@Override
    public String toString() {
        String toStringStr = "{}";
        switch (bitmapType) {
            case EMPTY:
                break;
            case SINGLE_VALUE:
                toStringStr = String.format("{%s}", singleValue);
                break;
            case BITMAP_VALUE:
                toStringStr = this.bitmap.toString();
                break;
            case SET_VALUE:
                toStringStr = String.format("{%s}", setToString());
                break;

        }
        return toStringStr;
    }
@Test
    public void testToString() {
        Assert.assertEquals(emptyBitmap.toString(), "{}");
        Assert.assertEquals(singleBitmap.toString(), "{1}");
        Assert.assertEquals(mediumBitmap.toString(), "{0,1,2,3,4,5,6,7,8,9}");
        Assert.assertEquals(largeBitmap.toString(), "{0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22," +
                "23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39}");
    }