Multi-stream: merge two sources and let one win on a business key

Two feeds carrying the same entity need a precedence rule, not a distinct(). toMap's merge function is where that rule belongs - and it is evaluated in stream order, so the first source wins by construction.

Code

record Contact(String email, String name, String source) {}

var crm = List.of(new Contact("ava@x.io", "Ava", "crm"),
                  new Contact("ben@x.io", "Ben", "crm"));
var web = List.of(new Contact("ben@x.io", "Ben R", "web"),
                  new Contact("cara@x.io", "Cara", "web"));

var merged = Stream.concat(crm.stream(), web.stream())
        .collect(Collectors.toMap(Contact::email, c -> c,
                                  (first, second) -> first,   // CRM was streamed first
                                  TreeMap::new));

merged.values().forEach(c ->
        System.out.println(c.email() + " " + c.name() + " (" + c.source() + ")"));

Output

ava@x.io Ava (crm)
ben@x.io Ben (crm)
cara@x.io Cara (web)