@Override
    public boolean hasNextBarrier() {
        if (this.barrier.isEmpty()) {
            this.processAllStarts();
        }
        return !this.barrier.isEmpty();
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/sideEffect/AggregateGlobalStep.java