java 30 lines · 5 steps

Grouping streams with Java Collectors

Four flavors of Collectors.groupingBy show how downstream collectors reshape each group.

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Intermediate streams grouping collectors functional aggregation

1public Map<Long, List<Order>> ordersByCustomer(List<Order> orders) {
2    return orders.stream()
3            .collect(Collectors.groupingBy(Order::getCustomerId));
4}
5 
6public Map<Long, BigDecimal> totalSpentByCustomer(List<Order> orders) {
7    return orders.stream()
8            .filter(Order::isPaid)
9            .collect(Collectors.groupingBy(
10                    Order::getCustomerId,
11                    Collectors.reducing(
12                            BigDecimal.ZERO,
13                            Order::getTotal,
14                            BigDecimal::add)));
15}
16 
17public Map<Long, Long> orderCountByCustomer(List<Order> orders) {
18    return orders.stream()
19            .collect(Collectors.groupingBy(
20                    Order::getCustomerId,
21                    TreeMap::new,
22                    Collectors.counting()));
23}
24 
25public Map<OrderStatus, List<String>> orderNumbersByStatus(List<Order> orders) {
26    return orders.stream()
27            .collect(Collectors.groupingBy(
28                    Order::getStatus,
29                    Collectors.mapping(Order::getOrderNumber, Collectors.toList())));
30}

01 / 01

STEP 01

Walkthrough

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Three takeaways

1The classifier function of groupingBy determines the map's keys, one bucket per distinct value.
2A downstream collector transforms each group from a raw list into a count, sum, or projected list.
3A supplier argument lets you control the concrete map type, such as a sorted TreeMap.

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