So I have this function, and it's not compiling. What is the problem?
The compiling error is Line 4: error: incompatible types
public List<List<Integer>> myfunc(int[] num) {
List<List<Integer>> r = new ArrayList<ArrayList<Integer>>(); //line 4
return r;
}
Thanks!!
An ArrayList<ArrayList<Integer>> isn't a List<List<Integer>>, for the same reason that an ArrayList<Apple> isn't a List<Fruit>, and it's fairly easy to given an example of why that's the case:
ArrayList<ArrayList<Integer>> arrayLists = new ArrayList<ArrayList<Integer>>();
// This won't work, but imagine that it did...
List<List<Integer>> lists = arrayLists;
// This is fine - a LinkedList<Integer> is a List<Integer>
lists.add(new LinkedList<Integer>());
// This should be fine too
ArrayList<Integer> arrayList = arrayLists.get(0);
So what happens at that last line? Somehow arrayList would have a reference to a LinkedList, even though its type is ArrayList<Integer>. Eek!
That's a demonstration of why the code wouldn't be safe if it were allowed - which is why it's not allowed. A simpler example is with fruits as I mentioned earlier:
List<Apple> apples = new List<Apples>();
// Again, not allowed
List<Fruit> fruit = apples;
// Because this would be odd...
fruit.Add(new Banana());
// It's a Banapple!
Apple apple = apples.get(0);
There are various things you could do here - but the simplest is probably just to create an ArrayList<List<Integer>> instead of an ArrayList<ArrayList<Integer>>.
The compiler is matching the element type of the declared List with the element type of the created instance of ArrayList. They should be the same:
List<ArrayList<Integer>> r = new ArrayList<ArrayList<Integer>>();
Or
List<List<Integer>> r = new ArrayList<List<Integer>>();
The second one is better since the element type is an interface.
List<List<Integer>> r = new ArrayList<List<Integer>>();List<? extends List<Integer>> r = new ArrayList<ArrayList<Integer>>();