Solution

src/main/java/mate/academy/service/AuthenticationService.java

@@ -1,16 +1,23 @@
 package mate.academy.service;
 
+import mate.academy.model.User;
+
 public class AuthenticationService {
     /**
      * Imagine that some user wants to login to your site.
      * You should check if user credentials (login and password) are valid or not.
      * All users are stored in <code>UserService</code> class.
-     * @param email - user's email
+     *
+     * @param email    - user's email
      * @param password - user's password
      * @return true if user by email exists and passed password is equal to user's password.
      * Return false in any other cases.
      */
     public boolean login(String email, String password) {
+        User user = UserService.findByEmail(email);
+        if (user != null && user.getPassword().equals(password)) {
+            return true;
+        }
         return false;
     }
 }

src/main/java/mate/academy/service/UserService.java

@@ -14,7 +14,12 @@ public class UserService {
      * @return - user if his email is equal to passed email.
      * Return <code>null</code> if there is no suitable user
      */
-    public User findByEmail(String email) {
+    public static User findByEmail(String email) {
+        for (User user : users) {
+            if (user.getEmail().equals(email)) {
+                return user;
+            }
+        }
         return null;
     }
 }