java 30 lines · 5 steps

Grouping streams with Java Collectors

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

Explained by highlit
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

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

Related explainers

java 
public interface GitHubClient {
 
    @GetExchange("/users/{username}")
    GitHubUser getUser(@PathVariable String username);

Declarative HTTP clients in Spring

http-client declarative-api proxy
Intermediate 8 steps
java 
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

Building a trie for autocomplete in Java

trie prefix-tree recursion
Intermediate 8 steps
java 
@RestController
@RequestMapping("/api/products")
public class ProductSearchController {
 

Binding collection query params in Spring

rest-api query-parameters dependency-injection
Intermediate 6 steps
java 
import java.util.ArrayDeque;
import java.util.Deque;
import java.util.Map;
 

Evaluating math expressions with two stacks

stacks parsing operator-precedence
Intermediate 9 steps
java 
@Entity
@Table(name = "orders")
@SQLDelete(sql = "UPDATE orders SET deleted = true, deleted_at = now() WHERE id = ?")
@Where(clause = "deleted = false")

Soft deletes with Hibernate in Spring

soft-delete jpa hibernate
Intermediate 9 steps
php 
<?php
 
namespace App\Services\Reports;
 

Streaming a monthly revenue CSV in Laravel

csv-export lazy-collections eloquent-query
Intermediate 8 steps

Share this explainer

Here's the card — post it anywhere.

Grouping streams with Java Collectors — share card
Made with highlit — turn any snippet into a walkthrough like this in about a minute.
Explain your code