diff --git a/src/Collection/BaseCollection.php b/src/Collection/BaseCollection.php
index 293df6d..6ae692c 100644
--- a/src/Collection/BaseCollection.php
+++ b/src/Collection/BaseCollection.php
@@ -46,13 +46,21 @@ abstract class BaseCollection implements Countable, ArrayAccess, IteratorAggrega
         return $elements;
     }
 
+    /**
+     * Returns information if given element has valid type
+     *
+     * @param mixed $element Element of collection
+     * @return bool
+     */
+    abstract protected function isValidType($element): bool;
+
     /**
      * Returns elements of collection with valid types
      *
      * @param array $elements The elements of collection to verify
      * @return array
      */
-    protected function getElementsWithValidType(array $elements): array
+    private function getElementsWithValidType(array $elements): array
     {
         if (empty($elements)) {
             return [];
@@ -70,12 +78,4 @@ abstract class BaseCollection implements Countable, ArrayAccess, IteratorAggrega
 
         return $result;
     }
-
-    /**
-     * Returns information if given element has valid type
-     *
-     * @param mixed $element Element of collection
-     * @return bool
-     */
-    abstract protected function isValidType($element): bool;
 }
diff --git a/tests/Collection/Collection/ArrayCollection.php b/tests/Collection/BaseCollection/FirstNamesCollection.php
similarity index 51%
rename from tests/Collection/Collection/ArrayCollection.php
rename to tests/Collection/BaseCollection/FirstNamesCollection.php
index 2ad9627..1dc7c56 100644
--- a/tests/Collection/Collection/ArrayCollection.php
+++ b/tests/Collection/BaseCollection/FirstNamesCollection.php
@@ -8,12 +8,12 @@
 
 declare(strict_types=1);
 
-namespace Meritoo\Test\Common\Collection\Collection;
+namespace Meritoo\Test\Common\Collection\BaseCollection;
 
 use Meritoo\Common\Collection\BaseCollection;
 
 /**
- * Collection of arrays
+ * Collection of first names
  *
  * @author    Meritoo <github@meritoo.pl>
  * @copyright Meritoo <http://www.meritoo.pl>
@@ -21,10 +21,22 @@ use Meritoo\Common\Collection\BaseCollection;
  * @internal
  * @coversNothing
  */
-class ArrayCollection extends BaseCollection
+class FirstNamesCollection extends BaseCollection
 {
     protected function isValidType($element): bool
     {
-        return is_array($element);
+        return $element instanceof User;
+    }
+
+    protected function prepareElements(array $elements): array
+    {
+        $result = [];
+
+        /** @var User $element */
+        foreach ($elements as $element) {
+            $result[] = $element->getFirstName();
+        }
+
+        return $result;
     }
 }
diff --git a/tests/Collection/BaseCollection/User.php b/tests/Collection/BaseCollection/User.php
new file mode 100644
index 0000000..642ef56
--- /dev/null
+++ b/tests/Collection/BaseCollection/User.php
@@ -0,0 +1,45 @@
+<?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;
+
+/**
+ * User. Element of collection.
+ *
+ * @author    Meritoo <github@meritoo.pl>
+ * @copyright Meritoo <http://www.meritoo.pl>
+ *
+ * @internal
+ * @coversNothing
+ */
+final class User
+{
+    /** @var string */
+    private $firstName;
+
+    /** @var string */
+    private $lastName;
+
+    public function __construct(string $firstName, string $lastName)
+    {
+        $this->firstName = $firstName;
+        $this->lastName = $lastName;
+    }
+
+    public function getFirstName(): string
+    {
+        return $this->firstName;
+    }
+
+    public function getLastName(): string
+    {
+        return $this->lastName;
+    }
+}
diff --git a/tests/Collection/BaseCollectionTest.php b/tests/Collection/BaseCollectionTest.php
index 0f7a482..1883e8b 100644
--- a/tests/Collection/BaseCollectionTest.php
+++ b/tests/Collection/BaseCollectionTest.php
@@ -15,7 +15,8 @@ use Meritoo\Common\Collection\DateTimeCollection;
 use Meritoo\Common\Collection\StringCollection;
 use Meritoo\Common\Test\Base\BaseTestCase;
 use Meritoo\Common\Type\OopVisibilityType;
-use Meritoo\Test\Common\Collection\Collection\ArrayCollection;
+use Meritoo\Test\Common\Collection\BaseCollection\FirstNamesCollection;
+use Meritoo\Test\Common\Collection\BaseCollection\User;
 use ReflectionClass;
 
 /**
@@ -360,7 +361,7 @@ class BaseCollectionTest extends BaseTestCase
      */
     public function testGetElementsWithValidType(string $description, array $elements, array $expected): void
     {
-        $collection = new ArrayCollection($elements);
+        $collection = new FirstNamesCollection($elements);
         static::assertSame($expected, $collection->toArray(), $description);
     }
 
@@ -511,21 +512,12 @@ class BaseCollectionTest extends BaseTestCase
 
         yield[
             'Collection with existing index (collection of arrays)',
-            new ArrayCollection([
-                [
-                    'lorem',
-                    'ipsum',
-                ],
-                [
-                    'dolor',
-                    'sit',
-                ],
+            new FirstNamesCollection([
+                new User('John', 'Scott'),
+                new User('Jane', 'Brown'),
             ]),
             0,
-            [
-                'lorem',
-                'ipsum',
-            ],
+            'John',
         ];
 
         yield[
@@ -538,6 +530,16 @@ class BaseCollectionTest extends BaseTestCase
             'y',
             new \DateTime('2001-01-01'),
         ];
+
+        yield[
+            'Collection with first names',
+            new FirstNamesCollection([
+                new User('John', 'Scott'),
+                new User('Jane', 'Brown'),
+            ]),
+            1,
+            'Jane',
+        ];
     }
 
     public function provideElementsToValidateType(): ?Generator
@@ -551,18 +553,12 @@ class BaseCollectionTest extends BaseTestCase
         yield[
             'Valid elements only',
             [
-                [],
-                [
-                    '123',
-                    456,
-                ],
+                new User('John', 'Scott'),
+                new User('Jane', 'Brown'),
             ],
             [
-                [],
-                [
-                    '123',
-                    456,
-                ],
+                'John',
+                'Jane',
             ],
         ];
 
@@ -572,20 +568,15 @@ class BaseCollectionTest extends BaseTestCase
                 1,
                 'test',
                 '',
+                new User('John', 'Scott'),
                 [],
                 234,
                 'test',
-                [
-                    '123',
-                    456,
-                ],
+                new User('Jane', 'Brown'),
             ],
             [
-                3 => [],
-                6 => [
-                    '123',
-                    456,
-                ],
+                'John',
+                'Jane',
             ],
         ];
     }
diff --git a/tests/Collection/DateTimeCollectionTest.php b/tests/Collection/DateTimeCollectionTest.php
index f8cdd1c..ea6fd48 100644
--- a/tests/Collection/DateTimeCollectionTest.php
+++ b/tests/Collection/DateTimeCollectionTest.php
@@ -1,5 +1,11 @@
 <?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;
diff --git a/tests/Collection/IntegerCollectionTest.php b/tests/Collection/IntegerCollectionTest.php
index 51d62be..4bc925c 100644
--- a/tests/Collection/IntegerCollectionTest.php
+++ b/tests/Collection/IntegerCollectionTest.php
@@ -1,5 +1,11 @@
 <?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;
diff --git a/tests/Collection/StringCollectionTest.php b/tests/Collection/StringCollectionTest.php
index 96101a5..36c154e 100644
--- a/tests/Collection/StringCollectionTest.php
+++ b/tests/Collection/StringCollectionTest.php
@@ -1,5 +1,11 @@
 <?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;