What is an interface in Dart?

Dart interface

An interface specifies the syntax that all classes must follow. An interface defines a set of methods for an object. The most common usage of the interface is to impose compulsion on a class. When a class implements an interface, it must override all its methods and instance variables.

The class declaration is also an interface in Dart.

Why use an interface in Dart?

  • It is used to achieve abstraction.
  • It’s a mechanism to achieve multiple inheritances.

Note: We must use the implements keyword since Dart does not offer a simple means to build inherited classes.

Syntax

class Interface_class_name{
   ...
}

class Class_name implements Interface_class_name {
   ...
}

Code

The following code shows how to use the interface keyword in Dart:

// class Shape (interface)
class Shape {
  void display() {
    print('Method in Shape class');
  }
}
 
// class Triangle implementing class Shape
class Triangle implements Shape {
  void display() {
    print('This is the method of the impemented class');
  }
}
 
void main() {
  // creating instance of Class Triangle
  var triangle = Triangle();
  // Invoke method of class Shape 
  // using class Triangle instance 
  triangle.display();
}

Explanation

  • Lines 1 to 5: We create a Shape class with a method display().
  • Lines 10 to 15: We create another Triangle class that implements the class Shape.
  • Lines 17 to 24: In the main() function, we create an instance of class Triangle named triangle. Next, we call the method inherited from class Shape using triangle.

Note: To use an interface method, a class should use the implements keyword instead of the extends keyword.

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