@Override
    protected Class<?> mapClass(int id)
    {
        final RuntimeMapFactory factory = id < maps.size() ? maps.get(id) : null;
        if (factory == null)
            throw new UnknownTypeException("Unknown map id: " + id);

        return factory.typeClass();
    }
Register a {@link Delegate} and assign an id.
<p>
Delegate ids start at 1.
protostuff-runtime-registry/src/main/java/io/protostuff/runtime/IncrementalIdStrategy.java