diff --git a/src/main/java/edu/group5/app/App.java b/src/main/java/edu/group5/app/App.java
index b9d67da..6402156 100644
--- a/src/main/java/edu/group5/app/App.java
+++ b/src/main/java/edu/group5/app/App.java
@@ -35,6 +35,7 @@ public class App extends Application {
   NavigationController nav;
 
   private Logger logger;
+  static int MAX_ReTRIES = 3;
 
   @Override
   public void init() {
@@ -44,8 +45,14 @@ public void init() {
     this.dbWrapper = new DbWrapper(false);
     OrgApiWrapper orgApiWrapper = new OrgApiWrapper("https://app.innsamlingskontrollen.no/api/public/v1/all");
 
-    while (!dbWrapper.connect()) {
+    int retries = 0;
+    while (!dbWrapper.connect() && retries < MAX_ReTRIES) {
       this.logger.warning("Failed to connect to database");
+      retries++;
+    }
+    if (retries == MAX_ReTRIES) {
+      this.logger.severe("Failed to connect to database after " + MAX_ReTRIES + " attempts");
+      throw new RuntimeException("Failed to connect to database");
     }
 
     // Load data from database
diff --git a/src/main/java/edu/group5/app/control/AuthController.java b/src/main/java/edu/group5/app/control/AuthController.java
index 249a436..4911a3b 100644
--- a/src/main/java/edu/group5/app/control/AuthController.java
+++ b/src/main/java/edu/group5/app/control/AuthController.java
@@ -3,6 +3,7 @@
 import edu.group5.app.model.AppState;
 import edu.group5.app.model.user.User;
 import edu.group5.app.model.user.UserService;
+import edu.group5.app.utils.ParameterValidator;
 import edu.group5.app.view.loginpage.LoginPageView;
 import edu.group5.app.view.loginpage.SignUpPageView;
 import javafx.scene.control.Alert;
@@ -31,6 +32,9 @@ public class AuthController {
   private final UserService userService;
 
   public AuthController(AppState appState, NavigationController nav, UserService userService) {
+    ParameterValidator.objectChecker(appState, "AppState");
+    ParameterValidator.objectChecker(nav, "NavigationController");
+    ParameterValidator.objectChecker(userService, "UserService");
     this.appState = appState;
     this.nav = nav;
     this.userService = userService;
@@ -69,7 +73,7 @@ public void handleSignUp(SignUpPageView view, String firstName, String lastName,
     // Clears password char array after creating a hash.
     String hashedPassword = encoder.encode(new String(passwordChars));
     for (int i = 0; i < passwordChars.length; i++) {
-      passwordChars[0] = '0';
+      passwordChars[i] = '\u0000';
     }
 
     Alert privacyPolicy = new Alert(Alert.AlertType.CONFIRMATION);
@@ -86,9 +90,8 @@ public void handleSignUp(SignUpPageView view, String firstName, String lastName,
 
       if (success) {
         User user = userService.getUserByEmail(email);
-
-      appState.setCurrentUser(user);
-      nav.showHomePage();
+        appState.setCurrentUser(user);
+        nav.showHomePage();
     } else {
       view.showError("Registration failed. Email may already be in use.");
     }
diff --git a/src/main/java/edu/group5/app/control/DonationController.java b/src/main/java/edu/group5/app/control/DonationController.java
index 8a8ec79..61ab172 100644
--- a/src/main/java/edu/group5/app/control/DonationController.java
+++ b/src/main/java/edu/group5/app/control/DonationController.java
@@ -6,6 +6,7 @@
 import edu.group5.app.model.organization.Organization;
 import edu.group5.app.model.user.Customer;
 import edu.group5.app.model.user.User;
+import edu.group5.app.utils.ParameterValidator;
 
 import java.math.BigDecimal;
 import java.util.HashSet;
@@ -34,6 +35,9 @@ public class DonationController {
   private final DonationService service;
 
   public DonationController(AppState appState, NavigationController nav, DonationService service) {
+    ParameterValidator.objectChecker(appState, "AppState");
+    ParameterValidator.objectChecker(nav, "NavigationController");
+    ParameterValidator.objectChecker(service, "DonationService");
     this.appState = appState;
     this.nav = nav;
     this.service = service;
@@ -46,6 +50,7 @@ public DonationController(AppState appState, NavigationController nav, DonationS
    * @return a map of donations.
    */
   public Map<Integer, Donation> getUserDonations(int userId) {
+    ParameterValidator.intChecker(userId, "User ID");
     return service.getDonationRepository().filterByUser(userId);
   }
 
@@ -56,7 +61,11 @@ public Map<Integer, Donation> getUserDonations(int userId) {
    * @return a set of organization IDs
    */
   public Set<Integer> getUniqueOrganizationIDs() {
-    Map<Integer, Donation> userDonations = getUserDonations(appState.getCurrentUser().getUserId());
+    User currentUser = appState.getCurrentUser();
+    if (currentUser == null) {
+      throw new IllegalStateException("No user logged in");
+    }
+    Map<Integer, Donation> userDonations = getUserDonations(currentUser.getUserId());
 
     Set<Integer> uniqueOrganizations = new HashSet<>();
     for (Donation donation : userDonations.values()) {
@@ -146,19 +155,26 @@ private void handleDonate() {
     );
 
     if (success) {
-      System.out.println("Donation created: " + amount + " kr to " + currentOrg.name() + ", with payment method: " + paymentMethod);
+      System.out.println("Donation created: " 
+      + amount + " kr to " + currentOrg.name() 
+      + ", with payment method: " + paymentMethod);
     } else {
       System.err.println("Failed to create donation");
     }
 
     // Clear donation session state
     appState.setCurrentDonationAmount(null);
+    // Clear org
+    appState.setCurrentOrganization(null);  
+    // Clear payment method
+    appState.setCurrentPaymentMethod(null);   
 
     // Navigate to payment complete
     nav.showPaymentCompletePage();
   }
 
   private void showError(String message) {
+    ParameterValidator.stringChecker(message, "message");
     Alert errorAlert = new Alert(Alert.AlertType.WARNING);
     errorAlert.setTitle("Donation Error");
     errorAlert.setHeaderText("Cannot Process Donation");
diff --git a/src/main/java/edu/group5/app/control/NavigationController.java b/src/main/java/edu/group5/app/control/NavigationController.java
index 7bfbc8c..326389b 100644
--- a/src/main/java/edu/group5/app/control/NavigationController.java
+++ b/src/main/java/edu/group5/app/control/NavigationController.java
@@ -4,6 +4,7 @@
 import edu.group5.app.model.donation.DonationService;
 import edu.group5.app.model.organization.OrganizationService;
 import edu.group5.app.model.user.UserService;
+import edu.group5.app.utils.ParameterValidator;
 import edu.group5.app.view.Header;
 import edu.group5.app.view.causespage.CausesPageView;
 import edu.group5.app.view.donationpage.DonationPageView;
@@ -30,7 +31,14 @@ public class NavigationController {
   private final DonationController donationController;
   private final OrganizationController organizationController;
 
-  public NavigationController(BorderPane root, AppState appState, UserService userService, DonationService donationService, OrganizationService organizationService) {
+  public NavigationController(BorderPane root, AppState appState, UserService userService, 
+    DonationService donationService, OrganizationService organizationService) {
+    ParameterValidator.objectChecker(root, "Root BorderPane");
+    ParameterValidator.objectChecker(appState, "AppState");
+    ParameterValidator.objectChecker(userService, "UserService");
+    ParameterValidator.objectChecker(donationService, "DonationService");
+    ParameterValidator.objectChecker(organizationService, "OrganizationService");
+
     this.root = root;
     this.header = new Header(this);
     this.loginHeader = new LoginHeader();
diff --git a/src/main/java/edu/group5/app/control/OrganizationController.java b/src/main/java/edu/group5/app/control/OrganizationController.java
index fb65127..fc1252f 100644
--- a/src/main/java/edu/group5/app/control/OrganizationController.java
+++ b/src/main/java/edu/group5/app/control/OrganizationController.java
@@ -2,6 +2,8 @@
 
 import edu.group5.app.model.organization.Organization;
 import edu.group5.app.model.organization.OrganizationService;
+import edu.group5.app.utils.ParameterValidator;
+import tools.jackson.databind.deser.bean.CreatorCandidate.Param;
 
 import java.util.Map;
 import java.util.concurrent.CompletableFuture;
@@ -13,10 +15,12 @@ public class OrganizationController {
   private final OrganizationService service;
 
   public OrganizationController(OrganizationService service) {
+    ParameterValidator.objectChecker(service, "OrganizationService");
     this.service = service;
   }
 
   public Organization getOrganizationById(int orgId) {
+    ParameterValidator.intChecker(orgId, "Organization ID");
     return service.findByOrgNumber(orgId);
   }
 
diff --git a/src/main/java/edu/group5/app/model/DBRepository.java b/src/main/java/edu/group5/app/model/DBRepository.java
index 3cd1fa9..88ac3d9 100644
--- a/src/main/java/edu/group5/app/model/DBRepository.java
+++ b/src/main/java/edu/group5/app/model/DBRepository.java
@@ -2,6 +2,9 @@
 
 import java.util.HashMap;
 import java.util.Map;
+
+import edu.group5.app.utils.ParameterValidator;
+
 import java.util.List;
 
 /**
@@ -22,6 +25,7 @@ public abstract class DBRepository<K, V> extends Repository<K, V> {
    * @param content the HashMap used to store repository entities
    */
   protected DBRepository(Map<K, V> content) {
+    ParameterValidator.objectChecker(content, "Content"); 
     super(content);
     this.contentLock = new HashMap<>();
   }
diff --git a/src/main/java/edu/group5/app/model/Repository.java b/src/main/java/edu/group5/app/model/Repository.java
index 032f307..64d4b90 100644
--- a/src/main/java/edu/group5/app/model/Repository.java
+++ b/src/main/java/edu/group5/app/model/Repository.java
@@ -2,6 +2,8 @@
 
 import java.util.Map;
 
+import edu.group5.app.utils.ParameterValidator;
+
 /**
  * Represents a repository.
  */
@@ -13,6 +15,7 @@ public abstract class Repository<K, V> {
    * @param content the underlying data structure used to store entities
    */
   protected Repository(Map<K, V> content) {
+    ParameterValidator.objectChecker(content, "content");
     this.content = content;
   }
 
diff --git a/src/main/java/edu/group5/app/model/donation/DonationRepository.java b/src/main/java/edu/group5/app/model/donation/DonationRepository.java
index a55ea74..f1102ba 100644
--- a/src/main/java/edu/group5/app/model/donation/DonationRepository.java
+++ b/src/main/java/edu/group5/app/model/donation/DonationRepository.java
@@ -1,6 +1,7 @@
 package edu.group5.app.model.donation;
 
 import edu.group5.app.model.DBRepository;
+import edu.group5.app.utils.ParameterValidator;
 
 import java.util.Comparator;
 import java.util.HashMap;
@@ -32,9 +33,7 @@ public class DonationRepository extends DBRepository<Integer, Donation> {
    */
   public DonationRepository(List<Object[]> rows) {
     super(new HashMap<>());
-    if (rows == null) {
-      throw new IllegalArgumentException("The list of rows cannot be null");
-    }
+    ParameterValidator.objectChecker(rows, "List of donation rows");
     this.content = new HashMap<>();
     for (Object[] row : rows) {
       if (row == null || row.length != 6) {
@@ -88,9 +87,7 @@ public List<Object[]> export() {
    * @throws IllegalArgumentException if the donationId is not positive
    */
   public Donation getDonationById(int donationId) {
-    if (donationId <= 0) {
-      throw new IllegalArgumentException("Donation ID must be positive");
-    }
+    ParameterValidator.intChecker(donationId, "Donation ID");
     return content.get(donationId);
   }
 
@@ -102,7 +99,7 @@ public Donation getDonationById(int donationId) {
    */
   public int getNextDonationId() {
     return content.keySet().stream().max(Integer::compareTo).orElse(0) + 1;
-  } /* TODO change this when data database is introduced */
+  }
 
   public Map<Integer, Donation> getAllDonations() {
     return new HashMap<>(content);
@@ -122,9 +119,7 @@ public Map<Integer, Donation> getAllDonations() {
    */
   @Override
   public boolean addContent(Donation donation) {
-    if (donation == null) {
-      throw new IllegalArgumentException("Donation cannot be null");
-    }
+    ParameterValidator.objectChecker(donation, "Donation");
     if (content.containsKey(donation.donationId())) {
       return false;
     }
@@ -180,9 +175,7 @@ public HashMap<Integer, Donation> sortByAmount() {
    * @throws IllegalArgumentException if the orgNumber is not positive
    */
   public HashMap<Integer, Donation> filterByOrganization(int orgNumber) {
-    if (orgNumber <= 0) {
-      throw new IllegalArgumentException("Organization number must be positive");
-    }
+  ParameterValidator.intChecker(orgNumber, "Organization number");
     return content.entrySet()
         .stream()
         .filter(entry -> entry.getValue().organizationId() == orgNumber)
@@ -201,9 +194,7 @@ public HashMap<Integer, Donation> filterByOrganization(int orgNumber) {
    * @throws IllegalArgumentException if the userId is not positive
    */
   public HashMap<Integer, Donation> filterByUser(int userId) {
-    if (userId <= 0) {
-      throw new IllegalArgumentException("User ID must be positive");
-    }
+    ParameterValidator.intChecker(userId, "User ID");
     return content.entrySet().stream()
         .filter(entry -> entry.getValue().userId() == userId)
         .collect(Collectors.toMap(
diff --git a/src/main/java/edu/group5/app/model/donation/DonationService.java b/src/main/java/edu/group5/app/model/donation/DonationService.java
index 690a632..6f978b1 100644
--- a/src/main/java/edu/group5/app/model/donation/DonationService.java
+++ b/src/main/java/edu/group5/app/model/donation/DonationService.java
@@ -1,11 +1,11 @@
 package edu.group5.app.model.donation;
 import java.time.Instant;
-
 import java.math.BigDecimal;
 import java.sql.Timestamp;
 import edu.group5.app.model.organization.Organization;
 import edu.group5.app.model.organization.OrganizationRepository;
 import edu.group5.app.model.user.Customer;
+import edu.group5.app.utils.ParameterValidator;
 
 /**
  * DonationService class provides functionality for handling donations in the system.
@@ -27,12 +27,8 @@ public class DonationService {
    */
   public DonationService(DonationRepository donationRepository,
                         OrganizationRepository organizationRepository) {
-    if (donationRepository == null) {
-      throw new IllegalArgumentException("DonationRepository cannot be null");
-    }
-    if (organizationRepository == null) {
-      throw new IllegalArgumentException("OrganizationRepository cannot be null");
-    }
+    ParameterValidator.objectChecker(donationRepository, "DonationRepository");
+    ParameterValidator.objectChecker(organizationRepository, "OrganizationRepository");
     this.donationRepository = donationRepository;
     this.organizationRepository = organizationRepository;
   }
diff --git a/src/main/java/edu/group5/app/model/organization/Organization.java b/src/main/java/edu/group5/app/model/organization/Organization.java
index 7016567..4c66be0 100644
--- a/src/main/java/edu/group5/app/model/organization/Organization.java
+++ b/src/main/java/edu/group5/app/model/organization/Organization.java
@@ -2,6 +2,8 @@
 
 import java.util.Objects;
 
+import edu.group5.app.utils.ParameterValidator;
+
 /**
  * Represents an organization.
  *
@@ -45,9 +47,7 @@ public record Organization(
    */
   public Organization(int orgNumber, String name, boolean trusted, String websiteUrl, boolean isPreApproved,
       String description, String logoUrl) {
-    if (orgNumber < 0) {
-      throw new IllegalArgumentException("orgNumber cannot be negative");
-    }
+    ParameterValidator.intChecker(orgNumber, "Organization number");
     this.orgNumber = orgNumber;
     this.name = Objects.requireNonNull(name, "name cannot be null");
     this.trusted = trusted;
diff --git a/src/main/java/edu/group5/app/model/organization/OrganizationRepository.java b/src/main/java/edu/group5/app/model/organization/OrganizationRepository.java
index 61155b7..5da9a97 100644
--- a/src/main/java/edu/group5/app/model/organization/OrganizationRepository.java
+++ b/src/main/java/edu/group5/app/model/organization/OrganizationRepository.java
@@ -1,6 +1,7 @@
 package edu.group5.app.model.organization;
 
 import edu.group5.app.model.Repository;
+import edu.group5.app.utils.ParameterValidator;
 import tools.jackson.core.type.TypeReference;
 import tools.jackson.databind.ObjectMapper;
 
@@ -30,12 +31,8 @@ public class OrganizationRepository extends Repository<Integer, Organization> {
   public OrganizationRepository(Object[] input, OrganizationScraper scraper) {
     super(new HashMap<>());
     this.grandMap = new HashMap<>();
-    if (input == null)  {
-      throw new IllegalArgumentException("The input cannot be null");
-    }
-    if (scraper == null) {
-      throw new IllegalArgumentException("The scraper cannot be null");
-    }
+    ParameterValidator.objectChecker(input, "Input data");
+    ParameterValidator.objectChecker(scraper, "Scraper");
     this.scraper = scraper;
     
     ObjectMapper mapper = new ObjectMapper();
@@ -44,6 +41,11 @@ public OrganizationRepository(Object[] input, OrganizationScraper scraper) {
       HashMap<String, Object> contentMap =
       mapper.convertValue(obj, new TypeReference<HashMap<String, Object>>() {});
 
+      Object orgNumberObj = contentMap.get("org_number");
+      if (orgNumberObj == null) {
+        System.err.println("Skipping organization: missing org_number");
+        continue;
+      }
       String orgNumberStr = ((String) contentMap.get("org_number")).replaceAll("\\s", "");
       int orgNumber = Integer.parseInt(orgNumberStr);
       String name = (String) contentMap.get("name");
@@ -101,9 +103,7 @@ public Map<Integer, Organization> getTrustedOrganizations() {
    * @throws IllegalArgumentException if the organization number is not a positive integer
    */
   public Organization findByOrgNumber(int orgNumber) {
-    if (orgNumber <= 0) {
-      throw new IllegalArgumentException("The Organization number must be a positive integer");
-    }
+    ParameterValidator.intChecker(orgNumber, "Organization number");
     return grandMap.get(orgNumber);
   }
 
@@ -114,9 +114,7 @@ public Organization findByOrgNumber(int orgNumber) {
    * @throws IllegalArgumentException if the name is null or empty
    */
   public Organization findByOrgName(String name) {
-    if (name == null || name.isEmpty()) {
-      throw new IllegalArgumentException("The name cannot be null");
-    }
+    ParameterValidator.stringChecker(name, "Organization name");
     return grandMap.values().stream()
         .filter(org -> org.name().equalsIgnoreCase(name))
         .findFirst()
diff --git a/src/main/java/edu/group5/app/model/organization/OrganizationScraper.java b/src/main/java/edu/group5/app/model/organization/OrganizationScraper.java
index 6a8c230..fdeec82 100644
--- a/src/main/java/edu/group5/app/model/organization/OrganizationScraper.java
+++ b/src/main/java/edu/group5/app/model/organization/OrganizationScraper.java
@@ -3,13 +3,9 @@
 import org.jsoup.Jsoup;
 import org.jsoup.nodes.Document;
 import org.jsoup.nodes.Element;
-import org.jsoup.nodes.TextNode;
-import org.jsoup.select.Elements;
-
 import java.util.stream.Collectors;
-
-import java.util.HashMap;
 import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
 
 /**
  * Handles web scraping of organization information from Innsamlingskontrollen.
@@ -17,8 +13,8 @@
  * All results are cached to avoid redundant network requests.
  */
 public class OrganizationScraper {
-  private final Map<String, String> logoCache = new HashMap<>();
-  private final Map<String, String> descriptionCache = new HashMap<>();
+  private final Map<String, String> logoCache = new ConcurrentHashMap<>();
+  private final Map<String, String> descriptionCache = new ConcurrentHashMap<>();
 
   /**
    * Fetches the description for the given URL by scraping all text content
@@ -35,10 +31,6 @@ public class OrganizationScraper {
    * @return the description text, or null if not found or pageUrl is invalid
    */
   public String fetchDescription(String pageUrl) {
-    if (pageUrl == null || pageUrl.isBlank()) {
-      return null;
-    }
-
     if (descriptionCache.containsKey(pageUrl)) {
       return descriptionCache.get(pageUrl);
     }
@@ -88,10 +80,6 @@ public String fetchDescription(String pageUrl) {
    * @return the absolute logo URL, or null if not found or pageUrl is invalid
    */
   public String fetchLogoUrl(String pageUrl) {
-    if (pageUrl == null || pageUrl.isBlank()) {
-      return null;
-    }
-
     if (logoCache.containsKey(pageUrl)) {
       return logoCache.get(pageUrl);
     }
diff --git a/src/main/java/edu/group5/app/model/organization/OrganizationService.java b/src/main/java/edu/group5/app/model/organization/OrganizationService.java
index e19c273..acb1110 100644
--- a/src/main/java/edu/group5/app/model/organization/OrganizationService.java
+++ b/src/main/java/edu/group5/app/model/organization/OrganizationService.java
@@ -2,6 +2,9 @@
 
 import java.util.stream.Collectors;
 
+import edu.group5.app.utils.ParameterValidator;
+import tools.jackson.databind.deser.bean.CreatorCandidate.Param;
+
 import java.util.Comparator;
 import java.util.LinkedHashMap;
 import java.util.Map;
@@ -27,12 +30,8 @@ public class OrganizationService {
    * @throws IllegalArgumentException if organizationRepository or scraper is null
    */
   public OrganizationService(OrganizationRepository organizationRepository, OrganizationScraper scraper) {
-    if (organizationRepository == null) {
-      throw new IllegalArgumentException("OrganizationRepository cannot be null");
-    }
-    if (scraper == null) {
-      throw new IllegalArgumentException("OrganizationScraper cannot be null");
-    }
+    ParameterValidator.objectChecker(organizationRepository, "OrganizationRepository");
+    ParameterValidator.objectChecker(scraper, "Scraper");
     this.organizationRepository = organizationRepository;
     this.scraper = scraper;
   }
@@ -71,6 +70,7 @@ public Organization findByOrgNumber(int orgNumber) {
    * @return the Organization if found, null otherwise
    */
   public Organization findByOrgName(String name) {
+    ParameterValidator.stringChecker(name, "name");
     return organizationRepository.findByOrgName(name);
   }
 
diff --git a/src/main/java/edu/group5/app/model/user/Customer.java b/src/main/java/edu/group5/app/model/user/Customer.java
index 93f03ac..d04139d 100644
--- a/src/main/java/edu/group5/app/model/user/Customer.java
+++ b/src/main/java/edu/group5/app/model/user/Customer.java
@@ -1,6 +1,9 @@
 package edu.group5.app.model.user;
 
 import java.util.List;
+
+import edu.group5.app.utils.ParameterValidator;
+
 import java.util.ArrayList;
 
 /**
@@ -38,9 +41,7 @@ public String getRole() {
   }
 
   public void addPreference(int orgNumber) {
-    if (orgNumber <= 0) {
-      throw new IllegalArgumentException("Organization number must be a positive integer");
-    }
+    ParameterValidator.intChecker(orgNumber,"Organization number");
     if (preferences.contains(orgNumber)) {
       throw new IllegalArgumentException("Organization number already in preferences");
     }
@@ -48,9 +49,7 @@ public void addPreference(int orgNumber) {
   }
 
   public void removePreference(int orgNumber) {
-    if (orgNumber <= 0) {
-      throw new IllegalArgumentException("Organization number must be a positive integer");
-    }
+    ParameterValidator.intChecker(orgNumber, "Organization number");
     if (!preferences.contains(orgNumber)) {
       throw new IllegalArgumentException("Organization number not found in preferences");
     }
diff --git a/src/main/java/edu/group5/app/model/user/User.java b/src/main/java/edu/group5/app/model/user/User.java
index 411538d..54c9e16 100644
--- a/src/main/java/edu/group5/app/model/user/User.java
+++ b/src/main/java/edu/group5/app/model/user/User.java
@@ -1,5 +1,9 @@
 package edu.group5.app.model.user;
+import java.lang.reflect.Parameter;
+
 import org.springframework.security.crypto.bcrypt.BCryptPasswordEncoder;
+
+import edu.group5.app.utils.ParameterValidator;
 /**
  * User class represents a user in the system. It is an abstract class that will be extended by specific user types such as Donor, Recipient, and Admin.
  * Each user has a unique userId, a role that defines their permissions in the system, and personal information such as first name, last name, email, and password hash.
@@ -27,21 +31,12 @@ public abstract class User {
    */
   public User(int userId, String firstName, 
             String lastName, String email, String passwordHash) {
-    if (userId <= 0) {
-        throw new IllegalArgumentException("User ID must be positive");
-    }
-    if (firstName == null || firstName.trim().isEmpty()) {
-        throw new IllegalArgumentException("First name cannot be null or empty");
-    }
-    if (lastName == null || lastName.trim().isEmpty()) {
-        throw new IllegalArgumentException("Last name cannot be null or empty");
-    }
-    if (email == null || email.trim().isEmpty()) {
-        throw new IllegalArgumentException("Email cannot be null or empty");
-    }
-    if (passwordHash == null || passwordHash.trim().isEmpty()) {
-        throw new IllegalArgumentException("Password hash cannot be null or empty");
-    }
+    ParameterValidator.intChecker(userId, "User ID");
+    ParameterValidator.stringChecker(firstName, "First name");
+    ParameterValidator.stringChecker(lastName, "Last name");
+    ParameterValidator.stringChecker(email, "Email");
+    ParameterValidator.stringChecker(passwordHash, "Password hash");
+
     this.userId = userId;
     this.firstName = firstName.trim();
     this.lastName = lastName.trim();
diff --git a/src/main/java/edu/group5/app/model/user/UserRepository.java b/src/main/java/edu/group5/app/model/user/UserRepository.java
index 193f433..07f9ca8 100644
--- a/src/main/java/edu/group5/app/model/user/UserRepository.java
+++ b/src/main/java/edu/group5/app/model/user/UserRepository.java
@@ -5,6 +5,7 @@
 import java.util.Map;
 
 import edu.group5.app.model.DBRepository;
+import edu.group5.app.utils.ParameterValidator;
 
 public class UserRepository extends DBRepository<Integer, User> {
   public final static String ROLE_CUSTOMER = "Customer";
@@ -18,9 +19,7 @@ public class UserRepository extends DBRepository<Integer, User> {
    */
   public UserRepository(List<Object[]> rows) {
     super(new HashMap<>());
-    if (rows == null) {
-      throw new IllegalArgumentException("The list of rows cannot be null");
-    }
+    ParameterValidator.objectChecker(rows, "List of User rows");
     for (Object[] row : rows) {
       if (row == null || row.length != 6) {
         throw new IllegalArgumentException("Each row must contain exactly 6 elements");
@@ -83,9 +82,7 @@ public HashMap<Integer, User> getUsers() {
    * @throws IllegalArgumentException if the userId is not positive
    */
   public User getUserById(int userId) {
-    if (userId <= 0) {
-      throw new IllegalArgumentException("User ID must be positive");
-    }
+    ParameterValidator.intChecker(userId, "User ID");
     return content.get(userId);
   }
 
@@ -104,7 +101,7 @@ public int getNextUserId() {
         () -> new IllegalStateException("No keys found"));
     int nextId = maxKey + 1;
     return nextId;
-  } /* TODO change this when data database is introduced */
+  }
 
   /**
    * Adds a new user to the repository
@@ -120,9 +117,7 @@ public int getNextUserId() {
    */
   @Override
   public boolean addContent(User user) {
-    if (user == null) {
-      throw new IllegalArgumentException("User cannot be null");
-    }
+    ParameterValidator.objectChecker(user, "User");
     if (content.containsKey(user.getUserId())) {
       return false;
     }
@@ -138,9 +133,7 @@ public boolean addContent(User user) {
    *         user exists
    */
   public User findUserByEmail(String email) {
-    if (email == null || email.trim().isEmpty()) {
-      throw new IllegalArgumentException("Email cannot be null or empty");
-    }
+    ParameterValidator.stringChecker(email, "Email");
     return content.values().stream()
         .filter(user -> user.getEmail().equals(email))
         .findFirst()
diff --git a/src/main/java/edu/group5/app/model/user/UserService.java b/src/main/java/edu/group5/app/model/user/UserService.java
index b237eae..a6d316e 100644
--- a/src/main/java/edu/group5/app/model/user/UserService.java
+++ b/src/main/java/edu/group5/app/model/user/UserService.java
@@ -1,5 +1,7 @@
 package edu.group5.app.model.user;
 
+import edu.group5.app.utils.ParameterValidator;
+
 /**
  * Service class for managing user-related operations, such as registration and
  * login.
@@ -19,9 +21,7 @@ public class UserService {
    * @throws IllegalArgumentException if userRepository is null
    */
   public UserService(UserRepository userRepository) {
-    if (userRepository == null) {
-      throw new IllegalArgumentException("UserRepository cannot be null");
-    }
+    ParameterValidator.objectChecker(userRepository, "UserRepository");
     this.userRepository = userRepository;
   }
 
@@ -106,9 +106,7 @@ public User login(String email, char[] password) {
    * @return the User object if found, null otherwise
    */
   public User getUserByEmail(String email) {
-    if (email == null || email.trim().isEmpty()) {
-      return null;
-    }
+    ParameterValidator.stringChecker(email, "email");
     return this.userRepository.findUserByEmail(email);
   }
 }
diff --git a/src/main/java/edu/group5/app/utils/ParameterValidator.java b/src/main/java/edu/group5/app/utils/ParameterValidator.java
index 3bc3e5a..28a8dc0 100644
--- a/src/main/java/edu/group5/app/utils/ParameterValidator.java
+++ b/src/main/java/edu/group5/app/utils/ParameterValidator.java
@@ -24,7 +24,7 @@ public final class ParameterValidator {
   public static final void stringChecker(String stringArg, String variableName)
       throws IllegalArgumentException {
     nullCheck(stringArg, variableName);
-    if (stringArg.isBlank()) {
+    if (stringArg.trim().isBlank()) {
       throw new IllegalArgumentException(String.format("%s can't be blank", variableName));
     }
   }
@@ -104,7 +104,7 @@ public static final void exportChecker(
     if (!data.isEmpty()) {
       if (data.stream().anyMatch(i -> i.length != rowLength)) {
         throw new IllegalArgumentException(
-          String.format("%s's arrays must have a length of 6", dataName)
+          String.format("%s's arrays must have a length of %d", dataName, rowLength)
         );
       }
       if (data.stream().anyMatch(i -> Arrays.asList(i).contains(null))) {
diff --git a/src/main/java/edu/group5/app/view/causespage/CausesPageView.java b/src/main/java/edu/group5/app/view/causespage/CausesPageView.java
index 7db3ee9..346df19 100644
--- a/src/main/java/edu/group5/app/view/causespage/CausesPageView.java
+++ b/src/main/java/edu/group5/app/view/causespage/CausesPageView.java
@@ -1,6 +1,7 @@
 package edu.group5.app.view.causespage;
 
 import edu.group5.app.model.organization.Organization;
+import edu.group5.app.utils.ParameterValidator;
 import edu.group5.app.control.NavigationController;
 import edu.group5.app.control.OrganizationController;
 import edu.group5.app.model.AppState;
@@ -41,6 +42,10 @@ public class CausesPageView extends BorderPane {
   private Map<Integer, Organization> allOrganizations;
 
   public CausesPageView(AppState appState, NavigationController nav, OrganizationController orgController) {
+    ParameterValidator.objectChecker(appState, "AppState");
+    ParameterValidator.objectChecker(nav, "NavigationController");
+    ParameterValidator.objectChecker(orgController, "OrganizationController");
+
     this.appState = appState;
     this.nav = nav;
     this.orgController = orgController;
diff --git a/src/main/java/edu/group5/app/view/causespage/OrganizationCard.java b/src/main/java/edu/group5/app/view/causespage/OrganizationCard.java
index 2ca30bb..c24b32a 100644
--- a/src/main/java/edu/group5/app/view/causespage/OrganizationCard.java
+++ b/src/main/java/edu/group5/app/view/causespage/OrganizationCard.java
@@ -3,6 +3,7 @@
 import edu.group5.app.control.NavigationController;
 import edu.group5.app.model.AppState;
 import edu.group5.app.model.organization.Organization;
+import edu.group5.app.utils.ParameterValidator;
 import javafx.geometry.Pos;
 import javafx.scene.image.Image;
 import javafx.scene.image.ImageView;
diff --git a/src/main/java/edu/group5/app/view/donationpage/DonationPageView.java b/src/main/java/edu/group5/app/view/donationpage/DonationPageView.java
index 9444421..61ca29c 100644
--- a/src/main/java/edu/group5/app/view/donationpage/DonationPageView.java
+++ b/src/main/java/edu/group5/app/view/donationpage/DonationPageView.java
@@ -3,6 +3,7 @@
 import edu.group5.app.control.DonationController;
 import edu.group5.app.control.NavigationController;
 import edu.group5.app.model.AppState;
+import edu.group5.app.utils.ParameterValidator;
 import javafx.geometry.Insets;
 import javafx.geometry.Pos;
 import javafx.scene.control.TextField;
@@ -37,6 +38,10 @@ public class DonationPageView extends BorderPane {
   private Button donateBtn;
 
   public DonationPageView(AppState appState, NavigationController nav, DonationController donationController) {
+    ParameterValidator.objectChecker(appState, "AppState");
+    ParameterValidator.objectChecker(nav, "NavigationController");
+    ParameterValidator.objectChecker(donationController, "DonationController");
+
     this.appState = appState;
     this.nav = nav;
     this.donationController = donationController;