public List<DStreamExecutionGraph> getCombinableExecutionGraphs(){
		return this.combinableExecutionGraphs == null 
				? Collections.emptyList() 
						: Collections.unmodifiableList(this.combinableExecutionGraphs);
	}
public List < DStreamExecutionGraph > getCombinableExecutionGraphs ( ) { return this . combinableExecutionGraphs == null ? Collections . emptyList ( ) : Collections . unmodifiableList ( this . combinableExecutionGraphs ) ; }
Returns the {@link List} of combinable {@link DStreamExecutionGraph}
where each {@link DStreamExecutionGraph} implies some type of
combine functionality with current operation (i.e., join, union, unionAll etc.)