D - Interfaces

An interface is a way of forcing the classes that inherit from it to have to implement certain functions or variables. Functions must not be implemented in an interface because they are always implemented in the classes that inherit from the interface.
An interface is created using the interface keyword instead of the class keyword even though the two are similar in a lot of ways. When you want to inherit from an interface and the class already inherits from another class then you need to separate the name of the class and the name of the interface with a comma.
Let us look at an simple example that explains the use of an interface.

import std.stdio;

// Base class
interface Shape
{
   public:
      void setWidth(int w);
      void setHeight(int h);
}

// Derived class
class Rectangle: Shape
{
   int width;
   int height;
   public:
      void setWidth(int w)
      {
         width = w;
      }
      void setHeight(int h)
      {
         height = h;
      }

      int getArea()
      {
         return (width * height);
      }
}

void main()
{
   Rectangle Rect = new Rectangle();

   Rect.setWidth(5);
   Rect.setHeight(7);

   // Print the area of the object.
   writeln("Total area: ", Rect.getArea());

}

When the above code is compiled and executed, it produces the following result:

Total area: 35

Interface with Final and Static Functions

An interface can have final and static method for which definitions should be included in interface itself. These functions cannot be over-riden by the derived class. A simple example is shown below.

import std.stdio;

// Base class
interface Shape
{
   public:
      void setWidth(int w);
      void setHeight(int h);
      static void myfunction1()
      {
         writeln("This is a static method");
      }
      final void myfunction2()
      {
         writeln("This is a final method");
      }
}

// Derived class
class Rectangle: Shape
{
   int width;
   int height;
   public:
      void setWidth(int w)
      {
         width = w;
      }
      void setHeight(int h)
      {
         height = h;
      }

      int getArea()
      {
         return (width * height);
      }
}

void main()
{
   Rectangle rect = new Rectangle();

   rect.setWidth(5);
   rect.setHeight(7);

   // Print the area of the object.
   writeln("Total area: ", rect.getArea());
   rect.myfunction1();
   rect.myfunction2();
}

When the above code is compiled and executed, it produces the following result:

Total area: 35
This is a static method
This is a final method