Why can a final reference still change?

Goal & mental model verified

Separate primitive values from object references. An interface states a contract; a class implements behavior. Records express data aggregates with final component fields; they do not make referenced collections deeply immutable.

Worked example · illustrative, not executed synthesis

record QuoteId(String value) {}
record Quote(QuoteId id, java.util.List<String> covers) {
  Quote { covers = java.util.List.copyOf(covers); }
}
// The defensive copy prevents external list mutation.

Engineering decision synthesis

Use composition for replaceable pricing behavior and small value types for identifiers. This clarifies invariants at the cost of more explicit conversions.

Pitfall & diagnosis synthesis

A final reference can point to a mutable object. Value equality also needs a compatible hashCode; identity equality with == answers a different question.

Improve & validate synthesis

Move invalid-state checks into construction boundaries. Test equality, null handling and defensive copies where the business invariant depends on them.