FraudService solution has NPE and also doesn't aggregate first order in the method simpleMerge

[entrpn]

The solution for the FraudService causes an NPE in the aggregate because total.getValue() could be null.

https://github.com/confluentinc/examples/blob/5.3.1-post/microservices-orders/exercises/FraudService.java

Solution:

        final KTable<Windowed<Long>, OrderValue> aggregate = orders
                .groupBy((id, order) -> order.getCustomerId(), Grouped.with(Serdes.Long(), Schemas.Topics.ORDERS.getValueSerde()))
                .windowedBy(SessionWindows.with(Duration.ofHours(1)))
                .aggregate(OrderValue::new,
                        (custId, order, total) -> {
                            return new OrderValue(order, (total.getValue() == null ? 0 : total.getValue()) + order.getQuantity() * order.getPrice());
                        },
                        (k, a, b) -> simpleMerge(a, b),
                        Materialized.with(null, new JsonSerdes<OrderValue>(OrderValue.class)));

The simpleMerge also doesn't aggregate the first order because it sets the value to 0 if a is null.

  private OrderValue simpleMerge(final OrderValue a, final OrderValue b) {
    return new OrderValue(b.getOrder(), (a == null ? 0D : a.getValue()) + b.getValue());
  }

should be:

    private OrderValue simpleMerge(final OrderValue a, final OrderValue b) {
        return new OrderValue(b.getOrder(), ((a == null || a.getValue() == null) ? b.getValue() : a.getValue() + b.getValue()));
    }