@Test
public void testOnTabChange() {
final int ACTIVE_INDEX = 5;
TabChangeEvent event = mock(TabChangeEvent.class);
Tab tab = mock(Tab.class);
TabView parent = mock(TabView.class);
when(event.getTab()).thenReturn(tab);
when(tab.getParent()).thenReturn(parent);
when(parent.getActiveIndex()).thenReturn(ACTIVE_INDEX);
proband.onTabChange(event); // Test
assertEquals(ACTIVE_INDEX, proband.getActiveIndex());
}