@Test
 public void testUserChange() {
   when(view.getUser()).thenReturn(USER);
   // emulates the presenter method executed from the UI.
   mainPanel.onUserChange();
   verify(view, times(1)).getUser();
   // the handler should have been invoked and collected the expected value.
   assertEquals(USER, user);
 }
 @Test
 public void testGetUser() {
   when(view.getUser()).thenReturn(USER);
   assertEquals(USER, mainPanel.getUser());
 }