Immutability

An immutable class has final fields, no setters, defensive copies of mutable fields, and is declared final. Once created, its state never changes.

No, not without defensive copies. If a mutable field (like a List) is shared with external code, that code can modify the internal state, breaking immutability.

List.of() and List.copyOf() create truly immutable lists. Collections.unmodifiableList() is only an unmodifiable view – the underlying list can still change.

An immutable object has no mutable state, so there is nothing for threads to race on. The Java Memory Model guarantees final fields are visible to all threads after construction. No synchronization needed.