Just a thought question here. In C++, I could do the following:
vector<vector<string> > data;
// add data into data
//..
data[0].push_back( "somedata" );
And I would expect somedata to get written to the vector array because the [] notation gives me access to the object by reference. What about in Java? If I:
List<List<String>> data = new ArrayList<List<String>>();
// add data into data
//..
data.get(0).add( "somedata" );
Would this actually write somedata into the data object? Or would it create a new copy of the element at data(0), add somedata to that, and then that object disappears into GC sometime down the line?
ArrayList is a List backed-up by array (in order to enable random access) the list stores references to real elements so when you add a new element as you mentioned, the reference to it will be added to the ArrayList (and the backing Array will point to this List element).
ReplyDeleteAlmost. The pattern you need is:
ReplyDeleteList<List<String>> data = new ArrayList<List<String>>();
data.add(new List<String>());
data.get(0).add( "somedata" );
The first line creates only the "outer" List of Lists; you have to populate it with one or more Lists (of Strings) before you can add data to the inner lists.