.PI is multiplied by radius * radius.

5fill in blank

hard

Fill all three blanks to declare a mutable property with a backing field and custom setter.

Kotlin

class Temperature {
    var celsius: Double = [1]
        set(value) {
            field = if (value [2] [3]273.15) [3]273.15 else value
        }
}

Drag options to blanks, or click blank then click option'

A0.0

B<

C+

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition instead of subtraction for negative numbers.

Not using the backing field field inside the setter.

The property celsius is initialized to 0.0. The setter checks if the new value is less than absolute zero (-273.15) and sets it to -273.15 if so; otherwise, it sets the value normally.

Practice

1. What is the main difference between val and var properties in Kotlin?

easy

A. val can be changed anytime, var is read-only after initialization.

B. val is read-only after initialization, var can be changed anytime.

C. val and var both can be changed anytime.

D. val and var are exactly the same.

Solution

1. Step 1: Understand val behavior

   val properties are assigned once and cannot be reassigned later.

2. Step 2: Understand var behavior

   var properties can be reassigned multiple times after initialization.

3. Final Answer:

   val is read-only after initialization, var can be changed anytime. -> Option B

4. Quick Check:

   val = read-only, var = mutable [OK]

Hint: Remember: val = fixed, var = variable [OK]

Common Mistakes:

• Confusing val as mutable
• Thinking var is read-only
• Believing val and var behave the same

2. Which of the following is the correct way to declare a mutable property named age of type Int in Kotlin?

easy

A. var age: Int = 25

B. val age: Int = 25

C. val age = 25

D. var age: Int

Solution

1. Step 1: Identify mutable property keyword

   Mutable properties use var keyword.

2. Step 2: Check initialization

   Property must be initialized or declared nullable; here age is initialized with 25.

3. Final Answer:

   var age: Int = 25 -> Option A

4. Quick Check:

   Mutable property uses var [OK]

Hint: Use var for changeable properties [OK]

Common Mistakes:

• Using val for mutable property
• Not initializing non-nullable property
• Omitting type without initialization

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

class Person {
    val name = "Anna"
    var age = 30
}

fun main() {
    val p = Person()
    println(p.name)
    p.age = 31
    println(p.age)
}

medium

A. Anna
30

B. Anna
null

C. Error: Cannot assign to val property

D. Anna
31

Solution

1. Step 1: Check val name usage

   name is a val, so it prints "Anna" and cannot be changed.

2. Step 2: Check var age reassignment

   age is a var, so it can be changed from 30 to 31 before printing.

3. Final Answer:

   Anna 31 -> Option D

4. Quick Check:

   val prints original, var prints updated [OK]

Hint: val prints original, var can be updated [OK]

Common Mistakes:

• Expecting error on changing var
• Thinking val can be reassigned
• Confusing printed values

4. Identify the error in the following Kotlin code:

class Car {
    val model: String
    var year: Int

    init {
        model = "Tesla"
        year = 2020
    }
}

fun main() {
    val car = Car()
    car.model = "BMW"
    println(car.model)
}

medium

A. Cannot assign a new value to val property 'model'.

B. Property 'year' is not initialized.

C. Missing constructor parameters.

D. No error, code runs fine.

Solution

1. Step 1: Check val model reassignment

   model is declared as val, so it cannot be reassigned after initialization.

2. Step 2: Verify property initialization

   model and year are both initialized in init, so no initialization error.

3. Final Answer:

   Cannot assign a new value to val property 'model'. -> Option A

4. Quick Check:

   val properties cannot be reassigned [OK]

Hint: val properties cannot be changed after init [OK]

Common Mistakes:

• Trying to reassign val property
• Ignoring uninitialized var warnings
• Assuming val can be reassigned

5. You want to create a Kotlin class Book where the title should never change after creation, but the price can be updated. Which declaration is correct?

hard

A. class Book(val title: String, val price: Double)

B. class Book(var title: String, var price: Double)

C. class Book(val title: String, var price: Double)

D. class Book(var title: String, val price: Double)

Solution

1. Step 1: Determine immutability for title

   title should not change, so declare it with val.

2. Step 2: Determine mutability for price

   price can change, so declare it with var.

3. Final Answer:

   class Book(val title: String, var price: Double) -> Option C

4. Quick Check:

   Immutable title = val, mutable price = var [OK]

Hint: Use val for fixed, var for changeable properties [OK]

Common Mistakes:

• Using var for title which should be fixed
• Using val for price which should be changeable
• Mixing up val and var keywords