@Override
    public GraphComputer compute() {
        if (this.configuration.containsKey(Constants.GREMLIN_HADOOP_DEFAULT_GRAPH_COMPUTER)) {
            try {
                return this.compute((Class<? extends GraphComputer>) Class.forName(this.configuration.getString(Constants.GREMLIN_HADOOP_DEFAULT_GRAPH_COMPUTER)));
            } catch (final Exception e) {
                throw new IllegalStateException(e.getMessage(), e);
            }
        } else
            throw new IllegalArgumentException("There is no default GraphComputer for HadoopGraph. Use HadoopGraph.compute(class) or gremlin.hadoop.defaultGraphComputer to specify the GraphComputer to use.");
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
hadoop-gremlin/src/main/java/org/apache/tinkerpop/gremlin/hadoop/structure/HadoopGraph.java