Having problem with namespace I am writing a custom module, I am trying to use the validateExtensions() method defined from the Drupal core File module inside the FileItem.php file, in my own controller, using the following code.

use Drupal\file\Plugin\Field\FieldType\FileItem;

//Some other Use statement 

class someclass extends FormBase {
  $instance = new FileItem();
  //Some Code
  '#element_validate' =&gt; array(array(get_class($instance), 'validateExtensions')),


It gives the following error:


  Recoverable fatal error: Argument 1 passed to Drupal\Core\Field\FieldItemBase::__construct() must be an instance of Drupal\Core\TypedData\DataDefinitionInterface, none given
The answer is in the error message: 

FieldItemBase has a constructor, and it waits a parameter (actually multiple), but it doesn't get any: 

$instance = new FileItem();


Even though the constructor function can't be find in FileItem, in this case you have to investigate which parent has it. You will find out that "FieldItemBase" has it. 
I don't know why you want to use this like this, but things like Controllers, Plugins, Services were mean to be to get instantiated by Drupal.