Real world examples of Factory Method pattern

I just read Factory Method. I understand that it provides a way to delegate the instantiation to sub-classes. But I couldn't understand the possible uses in a real-world scenario.

Can anyone give one typical example showing how Factory method pattern开发者_开发知识库 can be used so that I can relate to what I have read.

A problem statement for which factory method pattern is the best solution would be sufficient to make it clear.

A class implementing factory design pattern works as bridge between multiple classes. Consider an example of using multiple database servers like SQL Server and Oracle. If you are developing an application using SQL Server database as backend, but in future need to change backend database to oracle, you will need to modify all your code, if you haven’t written your code following factory design pattern.

In factory design pattern you need to do very little work to achieve this. A class implementing factory design pattern takes care for you and lessen your burden. Switching from database server won’t bother you at all. You just need to make some small changes in your configuration file.

An example php code with static creation:

interface DbTable
{
    public function doSomething(): void;
}

class MySqlTable implements DbTable
{
    public function doSomething(): void
    { }
}

class OracleTable implements DbTable
{
    public function doSomething(): void
    { }
}

class TableFactory
{
    public static function createTable(string $type = null): DbTable
    {
        if ($type === 'oracle') {
            return new OracleTable();
        }
        return new MySqlTable(); // default is mysql
    }
}

// client
$oracleTable = TableFactory::createTable('oracle');
$oracleTable->doSomething();

To make it more dynamic (less modification later):

interface DbTable
{
    public function doSomething(): void;
}

class MySqlTable implements DbTable
{
    public function doSomething(): void
    { }
}

class OracleTable implements DbTable
{
    public function doSomething(): void
    { }
}

class TableFactory
{
    public static function createTable(string $tableName = null): DbTable
    {
        $className = __NAMESPACE__ . $tableName . 'Table';
        if (class_exists($className)) {
            $table = new $className();
            if ($table instanceof DbTable) {
               return $table;
            }
        }
        throw new \Exception("Class $className doesn't exists or it's not implementing DbTable interface");
    }
}

$tbl = TableFactory::createTable('Oracle');
$tbl->doSomething();

From API I'm developing right now:

WebGalleryFactory factory = WebGalleryFactory.newInstance (WebGalleryType.PICASA);
WebAlbum album = factory.createAlbum (title, description);

In this example I use Factory Method to create Abstract Factory of a certain type (PICASA in the example).

These two patterns are often used together.

Zend_Db uses it in it's Zend_Db_Adapter class to allow the creation of different kinds of database objects based on database settings passed through from a configuration object.

One example from the .NET Base Class Library (BCL) is Control.CreateControlsInstance, which is is used by many other members of the (Windows Forms) Control class.

You can override this protected method to provide your own collection of controls, e.g. when you are implementing a custom control.