Properties with val and var in Kotlin - Mini Project: Build & Apply

Properties with val and var

📖 Scenario: Imagine you are creating a simple app to store information about a book. Each book has a title and a number of pages.

🎯 Goal: You will create a Kotlin class called Book with properties using val and var. Then you will create an instance of this class and print its details.

📋 What You'll Learn

Create a class named Book with two properties: title and pages.

Make title a val property (cannot be changed after creation).

Make pages a var property (can be changed after creation).

Create an instance of Book with the title "Kotlin Basics" and 200 pages.

Change the number of pages to 250.

Print the book's title and pages.

💡 Why This Matters

🌍 Real World

Understanding <code>val</code> and <code>var</code> is essential for managing data that should or should not change in apps, like user profiles or settings.

💼 Career

Many Kotlin jobs require knowledge of properties to write safe and clear code, especially in Android app development.

1

Create the Book class with properties

Create a Kotlin class called Book with two properties: a val called title of type String and a var called pages of type Int. Use the primary constructor to set these properties.

Kotlin

class Book(val title: String, var pages: Int) {
    // Your code here
}

Hint

Use the primary constructor syntax: class Book(val title: String, var pages: Int).

2

Create an instance of Book

Create a variable called myBook and assign it a new Book object with the title "Kotlin Basics" and pages 200.

Kotlin

class Book(val title: String, var pages: Int)

// Create myBook instance here
// Your code here

Hint

Use val myBook = Book("Kotlin Basics", 200) to create the instance.

3

Change the pages property

Change the pages property of myBook to 250.

Kotlin

class Book(val title: String, var pages: Int)

val myBook = Book("Kotlin Basics", 200)

// Change pages to 250
// Your code here

Hint

Use myBook.pages = 250 to update the pages.

4

Print the book details

Print the title and pages of myBook using println. Use this exact format: "Title: Kotlin Basics, Pages: 250".

Kotlin

class Book(val title: String, var pages: Int)

val myBook = Book("Kotlin Basics", 200)

myBook.pages = 250

// Print book details
// Your code here

Hint

Use println("Title: ${myBook.title}, Pages: ${myBook.pages}") to print the details.

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