Describing similar things

Describing similar things

Before we answer these questions, we need to consider some new concepts and their use within Grace. Let's start by thinking about how we describe things in English. We often describe real world objects as being like something else but then describe what the difference is:

• What is a strobe light? It's a light that flashes. It has an extra behavior that most lights don't have.
• What is a poisonous snake? It's a snake whose bit is poisonous. All snakes bite (except probably our friend Nibbles), so "poisonous" does not describe a new behavior, but a different effect than that seen from other kinds of snakes when they bite.
• What is a textbook? It is a book that is used in a course. It contains questions that can be given out as assignments. It has properties that most books don't have (questions) that enable it to be used in ways that most books can't be used.

We'd like to be able to do similar things in Grace, and these are exactly the kinds of things we can do with the keyword inherits. The class we "inherit" is the class that we are like. The constants, variables, and methods that we define indicate how the new class differs from the one that we are extending.

We have done this all semester to create graphicApplications. For example, consider the header

def fallingLeaves: GraphicApplication = object {
  inherits graphicApplication.size(500,500)

from our falling leaves example. fallingLeaves is a graphicApplication. It has the same behaviors and properties as other graphicApplications, except for the things that we specify: 1) how it it draws the display and 2) how it responds to mouse clicks (creating a tree from which leaves fall).

Here, we call fallingLeaves the subobject and graphicApplication the superclass.

Describing similar things