generics - Java 1.6: Creating an array of List<T>

Question

Why can't I create an array of List ?

List<String>[] nav = new List<String>[] { new ArrayList<String>() };

Eclipse says "Cannot create a generic array of List"

or

ArrayList<String>[] nav = new ArrayList<String>[] { new ArrayList<String>() };

Eclipse says "Cannot create a generic array of ArrayList"

or

List<String>[] getListsOfStrings() {
    List<String> groupA = new ArrayList<String>();
    List<String> groupB = new ArrayList<String>();
    return new List<String>[] { groupA, groupB };
}

But I can do this:

List[] getLists() {
    return new List[] { new ArrayList(), new ArrayList() };
}

Eclipse says that List and ArrayList are raw types but it compiles...

Seems pretty simple, why won't it work?

Answer 1

Well, generics tutorial give the answer to your question.

The component type of an array object may not be a type variable or a parameterized type, unless it is an (unbounded) wildcard type.You can declare array types whose element type is a type variable or a parameterized type, but not array objects.

This is annoying, to be sure. This restriction is necessary to avoid situations like:

// Not really allowed.
List<String>[] lsa = new List<String>[10];
Object o = lsa;
Object[] oa = (Object[]) o;
List<Integer> li = new ArrayList<Integer>();
li.add(new Integer(3));
// Unsound, but passes run time store check
oa[1] = li;

// Run-time error: ClassCastException.
String s = lsa[1].get(0);

If arrays of parameterized type were allowed, the previous example would compile without any unchecked warnings, and yet fail at run-time. We've had type-safety as a primary design goal of generics.