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Calling a private method from a subclass - PHP

Welcome to the final chapter of the series, “Protecting the data of PHP 5 classes with member visibility.” Complemented by copious illustrative examples, this article series guides you through learning the key concepts regarding the use of public, protected, and private data members within PHP 5 classes. This way you can start defining the visibility of the properties and methods of your own classes in a very short time.

TABLE OF CONTENTS:
  1. More on Private Methods with PHP 5 Member Visibility
  2. Reintroducing a previous hands-on example
  3. Calling a private method from a subclass
  4. Another way to protect class methods: the final keyword
By: Alejandro Gervasio
Rating: starstarstarstarstar / 2
June 25, 2008

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As I expressed in the section that you just read, I’d like to finish discussing the use of private methods in PHP 5 by coding yet another demonstrative example. In this particular case, I want you to see for yourself what happens when a private method defined by a parent is invoked by one of its child classes.

So take a look at the following code sample to dissipate any possible doubts:


// define 'DataSaver' class (methods are defined private)


class DataSaver{

protected $filePath;

protected $data;

public function __construct($data,$filePath){

if(!$data||strlen($data)>1024){

throw new Exception('Invalid data for being saved to target file.');

}

if(!file_exists($filePath)){

throw new Exception('Invalid target file.');

}

$this->data=$data;

$this->filePath=$filePath;

}

// save data to target file

public function save(){

if(!$fp=fopen($this->filePath,'w')){

throw new Exception('Error opening target file.');

}

if(!fwrite($fp,$this->data)){

throw new Exception('Error writing data to target file.');

}

fclose($fp);

}

// get target file via an accessor

private function getFilePath(){

return $this->filePath;

}

// get data via an accessor

private function getData(){

return $this->data;

}

}


// extends 'DataSaver' class

class DataHandler extends DataSaver{

// fetch data from target file

public function fetch(){

if(!$data=file_get_contents($this->filePath)){

throw new Exception('Error reading data from target file.');

}

return $data;

}

}


try{

// create new instance of 'DataHandler' class

$dataHandler=new DataHandler('This string of data will be saved to a target file!','datafile.txt');

// save data to target file

$dataHandler->save();

// call private method

echo $dataHandler->getFilePath();

/* displays the following

Fatal error: Call to private method DataSaver::getFilePath() from context '' in path/to/file/

*/

}

catch(Exception $e){

echo $e->getMessage();

exit();

}


As you can see, I first derived a basic subclass from the “DataSaver” parent, then created a new instance of it, and finally called its “getFilePath()” method in the global scope. This demonstrates in a nutshell that a private method defined by a base class can’t also be invoked by any of its child classes. Not too difficult to understand, right?

So far, so good. Having explained how the prior hands-on example functions, it’s time to end this discussion of private class methods with PHP 5. But wait a minute! Before I finish, I’d like to point out that PHP 5 provides developers with yet another mechanism for preventing the methods of a base class from being overridden by one or more subclasses.

As you might have guessed, I’m talking about the “final” keyword, whose appropriate utilization will be the final subject of this article. Thus, if you’re interested in learning how to use it within your own classes, jump ahead and read the following section. It’s only one click away.



 
 
>>> More PHP Articles          >>> More By Alejandro Gervasio
 

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