Static Versus Regular Methods and Variables

You have already learned that a static method is one that can be called without needing an instantiated object. Actually, you can also have static variables as you saw in the last section. Static variables can be used to emulate constants, values that don't change. Constants are very useful. For example, you can use them for tax rates, mathematical constants, and things such as state abbreviations. Here is an example using a small Perl script, static.pl:

package Math;

    $math{'PI'} = 3.1415;



package main;

    print("The value of PI is $Math::math{'PI'}.\n");

This program displays:

The value of PI is 3.1415.

You can also do this:

package Math;
    $PI = 3.1415;
package main;
    print("The value of PI is $Math::PI.\n");

Because you have been using a static method all along-the new() method -- let's demonstrate a regular function.

The following code shows how to use the UNIVERSAL package to define a utility function that is available to all classes.

• Start a definition of the UNIVERSAL class.
• Define the lookup() method.
• Dereference the object reference (the first element of @_) and use the second parameter as the key into the anonymous hash.
• Return the value of the hash entry.
• Start a definition of the Inventory_item class.
• Define the constructor for the class.
• Assign the rest of the parameters to the %params hash.
• Bless the anonymous hash with the class name.
• Use %params to initialize the class properties.
• Start the main namespace.
• Call the constructor for the Inventory_item class.
• Assign the object reference to $item.
• Print the two property values using the lookup() method to verify that the property initialization worked.

The Perl Code for this is stat_univ.pl:

package UNIVERSAL;

    sub lookup {

        return(%{$_[0]}->{$_[1]});

    }



package Inventory_item;

    sub new {

        my($class)  = shift;

        my(%params) = @_;

        my($self)   = { };

                             

        $self->{"PART_NUM"}    = $params{"PART_NUM"};

        $self->{"QTY_ON_HAND"} = $params{"QTY_ON_HAND"};



        return(bless($self, $class));

    }



package main;



    $item = Inventory_item->new("PART_NUM"=>"12A-34", "QTY_ON_HAND"=>34);



    print("The part number is " . $item->lookup('PART_NUM')     . "\n");

    print("The quantity is "    . $item->lookup('QTY_ON_HAND')  . "\n");

Finally, the printAll() function shown here displays all the properties of a class, or you can specify one or more properties to display printAll.pl:

sub printAll {

    my($self) = shift;

    my(@keys) = @_ ? @_ : sort(keys(%{$self}));



    print("CLASS: $self\n");

    foreach $key (@keys) {

        printf("\t%10.10s => $self->{$key}\n", $key);

    }

}

If you put this function into the UNIVERSAL package, it will be available to any classes you define.

After constructing an inventory object, the statement $item->printAll(); might display

CLASS: Inventory_item=HASH(0x77ceac)

          PART_NUM => 12A-34

        QTY_ON_HAN => 34

and the statement

$item->printAll('PART_NUM');

might display

CLASS: Inventory_item=HASH(0x77ceac)

          PART_NUM => 12A-34