YouTipDart Variables And Types
Variables are the basic units for storing data in a program, and data types determine what values a variable can store.
This chapter introduces variable declaration methods in Dart, basic data types, and the important null safety mechanism.
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## Variable Declaration: var, final, const
Dart provides three keywords for declaring variables, each suitable for different scenarios.
Understanding their differences is the first step to learning Dart well.
### var β Type-Inferred Variable
Variables declared with var have their type automatically inferred from the initial value, and the type cannot be changed once determined.
## Instance
void main(){
// Type inference: name's type is inferred as String
var name ='TUTORIAL';
// Type inference: age's type is inferred as int
var age =10;
print('$name already / has been $age years');
// The following will cause an error β once the type is determined, you cannot assign a value of another type
// name = 123; // Error: cannot assign int to String
}
TUTORIAL already / has been 10 years
> var is not "no type" or "dynamic type" β its type is fixed at the moment of assignment. This is completely different from JavaScript's var.
### final β Runtime Constant
Variables declared with final can only be assigned once, but the assignment can be deferred to runtime.
## Instance
void main(){
// final variable: cannot be modified once assigned
final currentTime = DateTime.now();
print('Current time: $currentTime');
// Deferred assignment is also allowed
final String greeting;
greeting ='Hello, TUTORIAL!';// First assignment OK
// greeting = 'Hello again'; // Error: final variable cannot be assigned again
print(greeting);
}
Current time: 2026-06-12 10:30:00.000Hello, TUTORIAL!
### const β Compile-Time Constant
const is more strict than final: its value must be determinable at compile time.
## Instance
void main(){
// const must have a determinable value at compile time
const siteName ='TUTORIAL';
const maxUsers =100;
print('site / website: $siteName, maximum number of users: $maxUsers');
// The following will cause an error β DateTime.now() can only be determined at runtime
// const currentTime = DateTime.now(); // Error!
}
site / website: TUTORIAL, maximum number of users: 100
### Comparison Summary
| Keyword | Modifiable | Assignment Timing | Typical Scenarios |
| --- | --- | --- | --- |
| var | Yes | Any time | Regular variables whose values change |
| final | Not modifiable | Runtime-determined | Configuration from API, current time |
| const | Not modifiable | Compile-time-determined | Math constants, fixed configuration, color values |
> A practical tip: Use final by default. Change it to var only when you find you need to modify it. This habit makes code safer and reduces bugs from accidental modifications.
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## Numeric Types: int and double
Dart has two numeric types: int (integer) and double (floating-point number).
Their common parent class is num.
## Instance
void main(){
// int: integer, range is -2^63 to 2^63-1
int viewCount =10000;
int negativeNumber =-42;
// double: double-precision floating-point number
double price =9.99;
double pi =3.1415926;
// num can accept both int and double
num someNumber =10;
someNumber =10.5;// Valid
print('page views / view count: $viewCount');
print('Price: Β₯$price');
print('pi (Ο): $pi');
}
page views / view count: 10000Price: Β₯9.99pi (Ο): 3.1415926
### Common Numeric Methods
## Instance
import'dart:math';// Import math library
void main(){
int a =-10;
double b =3.7;
// Absolute value
print('a absolute value of: ${a.abs()}');// 10
// Round to integer
print('b round off / rounding: ${b.round()}');// 4
// Floor
print('b Round Down: ${b.floor()}');// 3
// Ceiling
print('b Round Up: ${b.ceil()}');// 4
// String to number
int parsed =int.parse('42');
double parsedDouble =double.parse('3.14');
print('parse result: $parsed, $parsedDouble');
// Generate random number (requires import 'dart:math')
var random = Random().nextInt(100);
print('random number (0-99): $random');
}
a absolute value of: 10 b round off / rounding: 4 b Round Down: 3 b Round Up: 4parse result: 42, 3.14random number (0-99): 67
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## Strings: String and Interpolation Syntax
String is used to represent text. Dart strings support both single quotes and double quotes.
String Interpolation is one of Dart's most convenient features.
## Instance
void main(){
// Strings can use single or double quotes
String singleQuoted ='Hello';
String doubleQuoted ="World";
// Multi-line strings: use three quotes
String multiLine ='''
This is First row
This is Second row
This is the third row - TUTORIAL
''';
// String interpolation: use $variableName or ${expression}
String site ='TUTORIAL';
int years =10;
// $variableName: direct variable reference
print('Welcome to $site');
// ${expression}: embed complex expressions
print('$site has been with us for ${years + 1} years');
// Escape characters
print('Newline character: First row\\
Second row');
print('tab character: column1\\\\t column2');
}
Welcome to TUTORIAL TUTORIAL has been with us for 11 yearsNewline character: First rowSecond rowtab character: column1column2
### Common String Methods
## Instance
void main(){
String text =' Hello, TUTORIAL! ';
// Trim leading and trailing whitespace
print('trim: [${text.trim()}]');
// To uppercase / To lowercase
print('uppercase: ${text.toUpperCase()}');
print('lowercase: ${text.toLowerCase()}');
// Whether it contains a substring
print('contains TUTORIAL? ${text.contains('TUTORIAL')}');
// Whether it starts with or ends with a string
print('withleading space? ${text.startsWith('')}');
print('with!End? ${text.trim().endsWith('!')}');
// String replacement
print('Replace: ${text.replaceAll('TUTORIAL', 'DART')}');
// String splitting
String csv ='Dart,Flutter,Google';
List items = csv.split(',');
print('split result: $items');
// String concatenation
print('concatenate / join: '+'Hello'+', '+'World');
}
trim: [Hello, TUTORIAL!]uppercase: HELLO, TUTORIAL!lowercase: hello, tutorial!contains TUTORIAL?