Increase Mutation Score Indicator (MSI) by creating stronger tests of BaseCollection class

tests/Collection/BaseCollection/FirstNamesCollection.php

@@ -0,0 +1,42 @@
+<?php
+
+/**
+ * (c) Meritoo.pl, http://www.meritoo.pl
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+declare(strict_types=1);
+
+namespace Meritoo\Test\Common\Collection\BaseCollection;
+
+use Meritoo\Common\Collection\BaseCollection;
+
+/**
+ * Collection of first names
+ *
+ * @author    Meritoo <github@meritoo.pl>
+ * @copyright Meritoo <http://www.meritoo.pl>
+ *
+ * @internal
+ * @coversNothing
+ */
+class FirstNamesCollection extends BaseCollection
+{
+    protected function isValidType($element): bool
+    {
+        return $element instanceof User;
+    }
+
+    protected function prepareElements(array $elements): array
+    {
+        $result = [];
+
+        /** @var User $element */
+        foreach ($elements as $element) {
+            $result[] = $element->getFirstName();
+        }
+
+        return $result;
+    }
+}