Question

I have the following statements :

    List<List<String>> myList  = new ArrayList<ArrayList<String>>();

    List<ArrayList<String>> myList  = new ArrayList<ArrayList<String>>();

The first one gives compile error. Shouldn’t we code to interface? Here I am forced to code by class not the interface. Is this a generics failure, or I am understanding it wrong?

Answer 1

This is a known problem. It would have other problems if you do this.

Consider this code:

List<List<String>> myList  = new ArrayList<ArrayList<String>>();
myList.push( new LinkedList<String>() );   // impossible to catch, because java don't have generic in runtime

ArrayList<ArrayList<String>> list2 = (ArrayList<ArrayList<String>>) myList; // ok

ArrayList<String> something = list2.get(0);  // doh! impossible to assign LinkedList to ArrayList