Dart Programming - Variables

A variable is “a named space in the memory” that stores values. In other words, it acts a container for values in a program. Variable names are called identifiers. Following are the naming rules for an identifier −

Identifiers cannot be keywords.

Identifiers can contain alphabets and numbers.

Identifiers cannot contain spaces and special characters, except the underscore (_) and the dollar ($) sign.

Variable names cannot begin with a number.

Type Syntax

A variable must be declared before it is used. Dart uses the var keyword to achieve the same. The syntax for declaring a variable is as given below −

var name =  Smith ;

All variables in dart store a reference to the value rather than containing the value. The variable called name contains a reference to a String object with a value of “Smith”.

Dart supports type-checking by prefixing the variable name with the data type. Type-checking ensures that a variable holds only data specific to a data type. The syntax for the same is given below −

String name =  Smith ; 
int num = 10;

Consider the following example −

void main() { 
   String name = 1; 
}

The above snippet will result in a warning since the value assigned to the variable doesn’t match the variable’s data type.

Output

Warning: A value of type  String  cannot be assigned to a variable of type  int  

All uninitiapzed variables have an initial value of null. This is because Dart considers all values as objects. The following example illustrates the same −

void main() { 
   int num; 
   print(num); 
}

Null 

void main() { 
   dynamic x = "tom"; 
   print(x);  
}

tom

final variable_name

final data_type  variable_name

const variable_name

const data_type variable_name

void main() { 
   final val1 = 12; 
   print(val1); 
}

12

void main() { 
   const pi = 3.14; 
   const area = pi*12*12; 
   print("The output is ${area}"); 
}

The output is 452.15999999999997

void main() { 
   final v1 = 12; 
   const v2 = 13; 
   v2 = 12; 
}

Unhandled exception: 
cannot assign to final variable  v2= .  
NoSuchMethodError: cannot assign to final variable  v2=  
#0  NoSuchMethodError._throwNew (dart:core-patch/errors_patch.dart:178) 
#1      main (file: Test.dart:5:3) 
#2    _startIsolate.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:261) 
#3    _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:148)