Defining Classes

Defining Classes

Classes are used when we need more than one object with the same kind of information and operations. Suppose, for example, that I wanted to have a basketball that could start where ever I wanted to on the screen of any application. (E.g., maybe I want two basketballs to lay next to the laundry baskets in this weeks lab.)

To create multiple objects of the same general kind, we can define a class which can be used to generate objects. We can get from an object to a class with only a few small changes:

• Replace def nm: Tp = object {...} by class otherName(...) -> Tp {...} where the first set of ... following otherName represents a collection of parameters that provide specialized information for each new object.

  For example, see the Basketball with a class and compare it to our previous version above. The line:

  def ball: Draggable = object {...}
  

  was replaced by

  class basketBall.at(startPosn:Point) size (size:Number)
                     on (canvas:DrawingCanvas) -> Draggable {...}
  

  The formal parameters startPosn, size, and canvas are the items that can vary between uses. For example, in the basketballGame object, the line

       def ball = basketBall.at(startLocn)size(ballSize)on(canvas)
  

  will create a ball at startLocn (which was earlier defined to be 190 @ 300), with size ballSize (which was set to 40), and on the canvas that was created for basketballGame.

  To help you understand the correspondence, we used the same names as in the definition of ball in the earlier version. Make sure you understand the correspondence.

• We've been using classes to create objects from the objectdraw library objects from the beginning. When we say framedRect.at(pt)size(w,h)on(canvas), our library knows what to do to make a framed rectangle appear on the screen because someone defined a class to construct framed rectangles. When we write a class, we will write a list of statements to construct the components of the object desired (the parts of the basketball) and associate them with the defs and instance variables declared in the new class.

At the end of class, we looked at a class to generate Tshirts: Tshirt

Defining Classes