@Override
    public final Set<NAME> names() {
        if (isEmpty()) {
            return ImmutableSet.of();
        }
        final ImmutableSet.Builder<NAME> builder = ImmutableSet.builder();
        Entry e = firstGroupHead.after;
        while (e != firstGroupHead) {
            builder.add(e.getKey());
            e = e.after;
        }
        return builder.build();
    }
Validates the given user-specified {@code value}.
core/src/main/java/com/linecorp/armeria/common/StringMultimap.java