public <E extends Element> Set<String> getIndexedKeys(final Class<E> elementClass) {
        if (Vertex.class.isAssignableFrom(elementClass)) {
            return null == this.vertexIndex ? Collections.emptySet() : this.vertexIndex.getIndexedKeys();
        } else if (Edge.class.isAssignableFrom(elementClass)) {
            return null == this.edgeIndex ? Collections.emptySet() : this.edgeIndex.getIndexedKeys();
        } else {
            throw new IllegalArgumentException("Class is not indexable: " + elementClass);
        }
    }
Return all the keys currently being index for said element class  ({@link Vertex} or {@link Edge}).

@param elementClass the element class to get the indexed keys for
@param <E>          The type of the element class
@return the set of keys currently being indexed
tinkergraph-gremlin/src/main/java/org/apache/tinkerpop/gremlin/tinkergraph/structure/AbstractTinkerGraph.java