What are the risks of using @Data with mutable collections?

A field like private List<String> tags; under @Data gets a plain getter that returns the actual internal list reference, not a defensive copy — meaning external code that calls the getter can mutate the object's internal state directly, bypassing any invariant the class was supposed to maintain.

@Data
public class Team {
    private List<String> members;
}

Team t = new Team(List.of("Ada"));
t.getMembers().add("Grace");   //  mutates Team's internal state from outside, if the list itself is mutable

This also interacts badly with @EqualsAndHashCode and HashSet/HashMap usage: if such an object is placed in a hash-based collection and then its internal collection field is mutated externally (changing its hash code), the object can become "lost" in that collection, unable to be found by a subsequent lookup even though it's still physically present. Defensive copying in the getter, or exposing an unmodifiable view, is something you'd have to add manually — Lombok doesn't do it for you.

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