Dart is an object-oriented language. It supports object-oriented programming features like classes, interfaces, etc. A class in terms of OOP is a blueprint for creating objects. A class encapsulates data for the object. Dart gives built-in support for this concept called class.

Declaring a Class

Use the class keyword to declare a class in Dart. A class definition starts with the keyword class followed by the class name; and the class body enclosed by a pair of curly braces. The syntax for the same is given below −

Syntax

class class_name {  
   <fields> 
   <getters/setters> 
   <constructors> 
   <functions> 
}

The class keyword is followed by the class name. The rules for identifiers must be considered while naming a class.

A class definition can include the following −

• Fields − A field is any variable declared in a class. Fields represent data pertaining to objects.

• Setters and Getters − Allows the program to initialize and retrieve the values of the fields of a class. A default getter/ setter is associated with every class. However, the default ones can be overridden by explicitly defining a setter/ getter.

• Constructors − responsible for allocating memory for the objects of the class.

• Functions − Functions represent actions an object can take. They are also at times referred to as methods.

These components put together are termed as the data members of the class.

Example: Declaring a class

class Car {  
   // field 
   String engine = "E1001";  
   
   // function 
   void disp() { 
      print(engine); 
   } 
}

The example declares a class Car. The class has a field named engine. The disp() is a simple function that prints the value of the field engine.

Creating Instance of the class

To create an instance of the class, use the new keyword followed by the class name. The syntax for the same is given below −

Syntax

var object_name = new class_name([ arguments ])

• The new keyword is responsible for instantiation.

• The right-hand side of the expression invokes the constructor. The constructor should be passed values if it is parameterized.

Example: Instantiating a class

var obj = new Car("Engine 1")

Accessing Attributes and Functions

A class’s attributes and functions can be accessed through the object. Use the ‘.’ dot notation (called as the period) to access the data members of a class.

//accessing an attribute 
obj.field_name  

//accessing a function 
obj.function_name()

Example

Take a look at the following example to understand how to access attributes and functions in Dart −

void main() { 
   Car c= new Car(); 
   c.disp(); 
}  
class Car {  
   // field 
   String engine = "E1001";  
   
   // function 
   void disp() { 
      print(engine); 
   } 
}

E1001

Class_name(parameter_list) { 
   //constructor body 
}

void main() { 
   Car c = new Car(''E1001''); 
} 
class Car { 
   Car(String engine) { 
      print(engine); 
   } 
}

E1001 

Class_name.constructor_name(param_list)

void main() {           
   Car c1 = new Car.namedConst(''E1001'');                                       
   Car c2 = new Car(); 
}           
class Car {                   
   Car() {                           
      print("Non-parameterized constructor invoked");
   }                                   
   Car.namedConst(String engine) { 
      print("The engine is : ${engine}");    
   }                               
}

The engine is : E1001 
Non-parameterized constructor invoked

void main() { 
   Car c1 = new Car(''E1001''); 
}  
class Car { 
   String engine; 
   Car(String engine) { 
      this.engine = engine; 
      print("The engine is : ${engine}"); 
   } 
} 

The engine is : E1001

Return_type  get identifier 
{ 
} 

set identifier 
{ 
}

class Student { 
   String name; 
   int age; 
    
   String get stud_name { 
      return name; 
   } 
    
   void set stud_name(String name) { 
      this.name = name; 
   } 
   
   void set stud_age(int age) { 
      if(age<= 0) { 
        print("Age should be greater than 5"); 
      }  else { 
         this.age = age; 
      } 
   } 
   
   int get stud_age { 
      return age;     
   } 
}  
void main() { 
   Student s1 = new Student(); 
   s1.stud_name = ''MARK''; 
   s1.stud_age = 0; 
   print(s1.stud_name); 
   print(s1.stud_age); 
} 

Age should be greater than 5 
MARK 
Null 

class child_class_name extends parent_class_name 

void main() { 
   var obj = new Circle(); 
   obj.cal_area(); 
}  
class Shape { 
   void cal_area() { 
      print("calling calc area defined in the Shape class"); 
   } 
}  
class Circle extends Shape {}

calling calc area defined in the Shape class

void main() { 
   var obj = new Leaf(); 
   obj.str = "hello"; 
   print(obj.str); 
}  
class Root { 
   String str; 
}  
class Child extends Root {}  
class Leaf extends Child {}  
//indirectly inherits from Root by virtue of inheritance

hello

void main() { 
   Child c = new Child(); 
   c.m1(12); 
} 
class Parent { 
   void m1(int a){ print("value of a ${a}");} 
}  
class Child extends Parent { 
   @override 
   void m1(int b) { 
      print("value of b ${b}"); 
   } 
}

value of b 12

import ''dart:io''; 
void main() { 
   Child c = new Child(); 
   c.m1(12); 
} 
class Parent { 
   void m1(int a){ print("value of a ${a}");} 
} 
class Child extends Parent { 
   @override 
   void m1(String b) { 
      print("value of b ${b}");
   } 
}

value of b 12

class StaticMem { 
   static int num;  
   static disp() { 
      print("The value of num is ${StaticMem.num}")  ; 
   } 
}  
void main() { 
   StaticMem.num = 12;  
   // initialize the static variable } 
   StaticMem.disp();   
   // invoke the static method 
}

The value of num is 12

void main() { 
   Child c = new Child(); 
   c.m1(12); 
} 
class Parent { 
   String msg = "message variable from the parent class"; 
   void m1(int a){ print("value of a ${a}");} 
} 
class Child extends Parent { 
   @override 
   void m1(int b) { 
      print("value of b ${b}"); 
      super.m1(13); 
      print("${super.msg}")   ; 
   } 
}

value of b 12 
value of a 13 
message variable from the parent class