Comp210 Lab 14: Objects

You must set the DrScheme language to "Full Scheme" to run the define-class.ss package!

Documentation for the define-class.ss package

Download the define-class package (not required for lab, but useful for working at home).

At the top of your definitions window, type the following line to load the define-class.ss package:

(load "/home/comp210/Lectures/define-class.ss")

Link to the class notes

Exercise 1:

Write the class Animal% with the following

• gettable sound attribute
• settable name attribute
• constructor that takes a name and a sound (note: constructor parameters must refer only to gettable or settable attributes)
• public speak method -- the speak method should simply return the sound by using the get-sound method that was automatically generated by "gettable".

Normally, the Animal% class would be considered abstract, and hence one would not be able to make and instance of it, but this Scheme implementation of won't stop you, so we'll take advantage of that here.

Create an "instance" of ("instantiate") Animal% :

(define animal (make-object Animal% "anAnimal" "no sound"))

Test that the animal does what it is supposed to do:

(send animal getname)

(send animal get-sound)

(send animal speak)

Exercise 2:

Create the class Dog% where:

• Dog% is a subclass of Animal%.
• the speak method is overriden to return the sound multiple times. Note, since get-sound is defined in the superclass, simply calling (get-sound) won't work. You need to use (send this get-sound). "this" refers to this instance of the the class.

Test your Dog class by creating an instance of one and testing ALL of its methods.

Exercise 3.

Create a Cat% class similarly to Dog, but whose speak method is different than Dog%

Now, write a function that takes an Animal% and return a string with the animal's name and what it speaks.

Test your function on both Dog% and Cat% instances. This is a demostration of the hugely important concept of "polymorphism", which is that if two objects are abstractly equivalent, they can be used interchangeably wherever the abstract class is needed, but that they will still have their own distinct behaviors.

Modify your function so that it works on a list. Test it with a list filled with both Dogs and Cats.

Exercise 4.

Create a Bird% (or whatever) class that

• Has additional methods and attrributes
• Uses private variables that are not accessible from the outside.

Note: to add more arguments to a sub-class's constructor than the superclass has, write the following super-class initializer as the first function of the body of the class:

(super-init arg1 arg2 ....)

where the args are the arguments of the superclass constructor.

Use your imagination!