Why Properties with val and var in Kotlin? - Purpose & Use Cases

The Big Idea

What if you could stop accidental changes in your code before they even happen?

The Scenario

Imagine you have a notebook where you write down your daily tasks. Sometimes you want to keep a task as it is, and other times you want to be able to change it later. Without a clear way to mark which tasks can be changed and which cannot, you might accidentally erase or alter important notes.

The Problem

Manually tracking which values should stay fixed and which can change is confusing and error-prone. You might overwrite important data by mistake or waste time checking if a value should be updated. This slows down your work and causes bugs.

The Solution

Kotlin's val and var keywords let you clearly mark properties as read-only or mutable. This way, the compiler helps you avoid accidental changes and makes your code safer and easier to understand.

Before vs After

✗ Before

class Task {
    var description: String = ""
    fun updateDescription(newDesc: String) {
        description = newDesc
    }
}

✓ After

class Task(val id: Int, var description: String)

What It Enables

This lets you write safer and clearer code by explicitly controlling which properties can change and which cannot.

Real Life Example

Think of a bank account number that should never change once set (val), versus the account balance that updates with each transaction (var).

Key Takeaways

val creates read-only properties that cannot be changed after assignment.

var creates mutable properties that can be updated anytime.

Using them properly prevents bugs and makes your code easier to read.

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