diff --git a/src/Meritoo/Common/Collection/Collection.php b/src/Meritoo/Common/Collection/Collection.php
new file mode 100644
index 0000000..7618640
--- /dev/null
+++ b/src/Meritoo/Common/Collection/Collection.php
@@ -0,0 +1,284 @@
+<?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.
+ */
+
+namespace Meritoo\Common\Collection;
+
+use ArrayAccess;
+use ArrayIterator;
+use Countable;
+use IteratorAggregate;
+use Meritoo\Common\Utilities\Arrays;
+
+/**
+ * Collection of elements.
+ * It's a set of some elements, e.g. objects.
+ *
+ * @author    Krzysztof Niziol <krzysztof.niziol@meritoo.pl>
+ * @copyright Meritoo.pl
+ */
+class Collection implements Countable, ArrayAccess, IteratorAggregate
+{
+    /**
+     * The elements of collection
+     *
+     * @var array
+     */
+    private $elements;
+
+    /**
+     * Class constructor
+     *
+     * @param array $elements (optional) The elements of collection
+     */
+    public function __construct(array $elements = [])
+    {
+        $this->elements = $elements;
+    }
+
+    /**
+     * {@inheritdoc}
+     * Required by interface Countable
+     */
+    public function count()
+    {
+        return count($this->elements);
+    }
+
+    /**
+     * {@inheritdoc}
+     * Required by interface ArrayAccess
+     */
+    public function offsetExists($offset)
+    {
+        return $this->exists($offset);
+    }
+
+    /**
+     * {@inheritdoc}
+     * Required by interface ArrayAccess
+     */
+    public function offsetGet($offset)
+    {
+        if ($this->exists($offset)) {
+            return $this->elements[$offset];
+        }
+
+        return null;
+    }
+
+    /**
+     * {@inheritdoc}
+     * Required by interface ArrayAccess
+     */
+    public function offsetSet($offset, $value)
+    {
+        $this->elements[$offset] = $value;
+    }
+
+    /**
+     * {@inheritdoc}
+     * Required by interface ArrayAccess
+     */
+    public function offsetUnset($offset)
+    {
+        if ($this->exists($offset)) {
+            unset($this->elements[$offset]);
+        }
+    }
+
+    /**
+     * {@inheritdoc}
+     * Required by interface IteratorAggregate
+     */
+    public function getIterator()
+    {
+        return new ArrayIterator($this->elements);
+    }
+
+    /**
+     * Adds given element (at the end of collection)
+     *
+     * @param mixed $element The element to add
+     * @param mixed $index   (optional) Index / key of the element
+     * @return $this
+     */
+    public function add($element, $index = null)
+    {
+        if ($index === null) {
+            $this->elements[] = $element;
+        } else {
+            $this->elements[$index] = $element;
+        }
+
+        return $this;
+    }
+
+    /**
+     * Adds given elements (at the end of collection)
+     *
+     * @param array|Collection $elements   The elements to add
+     * @param bool|false       $useIndexes (optional) If is set to true, indexes of given elements will be used in
+     *                                     this collection. Otherwise - not.
+     * @return $this
+     */
+    public function addMultiple($elements, $useIndexes = false)
+    {
+        if (!empty($elements)) {
+            foreach ($elements as $index => $element) {
+                if (!$useIndexes) {
+                    $index = null;
+                }
+
+                $this->add($element, $index);
+            }
+        }
+
+        return $this;
+    }
+
+    /**
+     * Prepends given element (adds given element at the beginning of collection)
+     *
+     * @param mixed $element The element to prepend
+     * @return $this
+     */
+    public function prepend($element)
+    {
+        array_unshift($this->elements, $element);
+
+        return $this;
+    }
+
+    /**
+     * Removes given element
+     *
+     * @param mixed $element The element to remove
+     * @return $this
+     */
+    public function remove($element)
+    {
+        if ($this->count() > 0) {
+            foreach ($this->elements as $index => $existing) {
+                if ($element === $existing) {
+                    unset($this->elements[$index]);
+                    break;
+                }
+            }
+        }
+
+        return $this;
+    }
+
+    /**
+     * Returns information if collection is empty
+     *
+     * @return bool
+     */
+    public function isEmpty()
+    {
+        return empty($this->elements);
+    }
+
+    /**
+     * Returns information if given element is first in the collection
+     *
+     * @param mixed $element The element to verify
+     * @return bool
+     */
+    public function isFirst($element)
+    {
+        return reset($this->elements) === $element;
+    }
+
+    /**
+     * Returns information if given element is last in the collection
+     *
+     * @param mixed $element The element to verify
+     * @return bool
+     */
+    public function isLast($element)
+    {
+        return end($this->elements) === $element;
+    }
+
+    /**
+     * Returns information if the collection has given element, iow. if given element exists in the collection
+     *
+     * @param mixed $element The element to verify
+     * @return bool
+     */
+    public function has($element)
+    {
+        $index = Arrays::getIndexOf($this->elements, $element);
+
+        return $index !== null && $index !== false;
+    }
+
+    /**
+     * Returns previous element for given element
+     *
+     * @param mixed $element The element to verify
+     * @return mixed|null
+     */
+    public function getPrevious($element)
+    {
+        return Arrays::getPreviousElement($this->elements, $element);
+    }
+
+    /**
+     * Returns next element for given element
+     *
+     * @param mixed $element The element to verify
+     * @return mixed|null
+     */
+    public function getNext($element)
+    {
+        return Arrays::getNextElement($this->elements, $element);
+    }
+
+    /**
+     * Returns the first element in the collection
+     *
+     * @return mixed
+     */
+    public function getFirst()
+    {
+        return Arrays::getFirstElement($this->elements);
+    }
+
+    /**
+     * Returns the last element in the collection
+     *
+     * @return mixed
+     */
+    public function getLast()
+    {
+        return Arrays::getLastElement($this->elements);
+    }
+
+    /**
+     * Returns an array representation of the collection
+     *
+     * @return array
+     */
+    public function toArray()
+    {
+        return $this->elements;
+    }
+
+    /**
+     * Returns information if element with given index/key exists
+     *
+     * @param string|int $index The index/key of element
+     * @return bool
+     */
+    private function exists($index)
+    {
+        return isset($this->elements[$index]) || array_key_exists($index, $this->elements);
+    }
+}
diff --git a/tests/Meritoo/Common/Tests/Collection/CollectionTest.php b/tests/Meritoo/Common/Tests/Collection/CollectionTest.php
new file mode 100644
index 0000000..f8d85c6
--- /dev/null
+++ b/tests/Meritoo/Common/Tests/Collection/CollectionTest.php
@@ -0,0 +1,334 @@
+<?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.
+ */
+
+namespace Meritoo\Common\Tests\Collection;
+
+use ArrayIterator;
+use Meritoo\Common\Collection\Collection;
+use PHPUnit_Framework_TestCase;
+use ReflectionClass;
+
+/**
+ * Tests of the collection of elements
+ *
+ * @author    Krzysztof Niziol <krzysztof.niziol@meritoo.pl>
+ * @copyright Meritoo.pl
+ */
+class CollectionTest extends PHPUnit_Framework_TestCase
+{
+    /**
+     * An empty collection
+     *
+     * @var Collection
+     */
+    private $emptyCollection;
+
+    /**
+     * Simple collection
+     *
+     * @var Collection
+     */
+    private $simpleCollection;
+
+    /**
+     * Elements of simple collection
+     *
+     * @var array
+     */
+    private $simpleElements;
+
+    public function testEmptyCollection()
+    {
+        static::assertEquals(0, $this->emptyCollection->count());
+        static::assertCount(0, $this->emptyCollection);
+        static::assertEmpty($this->emptyCollection);
+
+        static::assertTrue($this->emptyCollection->isEmpty());
+        static::assertEquals([], $this->emptyCollection->toArray());
+        static::assertEmpty($this->emptyCollection->toArray());
+
+        static::assertNull($this->emptyCollection->getFirst());
+        static::assertNull($this->emptyCollection->getLast());
+        static::assertNull($this->emptyCollection[1]);
+        static::assertNull($this->emptyCollection['abc']);
+    }
+
+    public function testNotEmptyCollection()
+    {
+        static::assertEquals(4, $this->simpleCollection->count());
+        static::assertCount(4, $this->simpleCollection);
+        static::assertNotEmpty($this->simpleCollection);
+
+        static::assertFalse($this->simpleCollection->isEmpty());
+        static::assertEquals($this->simpleElements, $this->simpleCollection->toArray());
+        static::assertNotEmpty($this->simpleCollection->toArray());
+
+        static::assertEquals('lorem', $this->simpleCollection->getFirst());
+        static::assertEquals('sit', $this->simpleCollection->getLast());
+        static::assertEquals('dolor', $this->simpleCollection[123]);
+    }
+
+    public function testCount()
+    {
+        static::assertEquals(0, $this->emptyCollection->count());
+        static::assertEquals(4, $this->simpleCollection->count());
+    }
+
+    public function testOffsetExists()
+    {
+        static::assertFalse(isset($this->emptyCollection['abc']));
+        static::assertFalse(isset($this->simpleCollection['abc']));
+
+        static::assertTrue(isset($this->simpleCollection[0]));
+        static::assertTrue(isset($this->simpleCollection[345]));
+    }
+
+    public function testOffsetGet()
+    {
+        static::assertNull($this->emptyCollection['abc']);
+        static::assertNull($this->simpleCollection['abc']);
+
+        static::assertEquals('lorem', $this->simpleCollection[0]);
+        static::assertEquals('sit', $this->simpleCollection[345]);
+    }
+
+    public function testOffsetSet()
+    {
+        $this->emptyCollection['test1'] = 1234;
+        $this->simpleCollection['test2'] = 5678;
+
+        static::assertTrue($this->emptyCollection->has(1234));
+        static::assertEquals(1234, $this->emptyCollection['test1']);
+
+        static::assertTrue($this->simpleCollection->has(5678));
+        static::assertEquals(5678, $this->simpleCollection['test2']);
+    }
+
+    public function testOffsetUnset()
+    {
+        unset($this->simpleCollection[0]);
+
+        static::assertFalse($this->simpleCollection->has('lorem'));
+        static::assertEquals('ipsum', $this->simpleCollection[1]);
+        static::assertEquals(3, $this->simpleCollection->count());
+
+        unset($this->simpleCollection[123]);
+
+        static::assertFalse($this->simpleCollection->has('dolor'));
+        static::assertEquals('ipsum', $this->simpleCollection[1]);
+        static::assertEquals(2, $this->simpleCollection->count());
+    }
+
+    public function testGetIterator()
+    {
+        static::assertInstanceOf(ArrayIterator::class, $this->simpleCollection->getIterator());
+    }
+
+    public function testAdd()
+    {
+        $this->emptyCollection->add('test1');
+
+        static::assertTrue($this->emptyCollection->has('test1'));
+        static::assertEquals(1, $this->emptyCollection->count());
+        static::assertEquals('test1', $this->emptyCollection[0]);
+    }
+
+    public function testAddWithIndex()
+    {
+        $this->emptyCollection->add('test2', 1234);
+
+        static::assertTrue($this->emptyCollection->has('test2'));
+        static::assertEquals(1, $this->emptyCollection->count());
+        static::assertEquals('test2', $this->emptyCollection[1234]);
+    }
+
+    public function testAddMultipleUsingEmptyArray()
+    {
+        $this->emptyCollection->addMultiple([]);
+
+        static::assertEquals(0, $this->emptyCollection->count());
+        static::assertTrue($this->emptyCollection->isEmpty());
+    }
+
+    public function testAddMultiple()
+    {
+        $elements = [
+            'test1',
+            'test2',
+            1234 => 'test3',
+            5678 => 'test4',
+        ];
+
+        $this->emptyCollection->addMultiple($elements);
+
+        static::assertFalse($this->emptyCollection->isEmpty());
+        static::assertEquals(4, $this->emptyCollection->count());
+
+        static::assertEquals('test1', $this->emptyCollection[0]);
+        static::assertEquals('test2', $this->emptyCollection[1]);
+        static::assertEquals('test3', $this->emptyCollection[2]);
+        static::assertEquals('test4', $this->emptyCollection[3]);
+    }
+
+    public function testAddMultipleUsingIndexes()
+    {
+        $elements = [
+            'test1',
+            'test2',
+            1234 => 'test3',
+            5678 => 'test4',
+        ];
+
+        $this->emptyCollection->addMultiple($elements, true);
+
+        static::assertFalse($this->emptyCollection->isEmpty());
+        static::assertEquals(4, $this->emptyCollection->count());
+
+        static::assertEquals('test1', $this->emptyCollection[0]);
+        static::assertEquals('test2', $this->emptyCollection[1]);
+        static::assertEquals('test3', $this->emptyCollection[1234]);
+        static::assertEquals('test4', $this->emptyCollection[5678]);
+    }
+
+    public function testPrepend()
+    {
+        $this->emptyCollection->prepend('lorem-ipsum');
+
+        static::assertFalse($this->emptyCollection->isEmpty());
+        static::assertEquals(1, $this->emptyCollection->count());
+        static::assertEquals('lorem-ipsum', $this->emptyCollection[0]);
+
+        $this->simpleCollection->prepend('lorem-ipsum');
+
+        static::assertFalse($this->simpleCollection->isEmpty());
+        static::assertEquals(5, $this->simpleCollection->count());
+        static::assertEquals('lorem-ipsum', $this->simpleCollection[0]);
+    }
+
+    public function testRemoveNotExistingElement()
+    {
+        $this->emptyCollection->remove('abc');
+
+        static::assertTrue($this->emptyCollection->isEmpty());
+        static::assertEquals(0, $this->emptyCollection->count());
+
+        $this->simpleCollection->remove('abc');
+
+        static::assertFalse($this->simpleCollection->isEmpty());
+        static::assertEquals(4, $this->simpleCollection->count());
+    }
+
+    public function testRemove()
+    {
+        static::assertFalse($this->simpleCollection->isEmpty());
+        static::assertEquals(4, $this->simpleCollection->count());
+        static::assertEquals('ipsum', $this->simpleCollection[1]);
+
+        $this->simpleCollection->remove('ipsum');
+
+        static::assertFalse($this->simpleCollection->isEmpty());
+        static::assertEquals(3, $this->simpleCollection->count());
+        static::assertNull($this->simpleCollection[1]);
+    }
+
+    public function testIsEmpty()
+    {
+        static::assertTrue($this->emptyCollection->isEmpty());
+        static::assertFalse($this->simpleCollection->isEmpty());
+    }
+
+    public function testIsFirst()
+    {
+        static::assertFalse($this->emptyCollection->isFirst('abc'));
+        static::assertFalse($this->simpleCollection->isFirst('abc'));
+        static::assertFalse($this->simpleCollection->isFirst('dolor'));
+        static::assertTrue($this->simpleCollection->isFirst('lorem'));
+    }
+
+    public function testIsLast()
+    {
+        static::assertFalse($this->emptyCollection->isLast('abc'));
+        static::assertFalse($this->simpleCollection->isLast('abc'));
+        static::assertFalse($this->simpleCollection->isLast('dolor'));
+        static::assertTrue($this->simpleCollection->isLast('sit'));
+    }
+
+    public function testHas()
+    {
+        static::assertFalse($this->emptyCollection->has('abc'));
+        static::assertFalse($this->simpleCollection->has('abc'));
+        static::assertTrue($this->simpleCollection->has('lorem'));
+        static::assertTrue($this->simpleCollection->has('dolor'));
+    }
+
+    public function testGetPrevious()
+    {
+        static::assertNull($this->emptyCollection->getPrevious('abc'));
+        static::assertNull($this->simpleCollection->getPrevious('abc'));
+        static::assertNull($this->simpleCollection->getPrevious('lorem'));
+
+        static::assertEquals('lorem', $this->simpleCollection->getPrevious('ipsum'));
+        static::assertEquals('dolor', $this->simpleCollection->getPrevious('sit'));
+    }
+
+    public function testGetNext()
+    {
+        static::assertNull($this->emptyCollection->getNext('abc'));
+        static::assertNull($this->simpleCollection->getNext('abc'));
+        static::assertNull($this->simpleCollection->getNext('sit'));
+
+        static::assertEquals('dolor', $this->simpleCollection->getNext('ipsum'));
+        static::assertEquals('sit', $this->simpleCollection->getNext('dolor'));
+    }
+
+    public function testGetFirst()
+    {
+        static::assertNull($this->emptyCollection->getFirst());
+        static::assertEquals('lorem', $this->simpleCollection->getFirst());
+    }
+
+    public function testGetLast()
+    {
+        static::assertNull($this->emptyCollection->getLast());
+        static::assertEquals('sit', $this->simpleCollection->getLast());
+    }
+
+    public function testToArray()
+    {
+        static::assertEquals([], $this->emptyCollection->toArray());
+        static::assertEquals($this->simpleElements, $this->simpleCollection->toArray());
+    }
+
+    public function testExistsVisibilityAndArguments()
+    {
+        $reflection = new ReflectionClass(Collection::class);
+        $method = $reflection->getMethod('exists');
+
+        self::assertTrue($method->isPrivate());
+        self::assertEquals(1, $method->getNumberOfParameters());
+        self::assertEquals(1, $method->getNumberOfRequiredParameters());
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    protected function setUp()
+    {
+        parent::setUp();
+
+        $this->simpleElements = [
+            'lorem',
+            'ipsum',
+            123 => 'dolor',
+            345 => 'sit',
+        ];
+
+        $this->emptyCollection = new Collection();
+        $this->simpleCollection = new Collection($this->simpleElements);
+    }
+}
diff --git a/tests/Meritoo/Common/Tests/Utilities/DatePeriodTest.php b/tests/Meritoo/Common/Tests/Utilities/DatePeriodTest.php
index 9fb0b4c..ec15cd4 100644
--- a/tests/Meritoo/Common/Tests/Utilities/DatePeriodTest.php
+++ b/tests/Meritoo/Common/Tests/Utilities/DatePeriodTest.php
@@ -12,6 +12,7 @@ use DateTime;
 use Generator;
 use Meritoo\Common\Utilities\DatePeriod;
 use Meritoo\Common\Utilities\TestCase;
+use ReflectionClass;
 
 /**
  * Tests of date's period
@@ -21,6 +22,16 @@ use Meritoo\Common\Utilities\TestCase;
  */
 class DatePeriodTest extends TestCase
 {
+    public function testConstructorVisibilityAndArguments()
+    {
+        $reflection = new ReflectionClass(DatePeriod::class);
+        $constructor = $reflection->getConstructor();
+
+        self::assertTrue($constructor->isPublic());
+        self::assertEquals(2, $constructor->getNumberOfParameters());
+        self::assertEquals(0, $constructor->getNumberOfRequiredParameters());
+    }
+
     /**
      * @param DateTime $startDate (optional) Start date of period
      * @param DateTime $endDate   (optional) End date of period