code: private boolean containsInterval(Interval i) {
          if (i != null) {
              return directedGraph.contains(i.getStart())
                      && directedGraph.contains(i.getFinish());
          } else {
              return false;
          }
      }
docs: Determine if an interval is contained in this graph.
  
  @param i
  an interval.
  @return <code>true</code> if the given interval is found,
  <code>false</code> otherwise.