OO Design: Abstract Classes Page 6

Example: Interfaces

For our second example, we will see how an abstract class can be used as an interface. We will start out
by designing a few objects that will be a part of a fictitious web application for home-automation (one of
my favorite hobbies). We want to build a web interface to control our various home devices (i.e. lights,
toaster, stereo). However, not all of our devices speak the same language, or protocol. While the X-10 protocol
still dominates the market, we find several other home-automation protocols creeping in. The four major
contenders among home-automation protocols are:

• X-10
• LONworks
• CEBus (EIA IS-60)
• Smart House

The implementation for each of these protocols varies significantly. Yet, amongst these protocols the same
common control functionality exists: (1) turning on a device, (2) turning off a device, and (3) getting the
status of a device. Our interface will provide a template for this common functionality. We’ll name our
interface Control. As you can see, the methods do not contain any code.

<?php
class Control
{
  function on() {}
  function off() {}
  function getStatus() {}
}
?>

Earlier I mentioned providing well-written comments in our code. This helps the implementer have a clear
understanding of what behavior is expected. Therefore, we’ll update our example with appropriate comments.

<?php
/*
 * Class:   Control
 *  Type:   Interface
 *
 * Purpose: A template for common control functionality
 *          of home automated devices.
 *
 * Methods: on()
 *          off()
 *          getStatus()
 *
 * Error Codes
 * -----------
 * 0 - No errors occurred during transmission.
 * 1 - An error occurred during transmission.
 *
 *
 * API Developers
 * --------------
 * Be sure to provide appropriate functionality for each
 * method. The application will dynamically instantiate
 * the appropriate protocol for a selected device, and will
 * expect consistent control functionality.
 */
class Control
{
    function Control()
  {
    /*
     * Do not allow instantiation of Control class.
     * Prevent non subclass from calling constructor.
     */
    if ( !is_subclass_of($this, "Control") ) {
      trigger_error("Control instantiation from non subclass.", E_USER_ERROR);
      return NULL;
    }
  }
  function on() {}         // Turn on a device.
  function off() {}        // Turn off a device.
  function getStatus() {}  // Get the status of a device.
}
?>

Now that we have our interface defined, let’s implement a protocol. We will work with the X-10 protocol,
since it is the most popular one. We will inherit from Control, and provide the appropriate
implementation in X10_Control.