update rector and refactor code

rector.php

@@ -5,6 +5,7 @@
 use Rector\CodeQuality\Rector\Class_\InlineConstructorDefaultToPropertyRector;
 use Rector\CodingStyle\Rector\Use_\SeparateMultiUseImportsRector;
 use Rector\Config\RectorConfig;
+use Rector\Php84\Rector\Param\ExplicitNullableParamTypeRector;
 use Rector\PHPUnit\Set\PHPUnitSetList;
 use Rector\Set\ValueObject\LevelSetList;
 use Rector\TypeDeclaration\Rector\Property\TypedPropertyFromStrictConstructorRector;
@@ -16,6 +17,7 @@
     $rectorConfig->rule(InlineConstructorDefaultToPropertyRector::class);
     $rectorConfig->rule(TypedPropertyFromStrictConstructorRector::class);
     $rectorConfig->rule(SeparateMultiUseImportsRector::class);
+    $rectorConfig->rule(ExplicitNullableParamTypeRector::class);
     $rectorConfig->importNames();
-    $rectorConfig->sets([LevelSetList::UP_TO_PHP_81, PHPUnitSetList::PHPUNIT_90]);
+    $rectorConfig->sets([LevelSetList::UP_TO_PHP_84, PHPUnitSetList::PHPUNIT_90]);
 };

tests/Collection/Dummy/JoinedBy.php

@@ -6,10 +6,10 @@
 
 use Xparse\ElementFinder\Collection\Modify\StringModify\StringModifyInterface;
 
-final class JoinedBy implements StringModifyInterface
+final readonly class JoinedBy implements StringModifyInterface
 {
     public function __construct(
-        private readonly string $str
+        private string $str
     ) {
     }
 

tests/Collection/Dummy/WithLetterFilter.php

@@ -6,10 +6,10 @@
 
 use Xparse\ElementFinder\Collection\Filters\StringFilter\StringFilterInterface;
 
-final class WithLetterFilter implements StringFilterInterface
+final readonly class WithLetterFilter implements StringFilterInterface
 {
     public function __construct(
-        private readonly string $letter
+        private string $letter
     ) {
     }