@Override
            protected void mergeFrom(Input input, T message) throws IOException
            {
                final Object value = input.mergeObject(null, getSchema());
                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