code: public GraphHopper setStoreOnFlush(boolean storeOnFlush) {
          ensureNotLoaded();
          if (storeOnFlush)
              dataAccessType = DAType.RAM_STORE;
          else
              dataAccessType = DAType.RAM;
          return this;
      }
docs: Only valid option for in-memory graph and if you e.g. want to disable store on flush for unit
  tests. Specify storeOnFlush to true if you want that existing data will be loaded FROM disc
  and all in-memory data will be flushed TO disc after flush is called e.g. while OSM import.
  
  @param storeOnFlush true by default