Multi-stream: co-group two feeds that each know only their own half

Clicks know nothing about purchases and vice versa. Index both, take the distinct union of keys, and read each side with a zero default - the pages that appear in only one feed are usually the interesting ones.

Code

record Click(String page, int n) {}
record Buy(String page, int n) {}

var clicks = List.of(new Click("/a", 100), new Click("/b", 40));
var buys = List.of(new Buy("/a", 5), new Buy("/c", 2));

Map<String, Integer> c = clicks.stream().collect(Collectors.toMap(Click::page, Click::n));
Map<String, Integer> b = buys.stream().collect(Collectors.toMap(Buy::page, Buy::n));

Stream.concat(c.keySet().stream(), b.keySet().stream())
      .distinct()
      .sorted()
      .forEach(p -> System.out.println(p + " clicks=" + c.getOrDefault(p, 0)
              + " buys=" + b.getOrDefault(p, 0)));

Output

/a clicks=100 buys=5
/b clicks=40 buys=0
/c clicks=0 buys=2