@Override
    public String toString() {
        StringBuilder sb = new StringBuilder();
        sb.append('{');
        final AtomicBoolean first = new AtomicBoolean(true);
        forEach((item1, item2) -> {
            if (!first.getAndSet(false)) {
                sb.append(", ");
            }
            sb.append('[');
            sb.append(item1);
            sb.append(':');
            sb.append(item2);
            sb.append(']');
        });
        sb.append('}');
        return sb.toString();
    }
@Test
    public void testToString() {

        ConcurrentLongPairSet set = ConcurrentLongPairSet.newBuilder().build();

        set.add(0, 0);
        set.add(1, 1);
        set.add(3, 3);
        final String toString = "{[3:3], [0:0], [1:1]}";
        assertEquals(set.toString(), toString);
    }