Lesson 02

Immutable data and defensive copies

Protect domain snapshots from outside mutation by copying collections and exposing immutable views.

immutability
collections
defensive-copies

Good Code

src/main/java/dev/review/orders/OrderSnapshot.java

public record OrderSnapshot(OrderId id, List<LineItem> items) {
    public OrderSnapshot {
        Objects.requireNonNull(id, "id");
        // Copy the incoming list so outside code cannot mutate this snapshot.
        items = List.copyOf(items);
    }

    public Money total() {
        return items.stream()
            .map(LineItem::subtotal)
            .reduce(Money.zero(), Money::plus);
    }
}

Bad Code

src/main/java/OrderSnapshot.java

public class OrderSnapshot {
    private final List<LineItem> items;

    public OrderSnapshot(List<LineItem> items) {
        // This keeps the caller's mutable list reference.
        this.items = items;
    }

    public List<LineItem> items() {
        return items;
    }
}

Review Notes

What to review

Good Code

The good version copies the collection at construction time, so later caller mutation cannot change the snapshot. Its fields are final through the record shape.

Bad Code

The bad version stores and returns the same mutable list. Any code with a reference can add, remove, or reorder items after the object was created.

Takeaways

Objects that represent a completed decision should not change because a caller still holds a mutable collection reference.