Constant values with const val in Kotlin - Step-by-Step Execution

Concept Flow - Constant values with const val

Declare const val

↓

Assign compile-time value

↓

Use constant in code

↓

Value is inlined at compile time

↓

Cannot change value later

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Program runs using constant

This flow shows how a constant is declared with const val, assigned a fixed value, used in code, and remains unchanged during program execution.

Execution Sample

Kotlin

const val MAX_AGE = 100
fun main() {
  println("Max age is $MAX_AGE")
}

This code declares a constant MAX_AGE and prints it.

Execution Table

Step	Action	Evaluation	Result
1	Declare const val MAX_AGE = 100	MAX_AGE assigned 100 at compile time	MAX_AGE = 100
2	Enter main function	Ready to execute println	No change
3	Evaluate println("Max age is $MAX_AGE")	Substitute MAX_AGE with 100	Prints: Max age is 100
4	Program ends	No further actions	Execution stops

💡 Program ends after printing the constant value

Variable Tracker

Variable	Start	After Step 1	After Step 3	Final
MAX_AGE	undefined	100 (constant)	100 (used in print)	100 (unchanged)

Key Moments - 2 Insights

Why can't we change the value of MAX_AGE after declaring it with const val?

Because const val defines a compile-time constant, its value is fixed and inlined during compilation, so changing it later is not allowed (see execution_table step 1).

Is const val the same as a regular val in Kotlin?

No, const val must be assigned a compile-time constant and is inlined, while val can be assigned at runtime and is not inlined (refer to execution_table step 1 vs step 3).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of MAX_AGE after step 1?

A null

B undefined

C 100

D Cannot be determined

💡 Hint

Check the 'Result' column in execution_table row for step 1

At which step does the program print the constant value?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look at the 'Action' and 'Result' columns in execution_table for printing

If we try to assign MAX_AGE = 200 after declaration, what happens?

A Compile-time error

B Value changes to 200

C Runtime error

D Value changes but warning shown

💡 Hint

Recall that const val values cannot be changed after compile-time assignment (see key_moments)

Concept Snapshot

const val declares a compile-time constant in Kotlin
Value must be assigned immediately and be a constant expression
Value is inlined where used, no runtime overhead
Cannot be changed after declaration
Used for fixed values like MAX_AGE = 100

Full Transcript

This visual execution trace shows how Kotlin's const val works. First, a constant MAX_AGE is declared and assigned the value 100 at compile time. This value is fixed and cannot be changed later. When the main function runs, it prints the message including MAX_AGE, substituting the constant value directly. The program then ends. Key points include that const val values are inlined and immutable, different from regular val. The quizzes test understanding of when the value is assigned, when it is printed, and what happens if you try to change it.