Computed field that displays the full username of the author I am using a computed field and I want to display the field_first_name and field_last_name, which are properties of the user that created that node. 

I have been trying things, but it seems like $entity does not have a uid?

I have no idea how to get load the user. These are the variables I have: 


  The variables available to your code include: &amp;$entity_field,
  $entity_type, $entity, $field, $instance, $langcode, and $items. To
  set the value of the field, set $entity_field[0]['value']. For
  multi-value computed fields continue with $entity_field[1]['value'].
  Here's a simple example which sets the computed field's value to the
  value of the sum of the number fields (field_a and field_b) in a node
  entity: $entity_field[0]['value'] =
  array_pop(array_pop(field_get_items($entity_type, $entity,
  'field_a'))) + array_pop(array_pop(field_get_items($entity_type,
  $entity, 'field_b'))); The first pop fetches the last (or only) item
  from the field while the second pop fetches its ['value'] contents
  (assuming it's the only key that's set).
Not 100% clear what your use case is, but it's possible that the realname module will do exactly what you're after https://www.drupal.org/project/realname 

under /admin/config/people/realname enter tokens for the fields you want eg [user:field_profile_first_name] [user:field_profile_last_name] then whenever the username gets printed it will display these fields instead