Defining New Objects

Defining New Objects

We developed a fancier version of Basketball where we drag around an object that looked more like a real basketball.

The structure of an object expression is as follows:

    object {
        def, instance variable, and type declarations

        initialization code
       
        method definitions
    }

Of course you can (and usually do) give it a name using a def declaration:

   def ball: Draggable = object { ... }

Aside from the (long) definition of the object ball, the program is identical to what we had when the ball was simply an orange filled oval.

When we define such an object:

• We must declare whatever refs and instance variables that will be necessary to describe the parts and state of the object. A basketball object is composed of ovals, lines and arcs so the defs of the ball object refer to these objects.
• We need to provide code to initialize the features of the object. Defs are given values when they are defined, while variables may be given values whenever (and however many times) we like. In the ball example above, the initializations all occur in the def declarations. When we've defined objects for main programs we always include the extra initialization statement startGraphics to initialize and display the window.
• We need to define methods - i.e., lists of statement explaining how to perform the actions we want the new objects to know how to do.

Notice the difference between the move and contains method in ball. Also notice how moveTo cleverly uses the moveBy method of the class via writing self.moveBy(...,...). The self. prefix can be omitted as Grace assumes it is there if you don't provide another receiver for the message.

One subtle point is that the definitions of features body, border, vertical, horizontal, and many more depend on the canvas being available. The canvas is only available in objects that inherit graphicApplication. Thus to make it accessible, we must put the definition of the object inside the definition of the object forming the main program.

Defining New Objects