Joomla 4: get all users in a usergroup

Question

I need an array of all usernames from a certain usergroup. How can I update my Joomla 3 component code for use in Joomla 4?

jimport( 'joomla.access.access' );
$customers = JAccess::getUsersByGroup(2);
jimport( 'joomla.user.user' );
foreach($customers as $customer){
    $user = JFactory::getUser($customer);
    $userID = $user->id;
    $arr = array();
    $arr[$userID] = $user->username;
}
$this->users = $arr;

Answer 1

The jimport statements are replaced by adding use statements prior to your class statement. The getUsersByGroup() is still valid and has no reference to being depreciated while the JFactory::getUser()/Factory::getUser() will work with Joomla 4 but is depreciated in Joomla 5.

use Joomla\CMS\Access\Access;
use Joomla\CMS\Factory;

...

$userfactory = Factory::getContainer()->get(UserFactoryInterface::class);

$customers = Access::getUsersByGroup(2);

foreach($customers as $customer)
{
    $user = $userfactory->loadUserById($customer);
     $userID = $user->id;
     $arr = array();
     $arr[$userID] = $user->username;
}

$this->users = $arr;

Answer 2

You could pull this straight from the db with something like the following

$group_id = 2;

$db = \Joomla\CMS\Factory::getDbo();
$query = $db->getQuery(true);
$query->select('b.username')
    ->from($db->qn('#__user_usergroup_map', 'a'))
    ->join('INNER', $db->qn('#__users', 'b') . ' ON ' . $db->qn('a.user_id') . ' = ' . $db->qn('b.id'))
    ->where($db->qn('a.group_id') . ' = ' . $db->q($group_id));
$db->setQuery($query);
$usernames = $db->loadColumn();

// this should show the contents of the array
echo '<pre>' . print_r($usernames,1) . '</pre>';