[RED] Added UserService Test

src/test/java/com/safetypin/authentication/service/UserServiceTest.java

+package com.safetypin.authentication.service;
+
+import com.safetypin.authentication.model.User;
+import com.safetypin.authentication.repository.UserRepository;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+
+import java.util.Optional;
+import java.util.UUID;
+
+import static org.junit.jupiter.api.Assertions.*;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.*;
+
+@ExtendWith(MockitoExtension.class)
+class UserServiceTest {
+
+    @Mock
+    private UserRepository userRepository;
+
+    private UserService userService;
+
+    @BeforeEach
+    void setUp() {
+        userService = new UserService(userRepository);
+    }
+
+    @Test
+    void testFindById_WhenUserExists_ReturnUser() {
+        // Arrange
+        UUID userId = UUID.randomUUID();
+        User expectedUser = new User();
+        expectedUser.setId(userId);
+
+        when(userRepository.findById(userId)).thenReturn(Optional.of(expectedUser));
+
+        // Act
+        Optional<User> result = userService.findById(userId);
+
+        // Assert
+        assertTrue(result.isPresent());
+        assertEquals(expectedUser, result.get());
+        verify(userRepository).findById(userId);
+    }
+
+    @Test
+    void testFindById_WhenUserDoesNotExist_ReturnEmptyOptional() {
+        // Arrange
+        UUID userId = UUID.randomUUID();
+        when(userRepository.findById(userId)).thenReturn(Optional.empty());
+
+        // Act
+        Optional<User> result = userService.findById(userId);
+
+        // Assert
+        assertFalse(result.isPresent());
+        verify(userRepository).findById(userId);
+    }
+
+    @Test
+    void testFindByEmail_WhenUserExists_ReturnUser() {
+        // Arrange
+        String email = "test@example.com";
+        User expectedUser = new User();
+        expectedUser.setEmail(email);
+
+        when(userRepository.findByEmail(email)).thenReturn(expectedUser);
+
+        // Act
+        Optional<User> result = userService.findByEmail(email);
+
+        // Assert
+        assertTrue(result.isPresent());
+        assertEquals(expectedUser, result.get());
+        verify(userRepository).findByEmail(email);
+    }
+
+    @Test
+    void testFindByEmail_WhenUserDoesNotExist_ReturnEmptyOptional() {
+        // Arrange
+        String email = "nonexistent@example.com";
+        when(userRepository.findByEmail(email)).thenReturn(null);
+
+        // Act
+        Optional<User> result = userService.findByEmail(email);
+
+        // Assert
+        assertFalse(result.isPresent());
+        verify(userRepository).findByEmail(email);
+    }
+
+    @Test
+    void testSave_ShouldCallRepositorySaveAndReturnUser() {
+        // Arrange
+        User userToSave = new User();
+        userToSave.setEmail("test@example.com");
+
+        when(userRepository.save(any(User.class))).thenReturn(userToSave);
+
+        // Act
+        User savedUser = userService.save(userToSave);
+
+        // Assert
+        assertNotNull(savedUser);
+        assertEquals(userToSave, savedUser);
+        verify(userRepository).save(userToSave);
+    }
+
+    @Test
+    void testConstructor_InitializesRepositoryCorrectly() {
+        // Arrange & Act
+        UserService service = new UserService(userRepository);
+
+        // Assert
+        assertNotNull(service);
+
+        // Verify that the repository is correctly initialized by calling a method
+        UUID randomId = UUID.randomUUID();
+        service.findById(randomId);
+        verify(userRepository).findById(randomId);
+    }
+}
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