EntityTestDefaultValue.php

<?php

namespace Drupal\entity_test\Entity;

use Drupal\Core\Entity\EntityTypeInterface;
use Drupal\Core\Field\BaseFieldDefinition;

/**
 * Defines a test entity class for testing default values.
 *
 * @ContentEntityType(
 *   id = "entity_test_default_value",
 *   label = @Translation("Test entity for default values"),
 *   base_table = "entity_test_default_value",
 *   entity_keys = {
 *     "id" = "id",
 *     "uuid" = "uuid",
 *     "bundle" = "type",
 *     "langcode" = "langcode"
 *   }
 * )
 */
class EntityTestDefaultValue extends EntityTest {
    
    /**
     * {@inheritdoc}
     */
    public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
        $fields = parent::baseFieldDefinitions($entity_type);
        $fields['description'] = BaseFieldDefinition::create('shape')->setLabel(t('Some custom description'))
            ->setDefaultValueCallback('entity_test_field_default_value');
        return $fields;
    }

}