feat[DonationRepository]: Add filterByUser() method and respective JU…

…nit test for it

src/main/java/edu/group5/app/model/donation/DonationRepository.java

@@ -154,9 +154,9 @@ public HashMap<Integer, Donation> sortByAmount() {
 
   /**
    * Returns all donations associated with a specific organization.
-   *
    * @param orgNumber the organization ID to filter by
    * @return a map containing all donations that belong to the given organization
+   * @throws IllegalArgumentException if the orgNumber is not positive
    */
   public HashMap<Integer, Donation> filterByOrganization(int orgNumber) {
     if (orgNumber <= 0) {
@@ -172,4 +172,24 @@ public HashMap<Integer, Donation> filterByOrganization(int orgNumber) {
             LinkedHashMap::new
         ));
   }
+
+  /**
+   * Returns all donations made by a specific user.
+   * @param userId the user ID to filter by
+   * @return a map containing all donations that belong to the given user
+   * @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");
+    }
+    return content.entrySet().stream()
+        .filter(entry -> entry.getValue().userId() == userId)
+        .collect(Collectors.toMap(
+            Map.Entry::getKey,
+            Map.Entry::getValue,
+            (e1, e2) -> e1,
+            LinkedHashMap::new
+        ));
+  }
 }

src/test/java/edu/group5/app/model/donation/DonationRepositoryTest.java

@@ -239,4 +239,28 @@ void filterByOrganizationHandlesDuplicateKeysWithLambda() {
     Map<Integer, Donation> filtered = repo.filterByOrganization(201);
     assertEquals(d1, filtered.get(1));
   }
+
+  @Test
+  void filterByUserIdMatches() {
+    repo.addDonation(donation1);
+    repo.addDonation(donation3);
+
+    Map<Integer, Donation> filtered = repo.filterByUser(102);
+    assertEquals(1, filtered.size());
+    assertTrue(filtered.containsKey(1));
+  }
+
+  @Test
+  void filterByUserIdNoMatch() {
+    repo.addDonation(donation1);
+    Map<Integer, Donation> filtered = repo.filterByUser(999);
+    assertTrue(filtered.isEmpty());
+  }
+
+  @Test
+  void filterByUserIdThrowsIfNegative() {
+    IllegalArgumentException ex = assertThrows(IllegalArgumentException.class,
+            () -> repo.filterByUser(0));
+    assertEquals("User ID must be positive", ex.getMessage());
+  }
 }