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Dart Programming - (Functions , Interfaces ,Class)

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Dart Programming - Functions


Functions are the building blocks of readable, maintainable, and reusable code. A function is a set of statements to perform a specific task. Functions organize the program into logical blocks of code. Once defined, functions may be called to access code. This makes the code reusable. Moreover, functions make it easy to read and maintain the program’s code.

A function declaration tells the compiler about a function's name, return type, and parameters. A function definition provides the actual body of the function.

Sr.NoFunctions & Description
1Defining a Function

A function definition specifies what and how a specific task would be done.

2Calling a Function

A function must be called so as to execute it.

3Returning Functions

Functions may also return value along with control, back to the caller.

4Parameterized Function

Parameters are a mechanism to pass values to functions.

Optional Parameters

Optional parameters can be used when arguments need not be compulsorily passed for a function’s execution. A parameter can be marked optional by appending a question mark to its name. The optional parameter should be set as the last argument in a function.

We have three types of optional parameters in Dart −

Sr.NoParameter & Description
1Optional Positional Parameter

To specify optional positional parameters, use square [] brackets.

2Optional named parameter

Unlike positional parameters, the parameter's name must be specified while the value is being passed. Curly brace {} can be used to specify optional named parameters.

3Optional Parameters with Default Values

Function parameters can also be assigned values by default. However, such parameters can also be explicitly passed values.

Recursive Dart Functions

Recursion is a technique for iterating over an operation by having a function call to itself repeatedly until it arrives at a result. Recursion is best applied when you need to call the same function repeatedly with different parameters from within a loop.

Example

 Live Demo
void main() { 
   print(factorial(6));
}  
factorial(number) { 
   if (number <= 0) {         
      // termination case 
      return 1; 
   } else { 
      return (number * factorial(number - 1));    
      // function invokes itself 
   } 
}

It should produce the following output −

720

Lambda Functions

Lambda functions are a concise mechanism to represent functions. These functions are also called as Arrow functions.

Syntax

[return_type]function_name(parameters)=>expression;

Example

 Live Demo
void main() { 
   printMsg(); 
   print(test()); 
}  
printMsg()=>
print("hello"); 

int test()=>123;                       
// returning function

It should produce the following output −



Dart Programming - Interfaces




An interface defines the syntax that any entity must adhere to. Interfaces define a set of methods available on an object. Dart does not have a syntax for declaring interfaces. Class declarations are themselves interfaces in Dart.

Classes should use the implements keyword to be able to use an interface. It is mandatory for the implementing class to provide a concrete implementation of all the functions of the implemented interface. In other words, a class must redefine every function in the interface it wishes to implement.

Syntax: Implementing an Interface

class identifier implements interface_name

Example

In the following program, we are declaring a class Printer. The ConsolePrinter class implements the implicit interface declaration for the Printer class. The main function creates an object of the ConsolePrinter class using the new keyword. This object is used to invoke the function print_data defined in the ConsolePrinter class.

 Live Demo
void main() { 
   ConsolePrinter cp= new ConsolePrinter(); 
   cp.print_data(); 
}  
class Printer { 
   void print_data() { 
      print("__________Printing Data__________"); 
   } 
}  
class ConsolePrinter implements Printer { 
   void print_data() {  
      print("__________Printing to Console__________"); 
   } 
}

It should produce the following output −

__________Printing to Console__________

Implementing Multiple Interfaces

A class can implement multiple interfaces. The interfaces are separated by a comma. The syntax for the same is given below −

class identifier implements interface-1,interface_2,interface_4…….

The following example shows how you can implement multiple interfaces in Dart −

 Live Demo
void main() { 
   Calculator c = new Calculator(); 
   print("The gross total : ${c.ret_tot()}"); 
   print("Discount :${c.ret_dis()}"); 
}  
class Calculate_Total { 
   int ret_tot() {} 
}  
class Calculate_Discount { 
   int ret_dis() {} 
}
class Calculator  implements Calculate_Total,Calculate_Discount { 
   int ret_tot() { 
      return 1000; 
   } 
   int ret_dis() { 
      return 50; 
   } 
}


It should produce the following output −

The gross total: 1000 
Discount:50



Dart Programming - Classes



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 −

 Live Demo
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 −

 Live Demo
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 −

 Live Demo
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 −

 Live Demo
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 −

 Live Demo
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.

 Live Demo
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 −

 Live Demo
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

 Live Demo
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 −

 Live Demo
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

 Live Demo
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 value of num is 12

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

 Live Demo
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



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