@Override
    public String toString() {
        return toStringHelper(this)
                .add("total", total)
                .add("start", start)
                .add("end", end)
                .toString();
    }
@Test
    public void toStringTest() {
        Counter tt = new Counter(100L, 300L, 200L);
        assertEquals("Counter{total=300, start=100, end=200}", tt.toString());
        Counter another = new Counter(200L, 500L, 300L);
        assertEquals("Counter{total=500, start=200, end=300}", another.toString());
    }