Question

In Java I am having trouble converting from a Set<Set<String>> to a List<List<String>> and then populating this list with the contents of the Set<Set<String>>

Here is my code:

Set<Set<String>> treeComps = compExtractor.transform(forest); // fine
List<List<String>> components = new List<List<String>>();     // does not work
components.addAll(treeComps);                                 // does not work

Answer 1

You can’t instantiate an instance of the List interface, you need to use one of the implementations like ArrayList. Then you can iterate over the outer set in treeComps, create a new ArrayList for each inner set, call addAll on this ArrayList and then add the list to components.

List<List<String>> components = new ArrayList<List<String>>();
for( Set<String> s : treeComps )
{
  List<String> inner = new ArrayList<String>();
  inner.addAll( s );
  components.add( inner );
}