Question

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?

Answer 1

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).