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.