diff --git a/Behavioral/Observer/User.php b/Behavioral/Observer/User.php
index 79a13f7..f42d0d4 100644
--- a/Behavioral/Observer/User.php
+++ b/Behavioral/Observer/User.php
@@ -22,23 +22,23 @@ class User implements SplSubject
         $this->observers = new SplObjectStorage();
     }
 
-    public function attach(SplObserver $observer)
+    public function attach(SplObserver $observer): void
     {
         $this->observers->attach($observer);
     }
 
-    public function detach(SplObserver $observer)
+    public function detach(SplObserver $observer): void
     {
         $this->observers->detach($observer);
     }
 
-    public function changeEmail(string $email)
+    public function changeEmail(string $email): void
     {
         $this->email = $email;
         $this->notify();
     }
 
-    public function notify()
+    public function notify(): void
     {
         /** @var SplObserver $observer */
         foreach ($this->observers as $observer) {