Why would you extend a class rather than just adding a new method to the original class?

Question

I finally had my Ah-ha moment when it came to classes in OOP. I've been struggling to get my mind around the idea. Now that I'm starting to get it I was wondering what the purpose of extending a class is. Would it not be simpler to just add another method into the class or am I just missing the point?

Answer 1

If you want to add functionality to a class you add a method. However, there are situations that you want to add functionality, but cannot do that in a base class.

The best is show with a example. When adding a method Radius to a class makes only sense if the class is a Circle, a Rect has no radius. Adding a method Radius to the base class also makes no sense, because the Rect class then also gets a method Radius.

Example:

abstract class Shape
{
  abstract public function Draw();
}

class Rect extends Shape
{
  public function Draw()
  {
  }
}

class Circle extends Shape
{
  public function Draw()
  {
  }

  public function Radius()
  {
  }
}