@Override
            protected void writeTo(Output output, T message) throws IOException
            {
                final Collection<Object> existing = accessor.get(message);
                if (existing != null && !existing.isEmpty())
                {
                    for (Object o : existing)
                    {
                        if (o == null)
                        {
                            // ignore
                        }
                        else if (!genericType.isAssignableFrom(o.getClass()))
                        {
                            throw new RuntimeException(f.getDeclaringClass() + "::" + 
                                    f.getName() + " - " + o.getClass() + 
                                    " is not assignable to " + genericType);
                        }
                        else
                        {
                            output.writeObject(number, o, schema, true);
                        }
                    }
                }
            }
For lazy initialization called by {@link RuntimeFieldFactory}.
protostuff-runtime/src/main/java/io/protostuff/runtime/RuntimeRepeatedFieldFactory.java