Hi all,
I stumbled across something called BlueJ which provides a user-friendly
graphical environment in which to learn OO-programming, Java-style. I think
this is a great "parallel universe" to go along with the Perl Objects book.
I started re-writing one of the first exercises as a Perl package:
[question follows below]
package TicketMachine;
# etc.
=head1 DESCRIPTION
TicketMachine models a naive ticket machine that issues
flat-fare tickets.
The price of a ticket is specified via the constructor.
[Based on an exercise from "Objects First With Java:
A Practical Introduction Using BlueJ.]
=cut
# my $price; # Price of a ticket from this machine.
# my $balance; # Amount of money entered by customer
# my $total; # Total amount of money collected by machine
# Create a machine that issues tickets of the given price.
# Price must be greater than zero (but no checks done).
sub new {
my $class = shift;
# In the BlueJ environment instances are automatically
# named "behind the scenes" (so far)...
my $instance = "tm" . int(rand(25));
my $self = { Name => $instance, Price => shift };
bless $self, $class;
}
sub getPrice {
my $self = shift;
return $self->{Price};
}
# more methods to come...
1;
__END__
In the Java version, the three variables are declared in the class first:
private int price;
private int balance;
private int total;
So in Perl I tried declaring these in the package using my, but they seemed
to remain class variables, so if I would do something like:
for (@ticket_machines) {
print "A ticket costs ", $_->getPrice(), " cents.\n";
}
I would then get (for two machines):
A ticket costs 100 cents.
A ticket costs 100 cents.
Then I tried this with Perl...
In the "new" constructor:
my $class = shift;
my $price = shift;
my $self = $instance;
bless \$self, $class, $price;
...I was told "too many arguments to bless". The Perl Objects book says the
easiest way to use more data with an instance is with a blessed hash
reference, and that did work fine (and maybe that's the best way).
But I'm wondering if there is another way (like the Java "private variable")
to say "all class variables declared here are unique to each instance"?
It occurs to me that perhaps the difference is that in Perl you can call
methods on a class (i.e. the "an unnamed horse" from the Perl Objects book)
but not in Java (other than the contructor method)?
(If anyone is interested in BlueJ -> www.bluej.org)
-K
--
Kevin Pfeiffer
International University Bremen
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