Builder: defensive copies in build() stop later mutation

build() wraps the builder's own mutable list with List.copyOf so the finished object holds an independent snapshot. Continuing to mutate the builder afterwards has no effect on the object that was already built.

Code

class Team {
    private final List<String> members;
    private Team(List<String> members) { this.members = members; }
    List<String> members() { return members; }
    static class Builder {
        private final List<String> members = new ArrayList<>();
        Builder add(String name) { members.add(name); return this; }
        Team build() { return new Team(List.copyOf(members)); }
    }
}
Team.Builder builder = new Team.Builder().add("Ana").add("Ben");
Team team = builder.build();
builder.add("Cid");
System.out.println("Team after the builder kept mutating: " + team.members());

Output

Team after the builder kept mutating: [Ana, Ben]