Data Clumps — Tasks¶

Eight exercises, increasing in difficulty. Each one asks you to detect, justify, refactor, and verify.

Work through them in order. The validation table at the bottom tells you whether your solution is acceptable. One worked solution sketch is included at the end to anchor expectations.

How to attempt each exercise¶

For every task, deliver:

1. Detection — name the clump, list the types and the methods where PRC >= 3.
2. Refactor — extract one or more VOs, prefer records (JEP 395), enforce invariants in the compact constructor.
3. Tests — at least one test that would fail before the refactor and pass after.
4. Architectural guard — an ArchUnit rule or comment explaining how you would prevent regression.

Exercise 1 — The textbook trio¶

public class PricingService {
    public BigDecimal taxFor(BigDecimal amount, String currency, String country) { ... }
    public BigDecimal discountFor(BigDecimal amount, String currency, String customerTier) { ... }
    public Receipt charge(BigDecimal amount, String currency, String accountId) { ... }
}

Extract the obvious VO. Bonus: explain why country, customerTier, and accountId are not part of the same clump.

Exercise 2 — Two clumps in one class¶

public class Trip {
    LocalDate startDate;
    LocalDate endDate;
    double originLat;
    double originLng;
    double destinationLat;
    double destinationLng;
}

Identify both clumps. Extract appropriate VOs with the relevant invariants. State the invariants explicitly in the compact constructors.

Exercise 3 — Parallel collections¶

public class GradeBook {
    private List<String> studentIds = new ArrayList<>();
    private List<String> courseIds = new ArrayList<>();
    private List<Integer> scores = new ArrayList<>();

    public void record(String studentId, String courseId, int score) {
        studentIds.add(studentId);
        courseIds.add(courseId);
        scores.add(score);
    }
}

Refactor to eliminate the parallel-list anti-pattern. Add a test that demonstrates the prior bug class (inconsistent list lengths) cannot occur.

Exercise 4 — JPA embeddable¶

Given:

@Entity
public class Customer {
    @Id Long id;
    String shippingStreet;
    String shippingCity;
    String shippingZip;
    String shippingCountry;
    String billingStreet;
    String billingCity;
    String billingZip;
    String billingCountry;
}

Extract an Address @Embeddable (or a record-backed embeddable on Hibernate 6.2+). Migrate the schema if your DB supports ALTER ... ADD CHECK; otherwise document the migration plan. Demonstrate that loading and saving still works through a JUnit integration test or Testcontainers setup.

Exercise 5 — The currency mismatch bug¶

Build on Exercise 1. Add a test that calls taxFor with two different currencies in the same business operation:

service.charge(new BigDecimal("100"), "USD", "acct-1");
service.refund(new BigDecimal("100"), "EUR", "acct-1");

Make this test fail by extending the Money VO with a convert(ExchangeRate) method, and force callers through it.

Exercise 6 — Stringly-typed time window¶

Given:

public List<Event> search(String fromIso, String toIso, String timezone) { ... }

Replace with a TimeWindow(ZonedDateTime from, ZonedDateTime to) VO. Add a static parse(String fromIso, String toIso, String zone) factory. Show in tests that a daylight-saving boundary now produces deterministic results.

Exercise 7 — ArchUnit guard¶

Pick the codebase you used for any of the previous exercises. Write an ArchUnit rule that fails the build if any method in the domain package accepts (BigDecimal, Currency) as separate parameters. Run it. Confirm it fires on the unrefactored code and passes after refactor.

Exercise 8 — Production case: a half-anemic VO¶

public record Address(String street, String city, String postalCode, String country) {}

This record is immutable and value-equal, yet anemic. Add three pieces of behavior that justify its existence as a VO rather than a struct:

1. A factory or compact-constructor invariant that validates country against ISO-3166 alpha-2.
2. A formatting method formatPostalLabel() returning a multi-line string suitable for printing.
3. A withCountry(String) method that returns a new instance (and a test for non-mutation).

State explicitly which DDD principle each addition supports.

Validation table¶

If your solution misses an item from the "must contain" column, revise before moving on.

Worked solution sketch — Exercise 2¶

Detection. Method-level PRC is low (the class itself is small), but the field-group PRC across Trip, the TripRepository.findByOriginCity, and the RouteCalculator.distance methods is >= 3 for the (double lat, double lng) tuple. Similarly, (LocalDate start, LocalDate end) appears in Trip, Booking, and Invoice.

Refactor.

public record Latitude(double value) {
    public Latitude { if (value < -90 || value > 90) throw new IllegalArgumentException(); }
}

public record Longitude(double value) {
    public Longitude { if (value < -180 || value > 180) throw new IllegalArgumentException(); }
}

public record GeoPoint(Latitude lat, Longitude lng) {
    public GeoPoint {
        Objects.requireNonNull(lat);
        Objects.requireNonNull(lng);
    }
    public double distanceTo(GeoPoint other) { /* Haversine */ }
}

public record DateRange(LocalDate start, LocalDate end) {
    public DateRange {
        Objects.requireNonNull(start);
        Objects.requireNonNull(end);
        if (end.isBefore(start)) throw new IllegalArgumentException();
    }
    public long days() { return ChronoUnit.DAYS.between(start, end) + 1; }
}

public class Trip {
    DateRange when;
    GeoPoint origin;
    GeoPoint destination;
}

Tests.

@Test void rejects_swapped_dates() {
    assertThrows(IllegalArgumentException.class,
        () -> new DateRange(LocalDate.of(2026, 5, 20), LocalDate.of(2026, 5, 19)));
}

@Test void latitude_out_of_range() {
    assertThrows(IllegalArgumentException.class, () -> new Latitude(91.0));
}

Architectural guard. ArchUnit rule:

@ArchTest
static final ArchRule no_lat_lng_pairs =
    noMethods().that().areDeclaredInClassesThat().resideInAPackage("..domain..")
        .should().haveRawParameterTypes(double.class, double.class);

False positives (e.g., a Range(double, double) for non-geographic values) are silenced by class-level allow-list, not by weakening the rule.

What's next¶

• ../07-primitive-obsession/tasks.md — overlapping exercises focused on type-level errors.
• ../../06-anemic-domain-model/tasks.md — exercises pushing behavior onto extracted VOs.
• ./interview.md — twenty interview-style questions to consolidate the topic.