L548: Session 13

Object-oriented programming II

Use "ezperl" for these exercises. (Click here if you should be interested in the Perl code.)

class Cat
   properties
        $name
        $color
   /properties

   method meow
        print "meow\n"
   /method

   method printDetails
        print "Cat.name\n"
        print "Cat.color\n"
   /method
/class
###################################
main

$cat = new Cat (name "Fred", color "white")
$cat.meow
$cat.printDetails

/main

object or instance: $cat
class: Cat
attributes: name, color
behaviour or methods: new(), meow(), printDetails()
constructor method: new()
instance method: printDetails()
class method: meow()

Exercise

In analogy to the example above, create a class for another type of pet besides cat or dog (see below).

Encapsulation, inheritance, and polymorphism

class Animal
    properties
        $name
        $color
    /properties

    method printDetails
        print "Animal.name\n"
        print "Animal.color\n"
    /method
/class
###################################
class Cat
    ISA Animal
    properties
        $name
        $color
    /properties

    method speak
        print "meow\n"
    /method
/class            
###################################
class Dog
    ISA Animal
    properties
        $name
        $color
    /properties

    method speak
        print "woof\n"
    /method
/class
###################################
main

    $cat = new Cat(name "Fred", color "white")
    $dog = new Dog(name "Max", color "black")
    @pets = ($cat,$dog)

    foreach $pet (@pets)
        $pet.printDetails
        $pet.speak  
    /foreach
/main

encapsulation: the implementation of printDetails() is hidden from Cat and Dog
inheritance: Cat and Dog inherit printDetails()
polymorphism: $pet refers to instances of different types()

Exercises

Add your pet from the previous exercise to the Animal class.

Let users input a type of animal (Cat, Dog, etc) and then print the sound they make. Hint: "ref $pet" returns the class to which $pet belongs ("ref" stands for "reference" or "refers to"). For example, if $pet is a cat then "ref $pet" equals Cat. But the user has to ask for "Cat" not "cat". Of course, you could use a regular expression with "ignore case" to compare the user input with ref $pet...

Optional: Modules

Use Perl for these exercises not "ezperl". Use the code from the last exercise in the file (file.plx) that ezperl generated.

A module is a class (or "package" in Perl terminology) stored in a file by itself and named "class.pm". The last executed line in a module should be "return 1;". A class can be used in a different file by including "use class;" at the beginning of the file.

Exercise

Save Animal, Cat, Dog and the additional pet as modules by themselves. To use them you can either include "use" statements for all four of them in the main file or you use Cat, Dog and the other pet in the main file and Animal in Cat, Dog and the other pet. Try both possibilities.