Domain Modeling Essentials

Value objects with invariants, an entity with behaviour, and a sealed result type.

import java.util.*;

// VALUE OBJECT — immutable, validated, compared by value.
public record Money(long minor, String currency) {
  public Money {
    if (minor < 0) throw new IllegalArgumentException("negative money");
    Objects.requireNonNull(currency);
  }
  public Money plus(Money o) {
    requireSameCurrency(o);
    return new Money(Math.addExact(minor, o.minor), currency);
  }
  private void requireSameCurrency(Money o) {
    if (!currency.equals(o.currency)) throw new IllegalArgumentException("currency mismatch");
  }
}

public record TimeSlot(java.time.Instant start, java.time.Instant end) {
  public TimeSlot {
    if (!end.isAfter(start)) throw new IllegalArgumentException("end must be after start");
  }
  public boolean overlaps(TimeSlot o) { return start.isBefore(o.end) && o.start.isBefore(end); }
}

// ENTITY — identity + guarded state transitions ("tell, don't ask").
public final class Order {
  public enum Status { CREATED, PAID, SHIPPED, CANCELLED }

  private final UUID id;
  private final List<OrderLine> lines;
  private Status status = Status.CREATED;

  public Order(UUID id, List<OrderLine> lines) {
    if (lines.isEmpty()) throw new IllegalArgumentException("order needs a line");
    this.id = id;
    this.lines = List.copyOf(lines);          // defensive, immutable copy
  }

  public void markPaid() {
    if (status != Status.CREATED) throw new IllegalStateException("cannot pay in " + status);
    status = Status.PAID;
  }

  public void cancel() {
    if (status == Status.SHIPPED) throw new IllegalStateException("already shipped");
    status = Status.CANCELLED;
  }

  public Money total() {
    return lines.stream().map(OrderLine::subtotal)
        .reduce(new Money(0, "INR"), Money::plus);
  }

  @Override public boolean equals(Object o) { return o instanceof Order x && x.id.equals(id); }
  @Override public int hashCode() { return id.hashCode(); }
}

public record OrderLine(String sku, int qty, Money unitPrice) {
  public OrderLine { if (qty <= 0) throw new IllegalArgumentException("qty"); }
  public Money subtotal() { return new Money(unitPrice.minor() * qty, unitPrice.currency()); }
}

// SEALED RESULT — the compiler forces callers to handle every case.
public sealed interface ChargeResult {
  record Success(String txnId) implements ChargeResult {}
  record Declined(String reason) implements ChargeResult {}
  record Pending(String pollToken) implements ChargeResult {}
}
// String msg = switch (result) {
//   case ChargeResult.Success s  -> "ok " + s.txnId();
//   case ChargeResult.Declined d -> "declined: " + d.reason();
//   case ChargeResult.Pending p  -> "retry later";
// };