Functional Programming in Java, Second Edition: Functional Programming in Java, Second Edition: JUnit code improvements for Chapter 11, pages 193 ff “Refactoring Data Grouping Operations”

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());
    }
}