Dart

Variables & Data Types

1. What is a Variable?

A variable is a named place where we store a value.

| Type     | Example    | Use             |
| -------- | ---------- | --------------- |
| `String` | `"Hiruni"` | Text            |
| `int`    | `25`       | Whole numbers   |
| `double` | `99.99`    | Decimal numbers |
| `bool`   | `true`     | True/false      |

String

Text store කරන්න.

String name = "Hiruni";
String city = "Colombo";
String message = "Hello Flutter";

You can print:

void main() {
  String name = "Hiruni";
print(name);
}

Output:

Hiruni

int

Whole numbers.

int age = 25;
int quantity = 10;
int score = 100;

Example:

void main() {
  int age = 25;
print(age);
}

Output:

25

You can’t normally do:

int age = "25";

❌ Wrong.

Because "25" is a String, not an int.

double

Decimal numbers.

double price = 1500.50;
double rating = 4.5;
double temperature = 28.7;

Example

void main() {
  double price = 1500.50;

  print(price);
}

Output:

1500.5

bool

Boolean means true or false.

bool isLoggedIn = true;
bool isAdmin = false;
bool hasPermission = true;

Example:

void main() {
  bool isLoggedIn = true;

  print(isLoggedIn);
}

Output:

true

This becomes extremely important in Flutter.

For example:

bool isLoading = true;

Then Flutter can decide:

isLoading == true
       ↓
Show loading spinner

var

Instead of:

String name = "Hiruni";

You can write

var name = "Hiruni";

Dart automatically understands:

name → String

var vs JavaScript var

This is especially important for you because you know JavaScript.

JavaScript

var value = 10;

value = "Hello";

JavaScript allows this.

Dart

var value = 10;

value = "Hello";    ❌ Not allowed.

final

final String name = "Hiruni";

final means:

Assign the variable only once.

Example:

void main() {
  final name = "Hiruni";

  print(name);
}

This is fine.

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But:

final name = "Hiruni";

name = "Tharindu"; ❌ Error.

Because name is final.

variable එකට value එකක් එක් වතාවක් assign කළාට පස්සේ නැවත වෙනස් කරන්න බැහැ.

const

const appName = "SalesVision AI";

const also means the value cannot change.

But there’s an important difference between final and const.

final

Value is decided at runtime.

final currentTime = DateTime.now();

This is valid.

Why?

Because Dart only knows the current time when the program runs.

const

Value must be known at compile time.

const appName = "SalesVision AI";

This is valid because "SalesVision AI" is known before the application runs.

final vs const

Remember this:

final → assigned once
const → compile-time constant

Ex:

final currentTime = DateTime.now();

const appName = "My App";

final
 ↓
runtime value
 ↓
can be calculated when app runs

const
 ↓
compile-time value
 ↓
must already be known

dynamic

dynamic value = 10;

value = "Hello";

value = true;

value = 50.5;

This is allowed.

Because dynamic tells Dart:

Don’t enforce a specific static type here.

But don’t use dynamic everywhere.

It removes many of Dart’s type-safety benefits.

var
→ Dart figures out the type
→ type remains fixed

dynamic
→ type can change

Dart convention is camelCase:

firstName
productPrice
totalAmount
isLoggedIn
userId

not

first_name
product_price

Operators

An operator performs an operation on values.

For example:

int a = 10;
int b = 5;

print(a + b);   ---------> 15

Arithmetic Operators

| Operator | Meaning          | Example   | Result |
| -------- | ---------------- | --------- | ------ |
| `+`      | Add              | `10 + 5`  | `15`   |
| `-`      | Subtract         | `10 - 5`  | `5`    |
| `*`      | Multiply         | `10 * 5`  | `50`   |
| `/`      | Divide           | `10 / 5`  | `2.0`  |
| `~/`     | Integer division | `10 ~/ 3` | `3`    |
| `%`      | Remainder        | `10 % 3`  | `1`    |

Addition with Strings

+ can also join Strings.

String firstName = "Sri";
String lastName = "Lanka";

String fullName = firstName + " " + lastName;

print(fullName); -------->Sri Lanka

Comparison Operators

| Operator | Meaning               |
| -------- | --------------------- |
| `==`     | Equal                 |
| `!=`     | Not equal             |
| `>`      | Greater than          |
| `<`      | Less than             |
| `>=`     | Greater than or equal |
| `<=`     | Less than or equal    |

Logical Operators

&&
||
!