UserService mockUserService = mock(UserService.class); when(mockUserService.getUserById(1L)).thenReturn(new User("John")); @Test public void testGetUserById() { UserService mockUserService = mock(UserService.class); when(mockUserService.getUserById(1L)).thenReturn(new User("John")); User user = mockUserService.getUserById(1L); assertEquals("John", user.getName()); }