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 A;

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

Why does this last assignment fail? Sorry for the newbie question

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.