As usual, but the original code to be refactored was just too evil and had to be fixed.

package chapter11;

import org.junit.jupiter.api.Test;

import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;

import static java.util.stream.Collectors.groupingBy;
import static java.util.stream.Collectors.toSet;
import static org.junit.jupiter.api.Assertions.assertAll;
import static org.junit.jupiter.api.Assertions.assertEquals;

public class DataGroupingOperationsTest {

    interface Scores {

        Map<Integer, Set<String>> namesForScores(final Map<String, Integer> scores);

    }

    public static class ScoresBefore implements Scores {

        public Map<Integer, Set<String>> namesForScores(final Map<String, Integer> scores) {
            final Map<Integer, Set<String>> result = new HashMap<>();
            for (Map.Entry<String, Integer> entry : scores.entrySet()) {
                final String name = entry.getKey();
                final Integer score = entry.getValue();
                if (!result.containsKey(score)) {
                    result.put(score, new HashSet<>());
                }
                result.get(score).add(name);
            }
            return result;
        }
    }

    public static class ScoresAfter implements Scores {

        public Map<Integer, Set<String>> namesForScores(final Map<String, Integer> scores) {
            return scores.keySet().stream()
                    .collect(groupingBy(scores::get, toSet()));
        }
    }

    private static void commonNamesForScoresTests(final Scores scores) {
        assertAll(
                () -> assertEquals(Map.of(), scores.namesForScores(Map.of())),
                () -> assertEquals(
                        Map.of(1, Set.of("Jill")), scores.namesForScores(Map.of("Jill", 1))),
                () -> assertEquals(
                        Map.of(1, Set.of("Jill"), 2, Set.of("Paul")),
                        scores.namesForScores(Map.of("Jill", 1, "Paul", 2))),
                () -> assertEquals(
                        Map.of(1, Set.of("Jill", "Kate"), 2, Set.of("Paul")),
                        scores.namesForScores(Map.of("Jill", 1, "Paul", 2, "Kate", 1)))
        );
    }

    @Test
    void scoresBefore() {
        commonNamesForScoresTests(new ScoresBefore());
    }

    @Test
    void scoresAfter() {
        commonNamesForScoresTests(new ScoresAfter());
    }
}