Maps: group records into a lookup index with toMap

Turning a list into a map keyed by id is the standard way to avoid repeated linear searches later.

Code

record Topic(int id, String title) {}
var topics = List.of(new Topic(1, "Streams"), new Topic(2, "Maps"), new Topic(3, "Collections"));

Map<Integer, Topic> byId = topics.stream()
        .collect(Collectors.toMap(Topic::id, Function.identity(), (a, b) -> a, TreeMap::new));

System.out.println(byId.get(2));
System.out.println("ids = " + byId.keySet());

Output

Topic[id=2, title=Maps]
ids = [1, 2, 3]

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Published 2026-07-30