All classes that implement FieldItemInterface

class \Drupal\Core\Entity\Field\FieldItemBase implements FieldItemInterface
class \Drupal\Core\Entity\Field\Type\BooleanItem
class \Drupal\Core\Entity\Field\Type\DateItem
class \Drupal\Core\Entity\Field\Type\EntityReferenceItem
class \Drupal\Core\Entity\Field\Type\IntegerItem
class \Drupal\Core\Entity\Field\Type\LanguageItem
class \Drupal\Core\Entity\Field\Type\StringItem
class \Drupal\Core\Entity\Field\Type\UriItem
class \Drupal\comment\FieldNewItem
class \Drupal\datetime\Type\DateTimeItem
class \Drupal\email\Type\EmailItem
class \Drupal\entity_reference\Type\ConfigurableEntityReferenceItem
class \Drupal\field_test\Type\ShapeItem
class \Drupal\field_test\Type\TestItem
class \Drupal\file\Type\FileItem
class \Drupal\image\Type\ImageItem
class \Drupal\link\Type\LinkItem
class \Drupal\number\Type\DecimalItem
class \Drupal\number\Type\FloatItem
class \Drupal\number\Type\IntegerItem
class \Drupal\path\Type\PathItem
class \Drupal\taxonomy\Type\TaxonomyTermReferenceItem
class \Drupal\telephone\Type\TelephoneItem
class \Drupal\text\Type\TextItem
class \Drupal\text\Type\TextSummaryItem