#Lecture 008 — Constants with final & const

Some values must never change once set: a birth date, a tax rate, the number of days in a week. final and const lock them — and Dart stops anyone who tries to change them.

Contents

Prerequisites · Objectives · Lesson · Code Examples · Common Mistakes · Practice Questions · Answers · Takeaways

#Prerequisites

Lectures 006–007: variables and data types.

#Learning Objectives

• Make a value unchangeable with final
• Make a compile-time constant with const
• Tell final and const apart
• Know what a const list is
• Choose between var, final and const

#1. final: Set Once

final name = 'Usman';
final String city = 'Multan';   // the type can be written too

// name = 'Ali';   // error: 'name' can only be set once

A final value can come from anywhere — even something only known while the program runs:

final startedAt = DateTime.now();   // the current time, then locked

#2. const: Fixed Before the Program Runs

const daysInWeek = 7;
const pi = 3.14159;
const appName = 'Dart Basic';

// pi = 3;   // error: constant variables can't be assigned a value

A const value must be known when the code is compiled — written right there in the code, or calculated from other constants:

const minutesPerDay = 24 * 60;    // fine: made only of constants
// const now = DateTime.now();    // error: the time isn't known until it runs

#3. final vs const

#4. Constant Lists

final scores = [90, 85];
scores.add(70);           // fine: the variable is locked, the list isn't

const days = ['Mon', 'Tue', 'Wed'];
// days.add('Thu');       // crashes: a const list can't be changed

final locks the variable — it always points to the same list, but that list can grow. const freezes the value too — the list itself can never change.

#5. Which One to Use

1. Fixed and known in advance (7, 'Dart Basic', 0.05)? → const.
2. Set once, but calculated or loaded? → final.
3. Needs to change later (a counter, a score)? → a normal variable (var or a type).

Most variables turn out to be final. Start with final, and change it only when you really need to reassign.

#Code Examples

#Example 1 — A Bill With Fixed Rates

void main() {
  const taxRate = 0.05;
  const shopName = 'Corner Store';
  final itemName = 'Notebook';
  var quantity = 3;
  final unitPrice = 120.0;

  quantity = quantity + 1;   // fine: a normal variable
  // taxRate = 0.1;          // error: taxRate is const

  final subtotal = quantity * unitPrice;
  final total = subtotal + subtotal * taxRate;

  print(shopName);
  print('$quantity × $itemName');
  print('Subtotal: $subtotal');
  print('Total with tax: $total');
}

Output

Corner Store
4 × Notebook
Subtotal: 480.0
Total with tax: 504.0

#Common Beginner Mistakes

1. const with a runtime value — const now = DateTime.now(); is an error. Use final.

2. Thinking final freezes a list — final list = [1]; list.add(2); works. Use const for a list that must never change.

3. Writing final var — it's final name = ... or final String name = ..., never both keywords.

#Practice Questions

1. Which keyword fits a value read from the clock once? Which fits 365?
2. Is const total = 60 * 60; allowed?
3. After final list = [1, 2];, does list.add(3); work?
4. What happens if you try to change a const variable?

#Practice Answers

1. final; const.
2. Yes — it is made only of constants.
3. Yes — final locks the variable, not the list.
4. The code doesn't compile.

#Key Takeaways

• final = set once, value may be calculated while running.
• const = fixed when compiled; the value itself is frozen.
• A final list can still change; a const list can't.
• Default to final; use const for fixed values; plain variables only when they change.