Simple operations on list of lists with generics

Question

I'm trying to create a method that takes variables of type List<? extends List<T>> and makes all the lists the same size by chopping off the rear elements of the lists passed in. Here's what I'm trying, but the second for loop is throwing an exception
public static <T> List<? extends List<T>> standardizeNumberListsFromBack(List<? extends List<T>> input) {
          int minLength = input.get(0).size();
          for (List l: input){
               if (l.size() < minLength){
                    minLength = l.size();
          for (List l: input){
               l.subList(minLength, l.size()).clear();
          return input;
     }The call to clear() gives me an UnsupportedOperationException since the type of l is the List interface.
Any ideas how I can pull this off while preserving the generality of the method?
Thanks in advance.

Answer 1

The call to clear() gives me an
UnsupportedOperationException since the type of l is
the List interface.No you get an UnsupportedOperationException because you're calling clear on a list that doesn't support removing elements.
This most likely occurs because you're using something like Arrays.asList(...) (which returns a fixed size list backed by the given array) to generate your lists.
Calling your code withList<Integer> foo = Arrays.asList(1, 2, 3, 4);
List<Integer> bar = Arrays.asList(4, 5, 6);
System.out.println(standardizeNumberListsFromBack(Arrays.asList(foo, bar)));gives me the UnsupportedOperationException. While calling it with thisList<Integer> foo = new ArrayList<Integer>(Arrays.asList(1, 2, 3, 4));
List<Integer> bar = new ArrayList<Integer>(Arrays.asList(4, 5, 6));
// the following also works
// List<Integer> foo = new LinkedList<Integer>(Arrays.asList(1, 2, 3, 4));
// List<Integer> bar = new LinkedList<Integer>(Arrays.asList(4, 5, 6));
System.out.println(standardizeNumberListsFromBack(Arrays.asList(foo, bar)));doesn't.
Edit:
Oh and by the way, you should changefor (List l: input){tofor (List<T> l: input){in both loops to avoid type safety warnings.
Edit2:
My suggestion would be something likefor (List<T> l: input){
     try {
          l.subList(minLength, l.size()).clear();
     } catch (UnsupportedOperationException e) {
          // log the error in some way
          System.err.println("Can't remove elements from a list");
}Message was edited by:
dwg