Properties with val and var in Kotlin

Introduction

Properties let you store information inside objects. Using val means the value cannot change, while var means it can change.

When you want to keep some information fixed and not allow changes, like a person's birth date.

When you want to allow updates to information, like a user's current score in a game.

When you want to clearly show which data should stay constant and which can be modified.

When you want to prevent accidental changes to important data by using <code>val</code>.

When you want to create simple data holders with readable and writable properties.

Syntax

Kotlin

class MyClass {
    val readOnlyProperty: Type = value  // cannot be changed
    var readWriteProperty: Type = value // can be changed
}

val means the property is read-only after initialization.

var means the property can be changed anytime.

Examples

name cannot be changed once set, but age can be updated.

Kotlin

class Person {
    val name: String = "Alice"
    var age: Int = 30
}

Accessing a val property just reads it. A var property can be assigned a new value.

Kotlin

val car = Car()
println(car.model) // read-only
car.speed = 100   // can change speed

Sample Program

This program shows a book with a fixed title and a pagesRead property that can change. It prints the title and pages read before and after updating.

Kotlin

class Book {
    val title: String = "Kotlin Basics"
    var pagesRead: Int = 0
}

fun main() {
    val myBook = Book()
    println("Title: ${myBook.title}")
    println("Pages read: ${myBook.pagesRead}")
    
    myBook.pagesRead = 50
    println("Pages read after update: ${myBook.pagesRead}")
    
    // myBook.title = "New Title" // This line would cause an error because title is val
}

Important Notes

Trying to change a val property after it is set will cause a compile error.

You can think of val as a constant property and var as a variable property.

Summary

val properties are read-only after initialization.

var properties can be changed anytime.

Use val to protect data from accidental changes.

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