Interface Setter with List Generics

Hello All,
There may be some posts or other information out there but for whatever
reason I can't seem to find it. This is the closet I got, but did not find a
real answer.
http://forums.sun.com/thread.jspa?threadID=5142452

I have an interface for Orders and OrderLines. The Orders interface has a
method like so:
public void setOrderLines(List<? extends OrderLine> orderLines);

I was getting compile errors when attempting to compile the a CustomerOrder
class that implements the Order interface. The setOrderLines() method I
expected to be able to override as such:

public void setOrderLines(List<CustomerOrderLine> customerOrderLines) {
for (Iterator<CustomerOrderLine> iterator = customerOrderLines
.iterator(); iterator.hasNext();) {
CustomerOrderLine customerOrderLine = iterator.next();
this.customerOrderLines.put(customerOrderLine.getLineNumber(),
customerOrderLine);
}
}

But I ended up with this:
@SuppressWarnings("unchecked")
public void setOrderLines(List<? extends OrderLine> customerOrderLines)
{
for (Iterator<CustomerOrderLine> iterator =
(Iterator<CustomerOrderLine>) customerOrderLines
.iterator(); iterator.hasNext();) {
CustomerOrderLine customerOrderLine = iterator.next();
this.customerOrderLines.put(customerOrderLine.getLineNumber(),
customerOrderLine);
}
}

Am I doing something wrong? The getter method that returns a List I was able
to implement as expected.

Just thought one of you Java experts might have some insight for me.

Thanks in advance,
James R. Perkins