@Override
        public void emit(final OK key, OV value) {
            List<V> values = combineMap.get(key);
            if (null == values) {
                values = new ArrayList<>();
                combineMap.put((K) key, values);
            }
            values.add((V) value);
        }
@author Marko A. Rodriguez (http://markorodriguez.com)
spark-gremlin/src/main/java/org/apache/tinkerpop/gremlin/spark/process/computer/CombineIterator.java