@Override
            protected void doMergeFrom(Input input, Schema<Object> schema,
                    Object message) throws IOException
            {
                final Object value = schema.newMessage();
                if (input instanceof GraphInput)
                {
                    // update the actual reference.
                    ((GraphInput) input).updateLast(value, message);
                }

                schema.mergeFrom(input, value);
                Collection<Object> existing = accessor.get(message);
                if (existing == null)
                    accessor.set(message, existing = messageFactory.newMessage());
                
                existing.add(value);
            }
For lazy initialization called by {@link RuntimeFieldFactory}.
protostuff-runtime/src/main/java/io/protostuff/runtime/RuntimeRepeatedFieldFactory.java