diff --git a/src/Datastructure/Lists/ArrayLists/StringBuilder.php b/src/Datastructure/Lists/ArrayLists/StringBuilder.php
index b58c2af..766c704 100644
--- a/src/Datastructure/Lists/ArrayLists/StringBuilder.php
+++ b/src/Datastructure/Lists/ArrayLists/StringBuilder.php
@@ -162,7 +162,7 @@ private function arrayListToStringBuilder(ArrayList $arrayList, bool $reverse =
                 }
             }
             while (!$stack->isEmpty()) {
-                $stringBuilder->append($stack->peek());
+                $stringBuilder->append($stack->pop());
             }
         } else {
             $queue = new Queue();
diff --git a/src/Datastructure/Stackqueue/Stack.php b/src/Datastructure/Stackqueue/Stack.php
index f5af238..078787f 100644
--- a/src/Datastructure/Stackqueue/Stack.php
+++ b/src/Datastructure/Stackqueue/Stack.php
@@ -90,7 +90,7 @@ protected function isValid(): bool {
      *
      */
     public function peek() {
-        if (!$this->isValid()) {
+        if (null === $this->stack) {
             return null;
         }
         if (0 === $this->stackSize()) {
@@ -113,9 +113,8 @@ public function stackSize(): int {
     /**
      * pop() removes an item from the top of the stack.
      *
-     * @return bool
      */
-    public function pop(): bool {
+    public function pop() {
         if ($this->stack === null) {
             return false;
         }
@@ -128,7 +127,7 @@ public function pop(): bool {
          * of the array.
          */
         $return = array_pop($this->stack);
-        return $return !== null;
+        return $return;
     }
 
     /**
diff --git a/src/Datastructure/Stackqueue/StackSet.php b/src/Datastructure/Stackqueue/StackSet.php
index 4faeb37..875db6f 100644
--- a/src/Datastructure/Stackqueue/StackSet.php
+++ b/src/Datastructure/Stackqueue/StackSet.php
@@ -37,16 +37,29 @@ class StackSet {
     private $counter = 0;
     private $stackList = null;
 
+    /**
+     * StackSet constructor.
+     *
+     * @param int $maxSize
+     */
     public function __construct(int $maxSize = 128) {
         $this->maxSize = $maxSize;
         $this->stackList = new ArrayList();
     }
 
+    /**
+     * @param $element
+     * @throws \doganoo\PHPAlgorithms\Common\Exception\IndexOutOfBoundsException
+     */
     public function push($element) {
         $stack = $this->getLastStack();
         $this->addToStack($stack, $element);
     }
 
+    /**
+     * @return Stack
+     * @throws \doganoo\PHPAlgorithms\Common\Exception\IndexOutOfBoundsException
+     */
     private function getLastStack(): Stack {
         $modulo = $this->counter % $this->maxSize;
         if (0 === $modulo) {
@@ -57,6 +70,11 @@ private function getLastStack(): Stack {
         return $stack;
     }
 
+    /**
+     * @param Stack $stack
+     * @param       $element
+     * @throws \doganoo\PHPAlgorithms\Common\Exception\IndexOutOfBoundsException
+     */
     private function addToStack(Stack $stack, $element) {
         $stack->push($element);
         $this->counter++;
@@ -68,11 +86,15 @@ private function addToStack(Stack $stack, $element) {
         }
     }
 
+    /**
+     * @return mixed|null
+     * @throws \doganoo\PHPAlgorithms\Common\Exception\IndexOutOfBoundsException
+     */
     public function pop() {
         $index = $this->stackList->length();
         /** @var Stack $stack */
         $stack = $this->stackList->get($index - 1);
-        $element = $stack->peek();
+        $element = $stack->pop();
         $this->counter--;
         if (0 === $stack->stackSize()) {
             $this->stackList->remove($index - 1);
@@ -82,6 +104,9 @@ public function pop() {
         return $element;
     }
 
+    /**
+     * @return int
+     */
     public function stackCount(): int {
         return $this->stackList->length();
     }
diff --git a/tests/StackQueue/FixedStackTest.php b/tests/StackQueue/FixedStackTest.php
index 7aed93f..6798737 100644
--- a/tests/StackQueue/FixedStackTest.php
+++ b/tests/StackQueue/FixedStackTest.php
@@ -44,11 +44,11 @@ public function testStack() {
         $this->assertTrue($added === false);
         $this->assertTrue($stack->stackSize() === 2);
 
-        $class = $stack->peek();
+        $class = $stack->pop();
         $this->assertTrue($class instanceof Exception);
         $this->assertTrue($stack->isEmpty() == false);
 
-        $class = $stack->peek();
+        $class = $stack->pop();
         $this->assertTrue($class instanceof stdClass);
         $this->assertTrue($stack->isEmpty() == true);
     }
diff --git a/tests/StackQueue/StackSetTest.php b/tests/StackQueue/StackSetTest.php
index ff22741..489328f 100644
--- a/tests/StackQueue/StackSetTest.php
+++ b/tests/StackQueue/StackSetTest.php
@@ -1,12 +1,42 @@
 <?php
-
+/**
+ * MIT License
+ *
+ * Copyright (c) 2018 Dogan Ucar, <dogan@dogan-ucar.de>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
 
 namespace StackQueue;
 
-
 use doganoo\PHPAlgorithms\Datastructure\Stackqueue\StackSet;
+use PHPUnit\Framework\TestCase;
 
-class StackSetTest extends \PHPUnit\Framework\TestCase {
+/**
+ * Class StackSetTest
+ *
+ * @package StackQueue
+ */
+class StackSetTest extends TestCase {
+    /**
+     * @throws \doganoo\PHPAlgorithms\Common\Exception\IndexOutOfBoundsException
+     */
     public function testStackSet() {
         $stackSet = new StackSet(2);
         $stackSet->push("Hallo");
@@ -19,6 +49,9 @@ public function testStackSet() {
         $this->assertTrue($stackSet->stackCount() === 1);
     }
 
+    /**
+     * @throws \doganoo\PHPAlgorithms\Common\Exception\IndexOutOfBoundsException
+     */
     public function testHugeStackSet() {
         $setSize = 1024;
         $factor = 4;
diff --git a/tests/StackQueue/StackTest.php b/tests/StackQueue/StackTest.php
index fbde209..2ff94c2 100644
--- a/tests/StackQueue/StackTest.php
+++ b/tests/StackQueue/StackTest.php
@@ -25,11 +25,12 @@
 
 use doganoo\PHPAlgorithms\Datastructure\Stackqueue\Queue;
 use doganoo\PHPAlgorithms\Datastructure\Stackqueue\Stack;
+use PHPUnit\Framework\TestCase;
 
 /**
  * Class StackQueueTest class testing Stacks and Queues
  */
-class StackTest extends \PHPUnit\Framework\TestCase {
+class StackTest extends TestCase {
     /**
      * Stack class test
      */
@@ -37,13 +38,13 @@ public function testStack() {
         $stack = new Stack();
         $stack->push(new stdClass());
         $stack->push(new Exception());
-        $this->assertTrue($stack->isEmpty() == false);
+        $this->assertTrue(false === $stack->isEmpty());
 
-        $class = $stack->peek();
+        $class = $stack->pop();
         $this->assertTrue($class instanceof Exception);
         $this->assertTrue($stack->isEmpty() == false);
 
-        $class = $stack->peek();
+        $class = $stack->pop();
         $this->assertTrue($class instanceof stdClass);
         $this->assertTrue($stack->isEmpty() == true);
     }
@@ -64,7 +65,5 @@ public function testQueue() {
         $class = $queue->dequeue();
         $this->assertTrue($class instanceof stdClass);
         $this->assertTrue($queue->isEmpty() == true);
-
-
     }
 }
\ No newline at end of file