public void testEquals() {
        YIntervalSeriesCollection c1 = new YIntervalSeriesCollection();
        YIntervalSeriesCollection c2 = new YIntervalSeriesCollection();
        assertEquals(c1, c2);

        // add a series
        YIntervalSeries s1 = new YIntervalSeries("Series");
        s1.add(1.0, 1.1, 1.2, 1.3);
        c1.addSeries(s1);
        assertFalse(c1.equals(c2));
        YIntervalSeries s2 = new YIntervalSeries("Series");
        s2.add(1.0, 1.1, 1.2, 1.3);
        c2.addSeries(s2);
        assertTrue(c1.equals(c2));

        // add an empty series
        c1.addSeries(new YIntervalSeries("Empty Series"));
        assertFalse(c1.equals(c2));
        c2.addSeries(new YIntervalSeries("Empty Series"));
        assertTrue(c1.equals(c2));