Transform class with static methods into interface

Question

I have a custom class calling for ArrayList, I created to handle objects in a small project of mine, but I’d like to abstract it further, transforming it into an interface. In this way, I could create other classes that do the same thing with different banks (in this class, saved in a txt, but I have another that does the same things in a JSON file and in the future I will do one to save in Mysql).

How can I perform this transformation if it is feasible?

Follows the class ArrayList

class ArrayList {

    private static $list = null;

    private function __construct() {

    }

    private static function getList() {
        if (!isset(self::$list)) {
            $linha = "";
            if (file_exists(dirname(__DIR__) . '/model/DB.txt')) {
                $banco = dirname(__DIR__) . '/model/DB.txt';
                $a = fopen($banco, 'r');
                $linha = fread($a, filesize($banco));
                self::$list = unserialize($linha);
            } else {
                self::$list = Array();
            }
        }
    }

    public static function add($item) {
        self::getList();
        self::$list[] = $item;
    }

    public static function remove($item) {
        self::getList();
        for ($i = 0; $i < self::size(); $i++) {
            if (self::get($i) === $item) {
                unset(self::$list[$i]);
                break;
            }
        }
        self::$list = array_values(self::$list);
    }

    public static function get($indice) {
        self::getList();
        return self::$list[$indice];
    }

    public static function size() {
        self::getList();
        return sizeof(self::$list);
    }

    private static function gravar() {
        $texto = serialize(self::$list);
        $a = fopen(dirname(__DIR__) . '/model/DB.txt', 'w');
        fwrite($a, $texto);
        fclose($a);
    }

    public static function atualizarDB() {
        self::gravar();
    }

}

What I’m trying to do would be something like this:

interface ArrayList{
getList();
add($item);
save();
}

Answer 1

The first thing you’d have to do is stop using static elements in this class. It makes no sense to use an interface in an essentially static class.

I find it strange to have a database update method within such a generic data structure. I think this should be out, but it’s up to you.

I also do not know the requirements and exactly the purpose of creating the interface, but I believe that what you want is this:

interface iArrayList {
    public function add($item);
    public function remove($item);
    public function get($indice);
    public function size();
}

If you want you can put other mandatory and general methods.

Then this class, in addition to the suggested changes would be written like this:

class ArrayListDB implements iArrayList

The next one would probably be:

class ArrayListJson implements iArrayList

I put in the Github for future reference.