14.4 Freight Calculator Example

The examples we have shown so far in this chapter have been contrived to help illustrate the features of PHP. In this section, we apply these techniques and features discussed in this chapter to improve the freight calculator example we introduced in Chapter 4.

14.4.1 Review of the FreightCalculator

The FreightCalculator class defined in Example 4-9 is used to calculate the cost of delivering an online order. Example 4-9 also define the AirFreightCalculator class that redefines the two protected member functions that do the work of calculating the cost components: perKgTotal( ) and perCaseTotal( ).

FreightCalculator objects, including AirFreightCalculator objects, are constructed with two parameters, the total weight and the number of cases that make up a delivery:

    function _  _construct($numberOfCases, $totalWeight)

    {

        $this->numberOfCases = $numberOfCases;

        $this->totalWeight = $totalWeight;

    }

We also defined the CaseCounter class in Example 4-8 that extends the simple UnitCounter class defined in Example 4-4. A CaseCounter object can be used to accumulate units that make up a delivery and calculate the total weight and number of cases required to pack the order. The following example shows how a CaseCounter is created and used:

// Access to the CaseCounter class definition

require "example.4-8.php";



// Create a CaseCounter object where there are 

// 12 units to a case, and each unit weights 1.2 Kg.

$myOrder = new CaseCounter(12, 1.2);



// Add 5 bottles

$myOrder->add(5);



// Add 11 bottles

$myOrder->add(11);



// Add a case

$myOrder->addCase( );



// Now show me how many cases are required and the total weight

print "My order can be packed in {$myOrder->caseCount( )} and 

       weighs {$myOrder->totalWeight( ) Kg.";

The CaseCounter object is constructed with two parameters: the number of units that fit into a case, and the weight per unit. Once constructed, we can add units or cases to the counter. We describe how the CaseCounter works in Chapter 4.

To calculate the cost of delivering an order, we can create an AirFreightCalculator object using the output from the caseCount( ) and totalWeight( ) functions. Here's how it's done:

// Access to the CaseCounter class definition

require "example.4-8.php";



// Access to the AirFreightCalculator class definition

require "example.4-9.php";



// Create a CaseCounter object where there are 12 units to

// a case, and each unit weights 1.2 Kg.

$myOrder = new CaseCounter(12, 1.2);



// Add 28 bottles

$myOrder->add(28);



// Create an AirFreightCalculator

$air = new AirFreightCalculator($myOrder->caseCount( ), 

                                $myOrder->totalWeight( ))



// Now show me the cost

print "The cost of delivering $myOrder->numberOfUnits( ) bottles

       is $ {$air->totalFreight( )}.";

We can simplify the use of FreightCalculator objects by redefining the FreightCalculator class to use a CaseCounter object rather than passing in individual numeric values. However, a better solution is to use an interface that defines the functions that are required to calculate freight costs?an interface that can be used with classes other than CaseCounter.

14.4.2 Deliverable Interface

Example 14-4 shows the definition of the Deliverable interface that specifies the caseCount( ) and totalWeight( ) functions.

Example 14-4. Delivery interface
<?php



interface Deliverable

{

    function caseCount( );

    function totalWeight( );

}

?>

The CaseCounter class already supports the caseCount( ) and totalWeight( ) functions ?caseCount( ) is implemented in the CaseCounter definition, while the totalWeight( ) implementation is inherited from the base class UnitCounter?so the only change required is to use the Deliverable interface. This is shown in Example 14-5.

Example 14-5. The CaseCounter class implementing Deliverable
<?php



// Access to the UnitCounter class definition

require_once "example.4-4.php";



// Access to the Deliverable interface definition

require_once "example.14-4.php";





class CaseCounter extends UnitCounter implements Deliverable

{

    private $unitsPerCase;



    function addCase( )

    {

        $this->add($this->unitsPerCase);

    }



    function caseCount( )

    {

        return ceil($this->numberOfUnits( )/$this->unitsPerCase);

    }



    function _  _construct($caseCapacity, $unitWeight)

    {

        parent::_  _construct($unitWeight);

        $this->unitsPerCase = $caseCapacity;

    }

}

14.4.3 Improving the FreightCalculator

We now turn our attention to improving the FreightCalculator class. Improvements are made in two ways: we modify the class to use objects that support the Deliverable interface?thus simplifying the use of FreightCalculator objects?and we make the class abstract. The improved FreightCalculator class is shown in Example 14-6.

Example 14-6. The improved FreightCalculator class
<?php



// Access to the Deliverable interface definition

require_once "example.14-4.php";



abstract class FreightCalculator

{

    // The Deliverable item

    protected $item;



    function totalFreight( )

    {

        return $this->perCaseTotal( ) + $this->perKgTotal( );

    }



    abstract protected function perCaseTotal( );

    abstract protected function perKgTotal( );



    function _  _construct(Deliverable $item)

    {

        $this->item = $item;

    }

}



?>

While the FreightCalculator class defined in Example 4-9 implements a default pricing scheme, real schemes are implemented in descendant classes that redefine the protected perCaseTotal( ) and perKgTotal( ) functions. We designed the FreightCalculator class to be extended, creating a descendant class for each freight option that is offered by an online store. For example, the AirFreightCalculator class shown in Example 4-9 provides a pricing scheme appropriate for airfreight.

By defining the FreightCalculator class as abstract, we can prevent accidental creation of FreightCalculator objects. We can also remove the misleading implementation that doesn't correspond to any real pricing scheme by declaring the perCaseTotal( ) and perKgTotal( ) functions as abstract:

abstract protected function perCaseTotal( );

abstract protected function perKgTotal( );

PHP 5 allows the abstract and protected keywords to be used in either order.

The improved FreightCalculator class in Example 14-6 also replaces the protected member variables $numberOfCases and $totalWeight with the single protected variable $item. The _ _construct( ) function has also been modified to accept one parameter: a class type hint allowing only Deliverable objects to be passed:

    function _  _construct(Deliverable $item)

    {

        $this->item = $item;

    }

Descendant classes of FreightCalculator must also be modified to use the protected Deliverable member variable $item. Example 14-7 shows two descendant classes: a modified version of the AirFreightCalculator and a RoadFreightCalculator.

Example 14-7. AirFreightCalculator and RoadFreightCalculator
<?php



// Access to the FreightCalculator class

require_once "example.14-6.php";



class AirFreightCalculator extends FreightCalculator

{

     protected function perCaseTotal( )

     {

         return 15 + $this->item->caseCount( ) * 1.00;

     }



     protected function perKgTotal( )

     {

         return $this->item->totalWeight( ) * 0.40;

     }

}



class RoadFreightCalculator extends FreightCalculator

{



     protected function perCaseTotal( )

     {

         $numcases = $this->item->caseCount( );



         if ($numcases < 5)

             return 15;

         else

             return 15 + ($numcases - 5) * 1.50;

     }



     protected function perKgTotal( )

     {

         $weight = $this->item->totalWeight( );



         if ($weight < 50)

             return 0;

         else

             return ($weight - 50) * 0.10;

     }

}



?>

The class AirFreightCalculator calculates freight costs for a single delivery as $15 plus $1 per case and $0.40 per kilogram, while RoadFreightCalculator is a little more complicated with stepped rates for both case counts and total weight.

The member variables and functions that are available from $item can be accessed by chaining together -> access operators. For example, in the RoadFreightCalculator class definition, the number of cases is determined with the following code:

$numcases = $this->item->caseCount( );

14.4.4 Summary of Improvements

The classes we have defined in this section to calculate freight costs have real advantages over the FreightCalculator we presented in Chapter 4:

  • The FreightCalculator class is now defined as abstract, eliminating the risk of accidentally writing code that generate misleading freight costs.

  • We use the Deliverable interface allowing us to safely modify CaseCounter in the future without risk of breaking the FreightCalculator class.

  • Using the Deliverable interface, the FreightCalculator class can now be used with other, non-CaseCounter objects.

To illustrate the last point, consider the ChristmasHamper class in Example 14-8.

Example 14-8. A ChristmasHamper class
<?php



// access to the Deliverable interface

require_once "example.14-4.php";



class ChristmasHamper implements Deliverable

{

    function caseCount( )

    {

        return 1;

    }



    function totalWeight ( )

    {

        return 26.5;

    }



    function description( )

    {

        return "A hamper chock-full of Christmas goodies";

    }

}

?>

The ChristmasHamper class implements the Deliverable interface, so we can calculate the freight costs of ChristmasHamper objects using a FreightCalculator object. The following fragment shows how AirFreightCalculator objects can be created using CaseCounter and ChristmasHamper objects:

$wineOrder = new CaseCounter(12, 1.2);

$hamperOrder = new ChristmasHamper;



$wineOrder->add(10);



// Create two AirFreightCalculator objects

$a = new AirFreightCalculator($wineOrder);

$b = new AirFreightCalculator($hamperOrder);



// prints "Air freight on Christmas Hamper = 26.6"

print "Air freight on Christmas Hamper = {$b-> totalFreight( )}";

14.4.5 Using the Improved Freight Calculator

Example 14-9 demonstrates how the classes developed in this section can be used to build a simple application. The template that works with the script is shown in Example 14-10.

The application is a single web page that shows a table that compares the freight costs for different sized orders. This page might be included with an online store to help a customer choose delivery options when it comes time to purchase an order.

Example 14-9. Freight comparison table
<?php

require_once "HTML/Template/ITX.php";



// Access to the FreightCalculator classes

require_once"example.14-7.php";



// Access to the CaseCounter class

require_once"example.14-5.php";



// Access to the ChristmasHamper class

require_once"example.14-8.php";



$template = new HTML_Template_ITX("./templates");

$template->loadTemplatefile("example.14-10.tpl", true, true);



$exampleOrder = new CaseCounter(12, 1.2);



$air = new AirFreightCalculator($exampleOrder);

$road = new RoadFreightCalculator($exampleOrder);



for ($i = 0; $i < 10; $i++)

{

    $exampleOrder->add(6);



    $template->setCurrentBlock("order");

    $template->setVariable("UNITS", $exampleOrder->numberOfUnits( ));

    $template->setVariable("CASES", $exampleOrder->caseCount( ));

    $template->setVariable("WEIGHT", $exampleOrder->totalWeight( ));

    $template->setVariable("AIR", $air->totalFreight( ));

    $template->setVariable("ROAD", $road->totalFreight( ));

    $template->parseCurrentBlock( );

}



// Create a ChristmasHamper object

$hamper = new ChristmasHamper;

$air = new AirFreightCalculator($hamper);

$road = new RoadFreightCalculator($hamper);



// output the last row for the ChristmasHamper

$template->setCurrentBlock("order");

$template->setVariable("UNITS", "A Christmas hamper");

$template->setVariable("CASES", $hamper->caseCount( ));

$template->setVariable("WEIGHT", $hamper->totalWeight( ));

$template->setVariable("AIR", $air->totalFreight( ));

$template->setVariable("ROAD", $road->totalFreight( ));

$template->parseCurrentBlock( );



$template->show( );

?>

Example 14-10. The template used with Example 14-9
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"

                      "http://www.w3.org/TR/html401/loose.dtd">

<html>

<head>

  <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">

  <title>Freight Costs</title>

</head>

<body>

<h2>Freight Cost Comparison: Air vs. Road</h2>

<table border='1'>

    <tr>

      <th>Order size</th><th>Cases</th><th>Total Weight (kg)</th>

      <th>Air Freight</th><th>Road Freight</th>

    </tr>

<!-- BEGIN order -->

    <tr>

         <td>{UNITS}</td>

         <td>{CASES}</td>

         <td>{WEIGHT}</td>

         <td>${AIR}</td>

         <td>${ROAD}</td>

    </tr>

<!-- END order -->

</table>

</body>

</html>

To calculate the freight costs for the wine order, AirFreightCalculator and RoadFreightCalculator objects are constructed using the $exampleOrder CaseCounter object. Rates for different-sized orders can be calculated by adding units to the $exampleOrder object?we don't need to create new FreightCalculator objects for each order size. We do this inside the for loop that creates the table rows by adding 6 bottles to the $exampleOrder object:

        $exampleOrder->add(6);

The result of Example 14-9 is shown in Figure 14-2.

Figure 14-2. Freight comparison table
figs/wda2_1402.gif


14.4.6 Class Diagram

The freight calculation example now uses six classes, and one interface definition. Figure 14-3 shows how these definitions are related in a class diagram.

Figure 14-3. Class diagram
figs/wda2_1403.gif


As with Figure 14-1, inheritance is shown with connecting lines and solid arrowheads. In Figure 14-3 we've used additional notation to represent the Deliverable interface and the abstract class FreightCalculator.

The relationship between the FreightCalculator class and the Deliverable interface is shown with a connecting line with an open arrowhead. The open arrowhead indicates where one class uses the capabilities of another.



     
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