flatten_nested_map.java

import java.util.*;
import java.util.stream.Collectors;

class Main {
    static record Record(String asin, String sid, String wid) {}

    public static void main(String[] args) {
        var m =
            Map.ofEntries(
                Map.entry("k1", List.of(new Record("a1", "s1", "w1"), new Record("a2", "s1", "w1"))),
                Map.entry("k2", List.of(new Record("a1", "s2", "w1"), new Record("a2", "s2", "w1"))),
                Map.entry("k3", List.of(new Record("a1", "s3", "w1"), new Record("a2", "s3", "w1")))
            );

        var s = m.values().stream().
          flatMap(Collection::stream).
          map(Record::asin).
          collect(Collectors.toSet());

        System.out.println(s);

    }
}

two_level_groupBy.java

import java.util.List;
import java.util.stream.Collectors;

class Main {
    static record Record(String asin, String sid, String wid) {}

    public static void main(String[] args) {
        var l = List.of(
            new Record("a1", "s1", "w1"),
            new Record("a1", "s1", "w2"),
            new Record("a1", "s1", "w3"),

            new Record("a1", "s2", "w1"),
            new Record("a1", "s2", "w2"),
            new Record("a1", "s2", "w3"),

            new Record("a2", "s1", "w1"),
            new Record("a2", "s1", "w2"),
            new Record("a2", "s1", "w3"),

            new Record("a2", "s2", "w1"),
            new Record("a2", "s2", "w2"),
            new Record("a2", "s2", "w3")
        );

        var grp = l.stream()
            .collect(
                Collectors.groupingBy(
                    Record::sid,
                    Collectors.groupingBy(
                        Record::wid,
                        Collectors.mapping(Record::asin, Collectors.toSet())
                    )
                )
            );

        System.out.println(grp);
    }
}