@Override
    public void map(final Vertex vertex, final MapEmitter<Object, Double> emitter) {
        final Property pageRank = vertex.property(PageRankVertexProgram.PAGE_RANK);
        if (pageRank.isPresent()) {
            emitter.emit(vertex.id(), (Double) pageRank.value());
        }
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/computer/ranking/pagerank/PageRankMapReduce.java