docs: public static <K, V, T extends Map<K, V>> T unmarshallMap(ObjectInput in, MapBuilder<K, V, T> builder) throws IOException, ClassNotFoundException {
        final int size = unmarshallSize(in);
        if (size == NULL_VALUE) {
           return null;
        }
        final T map = Objects.requireNonNull(builder, "MapBuilder must be non-null").build(size);
        for (int i = 0; i < size; i++) //noinspection unchecked
           map.put((K) in.readObject(), (V) in.readObject());
        return map;
     }