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

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.

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.

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 −

Example

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