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

While at first glance you might think that

Class B extends A;

List  bList = new ArrayList();
List  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.