ComboBoxUI ui = comboBox.getUI();
// Verify that ui paints a rounded rect (per spec)
Graphics mockGraphics = Mockito.mock(Graphics.class, Mockito.RETURNS_MOCKS);
ui.paint(mockGraphics, comboBox);
Mockito.verify(mockGraphics).fillRoundRect(
Mockito.anyInt(), Mockito.anyInt(), Mockito.anyInt(),Mockito.anyInt(),Mockito.anyInt(),Mockito.anyInt());
}