Properties with val and var in Kotlin - Step-by-Step Execution

Concept Flow - Properties with val and var

Declare property with val

↓

Property is read-only

↓

Try to reassign?

Yes→Error: Cannot reassign val

No↓

Declare property with var

↓

Property is mutable

↓

Reassign allowed

↓

Use property value

This flow shows how val creates read-only properties and var creates mutable properties that can be reassigned.

Execution Sample

Kotlin

val name = "Alice"
var age = 30
// name = "Bob" // Error
age = 31
println("$name is $age years old")

This code declares a read-only name and a mutable age, then changes age and prints both.

Execution Table

Step	Action	Property	Value Before	Value After	Output/Note
1	Declare val name	name	undefined	"Alice"	name set to "Alice"
2	Declare var age	age	undefined	30	age set to 30
3	Attempt to reassign name	name	"Alice"	Error	Error: Cannot reassign val property
4	Reassign age	age	30	31	age changed to 31
5	Print statement	-	-	-	"Alice is 31 years old"

💡 Execution stops after print; reassignment to val causes error and is commented out.

Variable Tracker

Property	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
name	undefined	"Alice"	"Alice"	Error (reassign not allowed)	"Alice"	"Alice"
age	undefined	undefined	30	30	31	31

Key Moments - 2 Insights

Why can't we change the value of a property declared with val?

Because val properties are read-only after initialization, as shown in step 3 of the execution_table where reassignment causes an error.

Can we change the value of a property declared with var?

Yes, var properties are mutable and can be reassigned, as shown in step 4 where age changes from 30 to 31.

Visual Quiz - 3 Questions

Look at the execution_table at step 3. What happens when we try to reassign the val property 'name'?

A The program ignores the reassignment

B The value changes successfully

C An error occurs because val properties cannot be reassigned

D The value becomes null

💡 Hint

Check the 'Output/Note' column at step 3 in the execution_table.

According to variable_tracker, what is the value of 'age' after step 4?

A 31

B 30

C undefined

D Error

💡 Hint

Look at the 'age' row under 'After Step 4' in variable_tracker.

If we change 'val name' to 'var name', what would happen at step 3?

A An error would still occur

B Reassignment would succeed and name would change

C The program would crash

D The value would become null

💡 Hint

Recall that var properties are mutable as explained in the concept_flow and key_moments.

Concept Snapshot

val declares a read-only property (cannot be reassigned).
var declares a mutable property (can be reassigned).
Reassigning val causes a compile-time error.
Use val for constants and var for variables.
Access properties by name to get their current value.

Full Transcript

In Kotlin, properties declared with val are read-only and cannot be changed after they are set. Properties declared with var are mutable and can be reassigned. In the example, 'name' is declared with val and set to "Alice". Trying to change 'name' causes an error. 'age' is declared with var and initially set to 30, then changed to 31 successfully. The program prints the final values. This shows how val and var control mutability of properties.

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