diff --git a/src/test/java/com/linkurlshorter/urlshortener/user/UserServiceTest.java b/src/test/java/com/linkurlshorter/urlshortener/user/UserServiceTest.java
index 24ed572..5bb3aea 100644
--- a/src/test/java/com/linkurlshorter/urlshortener/user/UserServiceTest.java
+++ b/src/test/java/com/linkurlshorter/urlshortener/user/UserServiceTest.java
@@ -5,13 +5,13 @@
 import org.junit.jupiter.api.extension.ExtendWith;
 import org.mockito.InjectMocks;
 import org.mockito.Mock;
-import org.mockito.Mockito;
 import org.mockito.junit.jupiter.MockitoExtension;
 
 import java.util.Optional;
 import java.util.UUID;
 
-import static org.junit.jupiter.api.Assertions.*;
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
 import static org.mockito.ArgumentMatchers.*;
 import static org.mockito.Mockito.*;
 
@@ -36,7 +36,7 @@ class UserServiceTest {
      */
     @BeforeEach
     void setUp() {
-       user = User.builder()
+        user = User.builder()
                 .id(UUID.fromString("84991c79-f6a9-4b7b-b1b4-0d66c0b92c81"))
                 .email("test1@gmail.com")
                 .password("password1")
@@ -49,27 +49,45 @@ void setUp() {
      */
     @Test
     void saveTest() {
-        when(userRepository.save(Mockito.any(User.class))).thenReturn(user);
+        when(userRepository.save(any(User.class))).thenReturn(user);
         User savedUser = userService.save(user);
 
-        assertNotNull(savedUser);
-        assertEquals(user, savedUser);
+        assertThat(savedUser).isNotNull().isEqualTo(user);
         verify(userRepository, times(1)).save(user);
     }
 
+    /**
+     * Test case for the {@link UserService#save(User)} method when the user is null.
+     */
+    @Test
+    void saveNullTest() {
+        when(userRepository.save(any(User.class))).thenReturn(null);
+        assertThatThrownBy(() -> userService.save(null))
+                .isInstanceOf(NullUserPropertyException.class);
+    }
+
     /**
      * Test case for the {@link UserService#update(User)} method.
      */
     @Test
     void updateTest() {
-        when(userRepository.save(Mockito.any(User.class))).thenReturn(user);
+        when(userRepository.save(any(User.class))).thenReturn(user);
         User updatedUser = userService.update(user);
 
-        assertNotNull(updatedUser);
-        assertEquals(user, updatedUser);
+        assertThat(updatedUser).isNotNull().isEqualTo(user);
         verify(userRepository, times(1)).save(user);
     }
 
+    /**
+     * Test case for the {@link UserService#update(User)} method when the user is null.
+     */
+    @Test
+    void updateNullTest() {
+        when(userRepository.save(any(User.class))).thenReturn(null);
+        assertThatThrownBy(() -> userService.update(null))
+                .isInstanceOf(NullUserPropertyException.class);
+    }
+
     /**
      * Test case for the {@link UserService#updateByEmailDynamically(User, String)} method.
      */
@@ -78,7 +96,7 @@ void updateByEmailDynamicallyTest() {
         when(userRepository.updateUserByEmailDynamically(any(User.class), eq(user.getEmail()))).thenReturn(1);
         int updatedRecord = userService.updateByEmailDynamically(user, user.getEmail());
 
-        assertEquals(1, updatedRecord);
+        assertThat(updatedRecord).isEqualTo(1);
         verify(userRepository, times(1)).updateUserByEmailDynamically(user, user.getEmail());
     }
 
@@ -87,9 +105,8 @@ void updateByEmailDynamicallyTest() {
      */
     @Test
     void updateByEmailDynamicallyIsNullTest() {
-        assertThrows(NullEmailException.class, () -> {
-            userService.updateByEmailDynamically(user, null);
-        });
+        assertThatThrownBy(() -> userService.updateByEmailDynamically(user, null))
+                .isInstanceOf(NullEmailException.class);
     }
 
     /**
@@ -100,10 +117,18 @@ void findByIdTest() {
         when(userRepository.findById(user.getId())).thenReturn(Optional.of(user));
         User foundUser = userService.findById(user.getId());
 
-        assertEquals(user, foundUser);
+        assertThat(foundUser).isNotNull().isEqualTo(user);
         verify(userRepository, times(1)).findById(user.getId());
     }
 
+    /**
+     * Test case for the {@link UserService#findById(UUID)} method when the provided id is null.
+     */
+    @Test
+    void findByIdNullTest() {
+        assertThatThrownBy(() -> userService.findById(null))
+                .isInstanceOf(NullUserPropertyException.class);
+    }
     /**
      * Test case for the {@link UserService#findById(UUID)} method
      * when the user with provided id does not exist.
@@ -113,7 +138,8 @@ void findByIdNotFoundTest() {
         UUID nonExistentUserId = UUID.randomUUID();
 
         when(userRepository.findById(nonExistentUserId)).thenReturn(Optional.empty());
-        assertThrows(NoUserFoundByIdException.class, () -> userService.findById(nonExistentUserId));
+        assertThatThrownBy(() -> userService.findById(nonExistentUserId))
+                .isInstanceOf(NoUserFoundByIdException.class);
     }
 
     /**
@@ -124,10 +150,19 @@ void findByEmailTest() {
         when(userRepository.findByEmail(user.getEmail())).thenReturn(Optional.of(user));
         User foundUser = userService.findByEmail(user.getEmail());
 
-        assertEquals(user, foundUser);
+        assertThat(foundUser).isNotNull().isEqualTo(user);
         verify(userRepository, times(1)).findByEmail(user.getEmail());
     }
 
+    /**
+     * Test case for the {@link UserService#findByEmail(String)} method when the provided email is null.
+     */
+    @Test
+    void findByEmailNullTest() {
+        assertThatThrownBy(() -> userService.findByEmail(null))
+                .isInstanceOf(NullUserPropertyException.class);
+    }
+
     /**
      * Test case for the {@link UserService#findByEmail(String)} method
      * when the user with provided email does not exist.
@@ -137,7 +172,8 @@ void findByEmailNotFoundTest() {
         String nonExistentUserEmail = "nonexistent@gmail.com";
 
         when(userRepository.findByEmail(nonExistentUserEmail)).thenReturn(Optional.empty());
-        assertThrows(NoUserFoundByEmailException.class, () -> userService.findByEmail(nonExistentUserEmail));
+        assertThatThrownBy(() -> userService.findByEmail(nonExistentUserEmail))
+                .isInstanceOf(NoUserFoundByEmailException.class);
     }
 
     /**
@@ -146,6 +182,15 @@ void findByEmailNotFoundTest() {
     @Test
     void deleteByIdTest() {
         userService.deleteById(user.getId());
-        verify(userRepository, Mockito.times(1)).deleteById(user.getId());
+        verify(userRepository, times(1)).deleteById(user.getId());
+    }
+
+    /**
+     * Test case for the {@link UserService#deleteById(UUID)} method when the provided id is null.
+     */
+    @Test
+    void deleteByIdNullTest() {
+        assertThatThrownBy(() -> userService.deleteById(null))
+                .isInstanceOf(NullUserPropertyException.class);
     }
 }