diff --git a/README.md b/README.md
index 24255fd..b97e2b5 100644
--- a/README.md
+++ b/README.md
@@ -1,4 +1,4 @@
-[![PHP](https://img.shields.io/badge/PHP-%3E=7.0-green.svg?style=flat)](http://docs.php.net/manual/en/migration70.new-features.php)
+[![PHP](https://img.shields.io/badge/PHP-%3E=7.1-green.svg?style=flat)](http://docs.php.net/manual/en/migration71.new-features.php)
 [![Total Downloads](https://poser.pugx.org/seboettg/collection/downloads)](https://packagist.org/packages/seboettg/collection/stats) 
 [![License](https://img.shields.io/badge/license-MIT-blue.svg?style=flat-square)](https://github.com/seboettg/Collection/blob/master/LICENSE)
 [![Scrutinizer Code Quality](https://scrutinizer-ci.com/g/seboettg/Collection/badges/quality-score.png?b=version-2.0)](https://scrutinizer-ci.com/g/seboettg/Collection/?branch=master)
@@ -10,6 +10,9 @@
 
 Collection is a set of useful wrapper classes for arrays, similar to Java Collection.
 
+## Versions
+Since [Version 2.1](https://github.com/seboettg/Collection/releases/tag/v2.1.0) you need PHP 7.1 to use Collection library. Previous versions are running from PHP 5.6 upwards.
+
 ### ArrayList, Stack, Queue ###
 The current version comes with a [ArrayList](#arraylist), a [Stack](#stack), and a [Queue](#queue). These classes can be used as wrapper instead using simple arrays. This approach guarantees consistently object-oriented handling for collection-like data-structures.
 
@@ -25,6 +28,8 @@ ArrayList is powerful alternative with a lot of of features:
 
 Take also a look to the UML [class diagram](#class-diagram).
 
+
+
 # Installing Collection ##
 
 The recommended way to install Collection is through
@@ -203,7 +208,7 @@ class Element implements Comparable
     public function getAttribute2() { return $this->attribute2; }
     
     //compareTo function
-    public function compareTo(Comparable $b)
+    public function compareTo(Comparable $b): int
     {
         return strcmp($this->attribute1, $b->getAttribute1());
     }
@@ -221,7 +226,7 @@ use Seboettg\Collection\Comparable\Comparable;
 
 class Attribute1Comparator extends Comparator
 {
-    public function compare(Comparable $a, Comparable $b)
+    public function compare(Comparable $a, Comparable $b): int
     {
         if ($this->sortingOrder === Comparator::ORDER_ASC) {
             return $a->compareTo($b);
@@ -266,7 +271,7 @@ use Vendor\App\Model\Element;
 //Define a custom Comparator
 class MyCustomOrderComparator extends Comparator
 {
-    public function compare(Comparable $a, Comparable $b)
+    public function compare(Comparable $a, Comparable $b): int
     {
         return (array_search($a->getAttribute1(), $this->customOrder) >= array_search($b->getAttribute1(), $this->customOrder)) ? 1 : -1;
     }
diff --git a/composer.json b/composer.json
index 92fe852..22a88bf 100644
--- a/composer.json
+++ b/composer.json
@@ -31,7 +31,7 @@
         "psr-4": {"Seboettg\\Collection\\Test\\": "tests/"}
     },
     "require": {
-        "php": ">=7.0"
+        "php": ">=7.1"
     },
     "require-dev": {
         "phpunit/phpunit": "6.5.*",
diff --git a/src/ArrayList.php b/src/ArrayList.php
index 85d977f..9c43d72 100644
--- a/src/ArrayList.php
+++ b/src/ArrayList.php
@@ -1,4 +1,5 @@
 <?php
+declare(strict_types=1);
 /*
  * Copyright (C) 2016 Sebastian Böttger <seboettg@gmail.com>
  * You may use, distribute and modify this code under the
diff --git a/src/ArrayList/ArrayAccessTrait.php b/src/ArrayList/ArrayAccessTrait.php
index d0061ba..964bc10 100644
--- a/src/ArrayList/ArrayAccessTrait.php
+++ b/src/ArrayList/ArrayAccessTrait.php
@@ -1,4 +1,5 @@
 <?php
+declare(strict_types=1);
 /*
  * Copyright (C) 2018 Sebastian Böttger <seboettg@gmail.com>
  * You may use, distribute and modify this code under the
diff --git a/src/ArrayList/ArrayListInterface.php b/src/ArrayList/ArrayListInterface.php
index bf489d5..5588230 100644
--- a/src/ArrayList/ArrayListInterface.php
+++ b/src/ArrayList/ArrayListInterface.php
@@ -1,4 +1,5 @@
 <?php
+declare(strict_types=1);
 /*
  * Copyright (C) 2018 Sebastian Böttger <seboettg@gmail.com>
  * You may use, distribute and modify this code under the
@@ -37,7 +38,7 @@ public function get($key);
      * @param $element
      * @return ArrayListInterface
      */
-    public function set($key, $element);
+    public function set($key, $element): ArrayListInterface;
 
     /**
      * Returns the value of the array element that's currently being pointed to by the
@@ -69,17 +70,17 @@ public function prev();
 
     /**
      * @param array $array
-     * @return mixed
+     * @return ArrayListInterface
      */
-    public function replace(array $array);
+    public function replace(array $array): ArrayListInterface;
 
     /**
      * Appends the passed element to the end of this list.
      *
      * @param $element
-     * @return $this
+     * @return ArrayListInterface
      */
-    public function append($element);
+    public function append($element): ArrayListInterface;
 
     /**
      * Inserts the specified element at the specified position in this list. If another element already exist at the
@@ -88,17 +89,17 @@ public function append($element);
      *
      * @param $key
      * @param $element
-     * @return $this
+     * @return ArrayListInterface
      */
-    public function add($key, $element);
+    public function add($key, $element): ArrayListInterface;
 
     /**
      * Removes the element at the specified position in this list.
      *
      * @param $key
-     * @return $this
+     * @return ArrayListInterface
      */
-    public function remove($key);
+    public function remove($key): ArrayListInterface;
 
     /**
      * Returns true if an element exists on the specified position.
@@ -106,15 +107,15 @@ public function remove($key);
      * @param mixed $key
      * @return bool
      */
-    public function hasKey($key);
+    public function hasKey($key): bool;
 
     /**
      * Returns true if the passed element already exists in this list, otherwise false.
      *
      * @param string $element
-     * @return mixed
+     * @return bool
      */
-    public function hasElement($element);
+    public function hasElement($element): bool;
 
     /**
      * Returns the first element in this list
@@ -128,11 +129,59 @@ public function first();
      */
     public function last();
 
+    /**
+     * alias of replace function
+     * @param array $array
+     * @return ArrayListInterface
+     */
+    public function setArray(array $array): ArrayListInterface;
+
+    /**
+     * flush array list
+     *
+     * @return ArrayListInterface
+     */
+    public function clear(): ArrayListInterface;
+
+    /**
+     * Shuffles this list (randomizes the order of the elements in). It uses the PHP function shuffle
+     * @see http://php.net/manual/en/function.shuffle.php
+     * @return ArrayListInterface
+     */
+    public function shuffle(): ArrayListInterface;
+
+    /**
+     * returns a clone of this ArrayList, filtered by the given closure function
+     * @param callable $filterFunction
+     * @return ArrayListInterface
+     */
+    public function filter(callable $filterFunction): ArrayListInterface;
+
+    /**
+     * returns a clone of this ArrayList, filtered by the given array keys
+     * @param array $keys
+     * @return ArrayListInterface
+     */
+    public function filterByKeys(array $keys): ArrayListInterface;
+
     /**
      * Merges the elements of the passed list together with this list so that the values of the passed list are appended
      * to the end of the this list
      * @param ArrayListInterface $list
      * @return void
      */
-    public function merge(ArrayListInterface $list);
+    public function merge(ArrayListInterface $list): void;
+
+    /**
+     * returns a new ArrayList containing all the elements of this ArrayList after applying the callback function to each one.
+     * @param callable $mapFunction
+     * @return ArrayListInterface
+     */
+    public function map(callable $mapFunction): ArrayListInterface;
+
+    /**
+     * Returns a new ArrayList containing an one-dimensional array of all elements of this ArrayList. Keys are going lost.
+     * @return ArrayListInterface
+     */
+    public function flatten(): ArrayListInterface;
 }
diff --git a/src/ArrayList/ArrayListTrait.php b/src/ArrayList/ArrayListTrait.php
index fc48896..f062052 100644
--- a/src/ArrayList/ArrayListTrait.php
+++ b/src/ArrayList/ArrayListTrait.php
@@ -1,4 +1,5 @@
 <?php
+declare(strict_types=1);
 /*
  * Copyright (C) 2018 Sebastian Böttger <seboettg@gmail.com>
  * You may use, distribute and modify this code under the
@@ -10,8 +11,6 @@
 
 namespace Seboettg\Collection\ArrayList;
 
-use closure;
-
 /**
  * Trait ArrayListTrait
  * @package Seboettg\Collection
@@ -25,9 +24,9 @@ trait ArrayListTrait
     /**
      * flush array list
      *
-     * @return $this
+     * @return ArrayListInterface|ArrayListTrait
      */
-    public function clear()
+    public function clear(): ArrayListInterface
     {
         $this->array = [];
         return $this;
@@ -66,27 +65,32 @@ public function prev()
     }
 
     /**
-     * {@inheritdoc}
+     * @param $key
+     * @param $element
+     * @return ArrayListInterface|ArrayListTrait
      */
-    public function set($key, $element)
+    public function set($key, $element): ArrayListInterface
     {
         $this->array[$key] = $element;
         return $this;
     }
 
     /**
-     * {@inheritdoc}
+     * @param $element
+     * @return ArrayListInterface|ArrayListTrait
      */
-    public function append($element)
+    public function append($element): ArrayListInterface
     {
         $this->array[] = $element;
         return $this;
     }
 
     /**
-     * {@inheritdoc}
+     * @param $key
+     * @param $element
+     * @return ArrayListInterface|ArrayListTrait
      */
-    public function add($key, $element)
+    public function add($key, $element): ArrayListInterface
     {
 
         if (!array_key_exists($key, $this->array)) {
@@ -101,9 +105,10 @@ public function add($key, $element)
     }
 
     /**
-     * {@inheritdoc}
+     * @param $key
+     * @return ArrayListInterface|ArrayListTrait
      */
-    public function remove($key)
+    public function remove($key): ArrayListInterface
     {
         unset($this->array[$key]);
         return $this;
@@ -112,7 +117,7 @@ public function remove($key)
     /**
      * {@inheritdoc}
      */
-    public function hasKey($key)
+    public function hasKey($key): bool
     {
         return array_key_exists($key, $this->array);
     }
@@ -120,7 +125,7 @@ public function hasKey($key)
     /**
      * {@inheritdoc}
      */
-    public function hasElement($value)
+    public function hasElement($value): bool
     {
         $result = array_search($value, $this->array, true);
         return ($result !== false);
@@ -150,7 +155,7 @@ public function last()
     /**
      * {@inheritDoc}
      */
-    public function toArray()
+    public function toArray(): array
     {
         return $this->array;
     }
@@ -160,7 +165,7 @@ public function toArray()
      * @see http://php.net/manual/en/function.shuffle.php
      * @return ArrayListInterface|ArrayListTrait
      */
-    public function shuffle()
+    public function shuffle(): ArrayListInterface
     {
         shuffle($this->array);
         return $this;
@@ -168,28 +173,30 @@ public function shuffle()
 
     /**
      * returns a clone of this ArrayList, filtered by the given closure function
-     * @param closure $closure
+     * @param callable $filterFunction
      * @return ArrayListInterface|ArrayListTrait
      */
-    public function filter(closure $closure)
+    public function filter(callable $filterFunction): ArrayListInterface
     {
         $newInstance = new self();
-        $newInstance->setArray(array_filter($this->array, $closure));
+        $newInstance->setArray(array_filter($this->array, $filterFunction));
         return $newInstance;
     }
 
     /**
-     * {@inheritdoc}
+     * @param array $array
+     * @return ArrayListInterface|ArrayListTrait
      */
-    public function setArray(array $array)
+    public function setArray(array $array): ArrayListInterface
     {
         return $this->replace($array);
     }
 
     /**
-     * {@inheritdoc}
+     * @param array $data
+     * @return ArrayListInterface|ArrayListTrait
      */
-    public function replace(array $data)
+    public function replace(array $data): ArrayListInterface
     {
         $this->array = $data;
         return $this;
@@ -200,7 +207,7 @@ public function replace(array $data)
      * @param array $keys
      * @return ArrayListInterface|ArrayListTrait
      */
-    public function filterByKeys(array $keys)
+    public function filterByKeys(array $keys): ArrayListInterface
     {
         $newInstance = new self();
         $newInstance->setArray(array_filter($this->array, function ($key) use ($keys) {
@@ -211,10 +218,10 @@ public function filterByKeys(array $keys)
 
     /**
      * returns a new ArrayList containing all the elements of this ArrayList after applying the callback function to each one.
-     * @param closure $mapFunction
+     * @param callable $mapFunction
      * @return ArrayListInterface|ArrayListTrait
      */
-    public function map(closure $mapFunction)
+    public function map(callable $mapFunction): ArrayListInterface
     {
         $newInstance = new self();
         $newInstance->setArray(array_map($mapFunction, $this->array));
@@ -225,7 +232,7 @@ public function map(closure $mapFunction)
      * Returns a new ArrayList containing an one-dimensional array of all elements of this ArrayList. Keys are going lost.
      * @return ArrayListInterface|ArrayListTrait
      */
-    public function flatten()
+    public function flatten(): ArrayListInterface
     {
         $flattenedArray = [];
         array_walk_recursive($this->array, function ($item) use (&$flattenedArray) {
@@ -237,12 +244,10 @@ public function flatten()
     }
 
     /**
-     * Merges the elements of the passed list together with this list so that the values of the passed list are appended
-     * to the end of the this list
+     * @inheritDoc
      * @param ArrayListInterface $list
-     * @return void
      */
-    public function merge(ArrayListInterface $list)
+    public function merge(ArrayListInterface $list): void
     {
         $this->array = array_merge($this->array, $list->toArray());
     }
diff --git a/src/ArrayList/ToArrayInterface.php b/src/ArrayList/ToArrayInterface.php
index 9081e74..0a6dd8c 100644
--- a/src/ArrayList/ToArrayInterface.php
+++ b/src/ArrayList/ToArrayInterface.php
@@ -25,5 +25,5 @@ interface ToArrayInterface
      *
      * @return array
      */
-    public function toArray();
+    public function toArray(): array;
 }
diff --git a/src/CollectionInterface.php b/src/CollectionInterface.php
index b7785a5..9731e15 100644
--- a/src/CollectionInterface.php
+++ b/src/CollectionInterface.php
@@ -1,4 +1,5 @@
 <?php
+declare(strict_types=1);
 /*
  * Copyright (C) 2016 Sebastian Böttger <seboettg@gmail.com>
  * You may use, distribute and modify this code under the
diff --git a/src/Collections.php b/src/Collections.php
index 639a0d0..1f1d136 100644
--- a/src/Collections.php
+++ b/src/Collections.php
@@ -1,4 +1,5 @@
 <?php
+declare(strict_types=1);
 /*
  * Copyright (C) 2016 Sebastian Böttger <seboettg@gmail.com>
  * You may use, distribute and modify this code under the
@@ -10,9 +11,9 @@
 
 namespace Seboettg\Collection;
 
+use Seboettg\Collection\ArrayList\ArrayListInterface;
 use Seboettg\Collection\Comparable\Comparator;
 
-
 /**
  * Class Collections
  * @package Seboettg\Collection
@@ -25,11 +26,11 @@ class Collections
      * Sorts the specified list according to the order induced by the specified comparator. All elements in the list
      * must be mutually comparable.
      *
-     * @param ArrayList $list
+     * @param ArrayListInterface $list
      * @param Comparator $comparator
-     * @return ArrayList
+     * @return ArrayListInterface
      */
-    public static function sort(ArrayList &$list, Comparator $comparator)
+    public static function sort(ArrayListInterface $list, Comparator $comparator): ArrayListInterface
     {
         $array = $list->toArray();
         usort($array, [$comparator, "compare"]);
diff --git a/src/Comparable/Comparable.php b/src/Comparable/Comparable.php
index be938ce..587b213 100644
--- a/src/Comparable/Comparable.php
+++ b/src/Comparable/Comparable.php
@@ -1,4 +1,5 @@
 <?php
+declare(strict_types=1);
 /*
  * Copyright (C) 2016 Sebastian Böttger <seboettg@gmail.com>
  * You may use, distribute and modify this code under the
@@ -30,5 +31,5 @@ interface Comparable
      * @param Comparable $b
      * @return int
      */
-    public function compareTo(Comparable $b);
+    public function compareTo(Comparable $b): int;
 }
diff --git a/src/Comparable/Comparator.php b/src/Comparable/Comparator.php
index 356bf71..d0d19e8 100644
--- a/src/Comparable/Comparator.php
+++ b/src/Comparable/Comparator.php
@@ -1,4 +1,5 @@
 <?php
+declare(strict_types=1);
 /*
  * Copyright (C) 2016 Sebastian Böttger <seboettg@gmail.com>
  * You may use, distribute and modify this code under the
@@ -69,5 +70,5 @@ public function __construct($sortingOrder = self::ORDER_ASC, $customOrder = null
      * @param Comparable $b
      * @return int
      */
-    public abstract function compare(Comparable $a, Comparable $b);
+    public abstract function compare(Comparable $a, Comparable $b): int;
 }
diff --git a/src/Queue.php b/src/Queue.php
index 3af8112..337e71b 100644
--- a/src/Queue.php
+++ b/src/Queue.php
@@ -1,4 +1,5 @@
 <?php
+declare(strict_types=1);
 /*
  * Copyright (C) 2018 Sebastian Böttger <seboettg@gmail.com>
  * You may use, distribute and modify this code under the
diff --git a/src/Queue/QueueInterface.php b/src/Queue/QueueInterface.php
index f012c60..bfd1d54 100644
--- a/src/Queue/QueueInterface.php
+++ b/src/Queue/QueueInterface.php
@@ -1,4 +1,5 @@
 <?php
+declare(strict_types=1);
 /*
  * Copyright (C) 2018 Sebastian Böttger <seboettg@gmail.com>
  * You may use, distribute and modify this code under the
diff --git a/src/Queue/QueueTrait.php b/src/Queue/QueueTrait.php
index 1209c38..90c0fd1 100644
--- a/src/Queue/QueueTrait.php
+++ b/src/Queue/QueueTrait.php
@@ -1,4 +1,5 @@
 <?php
+declare(strict_types=1);
 /*
  * Copyright (C) 2018 Sebastian Böttger <seboettg@gmail.com>
  * You may use, distribute and modify this code under the
diff --git a/src/Stack.php b/src/Stack.php
index 4431387..d3e6713 100644
--- a/src/Stack.php
+++ b/src/Stack.php
@@ -1,4 +1,5 @@
 <?php
+declare(strict_types=1);
 /*
  * Copyright (C) 2018 Sebastian Böttger <seboettg@gmail.com>
  * You may use, distribute and modify this code under the
@@ -20,7 +21,6 @@
  */
 class Stack implements StackInterface
 {
-
     use StackTrait;
 
     /**
diff --git a/src/Stack/StackInterface.php b/src/Stack/StackInterface.php
index 79dc185..a41c735 100644
--- a/src/Stack/StackInterface.php
+++ b/src/Stack/StackInterface.php
@@ -1,4 +1,5 @@
 <?php
+declare(strict_types=1);
 /*
  * Copyright (C) 2018 Sebastian Böttger <seboettg@gmail.com>
  * You may use, distribute and modify this code under the
diff --git a/src/Stack/StackTrait.php b/src/Stack/StackTrait.php
index 7470468..dc24abd 100644
--- a/src/Stack/StackTrait.php
+++ b/src/Stack/StackTrait.php
@@ -1,4 +1,5 @@
 <?php
+declare(strict_types=1);
 /*
  * Copyright (C) 2018 Sebastian Böttger <seboettg@gmail.com>
  * You may use, distribute and modify this code under the
diff --git a/tests/ArrayListTest.php b/tests/ArrayListTest.php
index 5c5cafd..a304481 100644
--- a/tests/ArrayListTest.php
+++ b/tests/ArrayListTest.php
@@ -1,5 +1,4 @@
 <?php
-
 /*
  * Copyright (C) 2016 Sebastian Böttger <seboettg@gmail.com>
  * You may use, distribute and modify this code under the
@@ -11,6 +10,7 @@
 
 namespace Seboettg\Collection\Test;
 
+use Exception;
 use PHPUnit\Framework\TestCase;
 use Seboettg\Collection\ArrayList;
 use Seboettg\Collection\Collections;
@@ -139,7 +139,7 @@ public function testShuffle()
             ->append(new Element("z", "zz"));
 
         Collections::sort($this->numeratedArrayList, new class extends Comparator{
-            public function compare(Comparable $a, Comparable $b) {
+            public function compare(Comparable $a, Comparable $b): int {
                 return $a->compareTo($b);
             }
         });
@@ -190,6 +190,9 @@ public function testHasValue()
         $this->assertTrue($list->hasElement("a"));
     }
 
+    /**
+     * @throws Exception
+     */
     public function testGetIterator()
     {
         $it = $this->numeratedArrayList->getIterator();
@@ -382,7 +385,7 @@ public function setAttr2(string $attr2)
      * @param Comparable $b
      * @return int
      */
-    public function compareTo(Comparable $b)
+    public function compareTo(Comparable $b): int
     {
         /** @var Element $b */
         return strcmp($this->attr1, $b->getAttr1());
diff --git a/tests/CollectionsTest.php b/tests/CollectionsTest.php
index 51ad04b..287733a 100644
--- a/tests/CollectionsTest.php
+++ b/tests/CollectionsTest.php
@@ -73,7 +73,7 @@ class MyAscendingComparator extends Comparator
      * @param Comparable $b
      * @return int
      */
-    public function compare(Comparable $a, Comparable $b)
+    public function compare(Comparable $a, Comparable $b): int
     {
         return $a->compareTo($b);
     }
@@ -89,7 +89,7 @@ class MyDescendingComparator extends Comparator
      * @param Comparable $b
      * @return int
      */
-    public function compare(Comparable $a, Comparable $b)
+    public function compare(Comparable $a, Comparable $b): int
     {
         return $b->compareTo($a);
     }
@@ -106,7 +106,7 @@ class MyCustomOrderComparator extends Comparator
      * @param Comparable $b
      * @return int
      */
-    public function compare(Comparable $a, Comparable $b)
+    public function compare(Comparable $a, Comparable $b): int
     {
         /**
          * @var Element $a