Fixed structure of UI, Added some logic to Auth

src/main/java/repository/UserRepository.java

@@ -6,6 +6,7 @@ import java.util.Optional;
 
 public interface UserRepository {
     Optional<User> getUser(String username, String password);
+    Boolean userExists(String username);
     Optional<User> saveUser(String username, String email, String password);
     boolean deleteUser(int userId);
 }

src/main/java/repository/UserRepositoryJdbc.java

@@ -27,6 +27,9 @@ public class UserRepositoryJdbc implements UserRepository {
 
             preparedStatement.setString(1, username);
             ResultSet resultSet = preparedStatement.executeQuery();
+            if (!resultSet.next()) {
+                return Optional.empty();
+            }
 
             String passwordFromDB = resultSet.getString("password");
             String emailFromDB = resultSet.getString("email");
@@ -43,6 +46,28 @@ public class UserRepositoryJdbc implements UserRepository {
         }
     }
 
+    @Override
+    public Boolean userExists(String username) {
+        try {
+            PreparedStatement preparedStatement = connection.prepareStatement(
+                    "SELECT * FROM users WHERE name=(?);"
+            );
+
+            preparedStatement.setString(1, username);
+            ResultSet resultSet = preparedStatement.executeQuery();
+
+            if (resultSet.next()) {
+                return true;
+            }
+
+            return false;
+        } catch (SQLException e) {
+            e.printStackTrace();
+            return false;
+        }
+    }
+
+
     @Override
     public Optional<User> saveUser(String username, String email, String password) {
         try {