Cast LinkedList to ArrayList, enjoy the class cast exception

[Alana Sparx]

I should know better, but here goes....
Why does this throw a class cast exception at the last line?

  List list = new LinkedList();
  list.add(objA);
  list.add(objB);
  list.add(objC);
  list.add(objD);

  ArrayList aList = (ArrayList)list;

Both LinkedList and ArrayList implement List.
ArrayList extends AbstractList
LinkedList extends AbstractSequentialList which extends AbstractList

As ArrayList & Linked List are both of type �List�, and both share the common parent �AbstractList�, why does this fail. Is it LinkedList�s intermediate parent AbstractSequentialList spoiling the party?

Many thanks.

[Tony Morris]

Because the type of list (java.util.LinkedList) is not assignable to java.util.ArrayList. That they share a common supertype hierarchy is superfluous to the issue - the list refers to a type java.util.LinkedList, and this type is not (emphasis, not) assignable to type java.util.ArrayList.

This is the plain vanilla example of failed casts and their effect, so the question really is "why do you not think it will fail?"

[Jeff Albertson]

Given your discussion after the code, I thought you had meant to write this:

List list = new LinkedList();
...
[B]AbstractList aList = (AbstractList)list;[/B]

Not that casting to AbstractList is a *useful* thing to do, but it does demonstrate that LinkedList objects also have type AbstractList ...

[Alana Sparx]

Thanx

I see the error of my ways; so obvious now I re-look at it.