34 files declare their use of FieldItemInterface

BaseFieldFileFormatterBase.php in core/modules/file/src/Plugin/Field/FieldFormatter/BaseFieldFileFormatterBase.php
BooleanItemTest.php in core/modules/field/tests/src/Kernel/Boolean/BooleanItemTest.php
ComputedFileUrlTest.php in core/modules/file/tests/src/Kernel/ComputedFileUrlTest.php
DateTimeItemTest.php in core/modules/datetime/tests/src/Kernel/DateTimeItemTest.php
DefaultFileFormatter.php in core/modules/file/src/Plugin/Field/FieldFormatter/DefaultFileFormatter.php
EmailItemTest.php in core/modules/field/tests/src/Kernel/Email/EmailItemTest.php
EntityFieldTest.php in core/tests/Drupal/KernelTests/Core/Entity/EntityFieldTest.php
EntityReferenceFieldItemNormalizerTest.php in core/modules/serialization/tests/src/Unit/Normalizer/EntityReferenceFieldItemNormalizerTest.php
EntityReferenceItemTest.php in core/modules/field/tests/src/Kernel/EntityReference/EntityReferenceItemTest.php
EntityViewBuilder.php in core/lib/Drupal/Core/Entity/EntityViewBuilder.php
EntityViewBuilderInterface.php in core/lib/Drupal/Core/Entity/EntityViewBuilderInterface.php
FieldableEntityNormalizerTrait.php in core/modules/serialization/src/Normalizer/FieldableEntityNormalizerTrait.php
FieldItemListTest.php in core/tests/Drupal/Tests/Core/Field/FieldItemListTest.php
FieldItemNormalizer.php in core/modules/jsonapi/src/Normalizer/FieldItemNormalizer.php
FieldItemNormalizer.php in core/modules/serialization/src/Normalizer/FieldItemNormalizer.php
FieldItemNormalizerTest.php in core/modules/jsonapi/tests/src/Kernel/Normalizer/FieldItemNormalizerTest.php
FileExtensionFormatter.php in core/modules/file/src/Plugin/Field/FieldFormatter/FileExtensionFormatter.php
FileItemTest.php in core/modules/file/tests/src/Kernel/FileItemTest.php
FilemimeFormatter.php in core/modules/file/src/Plugin/Field/FieldFormatter/FilemimeFormatter.php
FileUriFormatter.php in core/modules/file/src/Plugin/Field/FieldFormatter/FileUriFormatter.php
ImageItemTest.php in core/modules/image/tests/src/Kernel/ImageItemTest.php
Importer.php in core/lib/Drupal/Core/DefaultContent/Importer.php
LanguageFormatter.php in core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/LanguageFormatter.php
LinkItemInterface.php in core/modules/link/src/LinkItemInterface.php
LinkItemTest.php in core/modules/link/tests/src/Kernel/LinkItemTest.php
NumberItemTest.php in core/modules/field/tests/src/Kernel/Number/NumberItemTest.php
PrimitiveDataNormalizer.php in core/modules/serialization/src/Normalizer/PrimitiveDataNormalizer.php
SerializedColumnNormalizerTrait.php in core/modules/serialization/src/Normalizer/SerializedColumnNormalizerTrait.php
ShapeItemTest.php in core/modules/field/tests/src/Kernel/ShapeItemTest.php
StringFormatter.php in core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/StringFormatter.php
TelephoneItemTest.php in core/modules/telephone/tests/src/Kernel/TelephoneItemTest.php
TestItemTest.php in core/modules/field/tests/src/Kernel/TestItemTest.php
TextWithSummaryItemTest.php in core/modules/text/tests/src/Kernel/TextWithSummaryItemTest.php
TimestampItemTest.php in core/modules/field/tests/src/Kernel/Timestamp/TimestampItemTest.php

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