@Override
    public void add(final ObjectWritable<A> v) {
        if (this.value.isEmpty())
            this.value = v;
        else if (!v.isEmpty())
            this.value = new ObjectWritable<>(this.memoryComputeKey.getReducer().apply(value.get(), v.get()));
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
@author Stephen Mallette (http://stephen.genoprime.com)
spark-gremlin/src/main/java/org/apache/tinkerpop/gremlin/spark/process/computer/MemoryAccumulator.java