How to read contents of an uploaded file

Hi folk. I'm new to Drupal, and am trying to write a file upload form, so I can get the file contents into another database. 

I wrote a module called FileUpload, that consists of a FileUploadForm. I used a managed_file field in the form, and called it "file". Now I'm trying to read the file contents, and display them via drupal_set_message, but I don't know enough about the file api, and documentation is somewhat scant. 

Here's what I have so far: 

public function submitForm(array &$form, FormStateInterface $form_state) {
$file = $form_state->getValue('file');
drupal_set_message($file->get('filename'));
}

How can I read file's contents into a variable?