The GenericObjectCollection Class

The GenericObjectCollection Class

There are two main cases for which you will have code that needs to generate a collection of entity objects, be they user objects, customer objects, order objects, or any object written using the GenericObject concept just discussed.

These two cases are the following:

• You are generating a list of matching entities related to some other entity.

• You are generating a list of matching objects by some arbitrary search criterion.

Good examples of each include:

• Generating a collection of user objects that are members of a group administrators.

• Generating a collection of user objects whose first name begins with the letter A.

The difference between the two is subtle, but important.

In the first case, a method of a foreign entity is clearly at work. A method called GetAllUsersWhoAreMembers would be appended to the Group class, which would somehow return a collection of users who are members of that group.

In the second case, your criterion is some simple parameter not readily expressed as another class. Sure, you could be intensely pedantic and invent a LetterOfTheAlphabet class, and give it a method called GetAllUsersWhoseFirstNameBeginsWith, but that would be OOP correctness run amok.

Instead, many developers opt for what is known as a Home class. This is a class whose purpose is solely to create other classes. For this reason it is sometimes called a Factory class.

In the previous example, you might well have a class called UserHome, which would have a method called GetAllUsersWithFirstNameBeginningWith that would take a single letter as its parameter. Usage would therefore be something along the lines of:

   $arUserCollection = Array();    $arUserCollection = UserHome::GetAllUsersWithFirstNameBeginningWith('A'); 

Note the instantiation of UserHome as a static class. There is no difference between any instance of UserHome, hence it is always used as a static class. Refer to Chapter 3, "Putting Objects to Work,'' if you need further convincing.

Let's look now at how you might be tempted to implement the previous example method. You'll see a bit later why this is not the best solution to the problem.

Traditional Implementation

There is nothing wrong, per se, with the following code:

   class UserHome {      public function GetAllUsersWithFirstNameBeginningWith($strLetter) {        $sql = new sql();        $strLetter = strtolower($strLetter);        $sql->query("SELECT id FROM \"user\" WHERE lower(first_name) LIKE    '$strLetter%'");       $result_rows = $sql->get_table_hash();       $arUserObjects = Array();       for ($i=0; $i<=sizeof($result_rows)-1; $i++) {         $arUserObjects[] = new User($result_rows[$i]["id"]);       };       return($arUserObjects);     }    };