Multi-stream: filter a feed by a rule list loaded at runtime

When the predicate itself comes from data, allMatch over the rule stream is the filter. A Map of field name to accessor keeps the rules declarative instead of a switch that has to be edited for every new field.

Code

record Rule(String field, String equalTo) {}
record Event(String type, String user, String region) {}

var rules = List.of(new Rule("type", "purchase"), new Rule("region", "east"));
var events = List.of(new Event("purchase", "u1", "east"),
                     new Event("purchase", "u2", "west"),
                     new Event("view", "u3", "east"));

Map<String, Function<Event, String>> readers =
        Map.of("type", Event::type, "user", Event::user, "region", Event::region);

var matched = events.stream()
        .filter(e -> rules.stream().allMatch(r -> readers.get(r.field()).apply(e).equals(r.equalTo())))
        .toList();

System.out.println("rules  : " + rules.size());
System.out.println("matched: " + matched);

Output

rules  : 2
matched: [Event[type=purchase, user=u1, region=east]]