@Override
            protected void writeTo(Output output, T message) throws IOException
            {
                final Collection<Object> existing = accessor.get(message);
                if (existing != null)
                    output.writeObject(number, existing, schema, false);
            }
For lazy initialization called by {@link RuntimeFieldFactory}.
protostuff-runtime/src/main/java/io/protostuff/runtime/RuntimeCollectionFieldFactory.java