Collectors deep-dive: groupingBy, toMap pitfalls, downstream collectors and writing your own.

record Order(String region, String product, BigDecimal total) {}

// revenue per region, per product — ONE pass
Map<String, Map<String, BigDecimal>> revenue = orders.stream()
    .collect(groupingBy(Order::region,
             groupingBy(Order::product,
             reducing(BigDecimal.ZERO, Order::total, BigDecimal::add))));

// top order per region — no sorting the whole stream
Map<String, Optional<Order>> top = orders.stream()
    .collect(groupingBy(Order::region,
             maxBy(comparing(Order::total))));

// toMap: ALWAYS decide the merge policy
Map<String, Order> byId = orders.stream()
    .collect(toMap(Order::product, o -> o, (a, b) -> a.total().compareTo(b.total()) >= 0 ? a : b));