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);
}
}
The output of the above code is as follows −
E1001
Dart Constructors
A constructor is a special function of the class that is responsible for initializing the variables of the class. Dart defines a constructor with the same name as that of the class. A constructor is a function and hence can be parameterized. However, unlike a function, constructors cannot have a return type. If you don’t declare a constructor, a default no-argument constructor is provided for you.
Syntax
Class_name(parameter_list) {
//constructor body
}
Example
The following example shows how to use constructors in Dart −
void main() {
Car c = new Car('E1001');
}
class Car {
Car(String engine) {
print(engine);
}
}
It should produce the following output −
E1001
Named Constructors
Dart provides named constructors to enable a class define multiple constructors. The syntax of named constructors is as given below −
Syntax : Defining the constructor
Class_name.constructor_name(param_list)
Example
The following example shows how you can use named constructors in Dart −
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}");
}
}
It should produce the following output −
The engine is : E1001
Non-parameterized constructor invoked
The this Keyword
The this keyword refers to the current instance of the class. Here, the parameter name and the name of the class’s field are the same. Hence to avoid ambiguity, the class’s field is prefixed with the this keyword. The following example explains the same −
Example
The following example explains how to use the this keyword in Dart −
void main() {
Car c1 = new Car('E1001');
}
class Car {
String engine;
Car(String engine) {
this.engine = engine;
print("The engine is : ${engine}");
}
}
It should produce the following output −
The engine is : E1001
Dart Class ─ Getters and Setters
Getters and Setters, also called as accessors and mutators, allow the program to initialize and retrieve the values of class fields respectively. Getters or accessors are defined using the get keyword. Setters or mutators are defined using the set keyword.
A default getter/setter is associated with every class. However, the default ones can be overridden by explicitly defining a setter/ getter. A getter has no parameters and returns a value, and the setter has one parameter and does not return a value.
Syntax: Defining a getter
Return_type get identifier
{
}
Syntax: Defining a setter
set identifier
{
}
Example
The following example shows how you can use getters and setters in a Dart class −
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);
}
This program code should produce the following output −
Age should be greater than 5
MARK
Null
Class Inheritance
Dart supports the concept of Inheritance which is the ability of a program to create new classes from an existing class. The class that is extended to create newer classes is called the parent class/super class. The newly created classes are called the child/sub classes.
A class inherits from another class using the ‘extends’ keyword. Child classes inherit all properties and methods except constructors from the parent class.
Syntax
class child_class_name extends parent_class_name
Note − Dart doesn’t support multiple inheritance.
Example: Class Inheritance
In the following example, we are declaring a class Shape. The class is extended by the Circle class. Since there is an inheritance relationship between the classes, the child class, i.e., the class Car gets an implicit access to its parent class data member.
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 {}
It should produce the following output −
calling calc area defined in the Shape class
Types of Inheritance
Inheritance can be of the following three types −
- Single − Every class can at the most extend from one parent class.
- Multiple − A class can inherit from multiple classes. Dart doesn’t support multiple inheritance.
- Multi-level − A class can inherit from another child class.
Example
The following example shows how multi-level inheritance works −
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
The class Leaf derives the attributes from Root and Child classes by virtue of multi-level inheritance. Its output is as follows −
hello
Dart – Class Inheritance and Method Overriding
Method Overriding is a mechanism by which the child class redefines a method in its parent class. The following example illustrates the same −
Example
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}");
}
}
It should produce the following output −
value of b 12
The number and type of the function parameters must match while overriding the method. In case of a mismatch in the number of parameters or their data type, the Dart compiler throws an error. The following illustration explains the same −
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}");
}
}
It should produce the following output −
value of b 12
The static Keyword
The static keyword can be applied to the data members of a class, i.e., fields and methods. A static variable retains its values till the program finishes execution. Static members are referenced by the class name.
Example
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
}
It should produce the following output −
The super Keyword
The super keyword is used to refer to the immediate parent of a class. The keyword can be used to refer to the super class version of a variable, property, or method. The following example illustrates the same −
Example
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}") ;
}
}
It should produce the following output −
value of b 12
value of a 13
message variable from the parent class