Java Generics — Assigning a list of subclass to a list of superclass

Question

I have a basic question regarding assignment of a list of subclass to a list of superclass.

So I have something like the following:

Class B extends          


        

Answer 1

To explain this, let me substitute "B" with Integer and "A" with Number. This is just to make it a little easier to explain.

Class Integer extends Number;

List <Integer> iList = new ArrayList<Integer>();
List <Number> nList = iList // will fail

The reason this would fail is because nList can take any Number -- it can take Integer, it can take Double, or for that matter any subclass of Number. However, this is not true for iList. You cannot add a Double to iList because it accepts only Integer and its subclasses. Hope this helps explain it to you.

Answer 2

Because generics are strict type safe.

You can have

List<? extends A> aList = bList;

It says aList can hold list of any type which is an A

Answer 3

Because List<B> does not extend List<A>. For example, Integer extends Number and so does Long. So List<Number> can contain both Integer and Long. So if you assign List<Integer> to List<Number> you will be able to add Long to your list of integers.

You can declare

List<? super B> superB;

And that would allow assignment to superB of any list that contains B and its super classes. But it's not the same as in your case aList=bList.

or

List<? extends A> extendsA;

Examples

    List<? super Integer> superA;
    superA = new ArrayList<Number>();

    List<? extends Number> extendsNumber;
    extendsNumber = new ArrayList<Integer>();

Answer 4

List<B> and List<A> are invariant type. What you need is covariant type. In this case, it is List<? extends A>.

Answer 5

While at first glance you might think that

Class B extends A;

List <B> bList = new ArrayList<B>();
List <A> aList = bList;

should work, the problem is obvious when you imagine actually using these lists:

A something = new A();
aList.add( something ); // Should work because aList is a list of A's

but aList was assigned to bList, so that should be the same as

bList.add( something ); // Here's the problem

bList.add() takes a B, but something is an A, and an A is not a B!

And that's why generics should be (and are) strict type safe.

Answer 6

When you declare a List of items of type A, only items of type A can be added or removed from the List. If you need to include subclasses of A, use the generic wildcard ? extends A to indicate so. Your code should therefore be:

List <? extends A> aList = bList;