subset based on index from google guava TreeBasedTable

Question

I have a TreeBasedTable<String,String,CustomType> structure from which I need to be able to get subsets based on start and end range indices, like fromindex to toindex. The cellSet method doesn't return a SortedSet. What would be the best approach for this?

I thought of doing Lists.newArrayList(structure.cellSet()).subList(start,end), but doesn't look like an efficient thing to do.

Answer 1

If startindex and endindex are integer positions, then your ArrayList implementation isn't actually that far off from the best that's feasible, though it'd be slightly more efficient to write

FluentIterable.from(table.cellSet()).skip(fromIndex).limit(toIndex).toList()

That implementation won't copy any more of the elements than it has to into the result list.

Generally speaking, there's not an efficient way to do this for an arbitrary SortedSet, SortedMap, or pretty much any of the sorted data structures that come with Java.

Answer 2

When using TreeBasedTable, the implementation of rowMap() actually returns a SortedMap.

So you should use:

@SuppressWarnings("unchecked") // safe cast because TreeBasedTable returns SortedMap
final SortedMap<String, Map<String, CustomType>> rowMap = (SortedMap<String, Map<String, CustomType>>) myTable.rowMap();
final SortedMap<String, Map<String, CustomType>> subRowMap = rowMap.subMap(start, end);

so start and end will work on rowMap just like subList(start,end) works for a List.