Re: Generics Problem

Hi,

I am trying to use generics in my code and am having a problem.
I have two classes that are quite similar, RootedNode and
UnrootedNode, so I decided to make a class called Node and have both
of these extend it. I have a similar structure with UnrootedTree,
RootedTree and Tree. In Tree I define:

protected List<? extends Node> allNodes;
protected List<? extends Node> internalNodes;
protected List<? extends Node> externalNodes;

Then in UnrootedTree I have this code:

internalNodes = new ArrayList<UnrootedNode>();
externalNodes = new ArrayList<UnrootedNode>();
allNodes = new ArrayList<UnrootedNode>();
NewickParser np = new NewickParser(t);
allNodes = np.getNodes(); //this method returns List<UnrootedNode>
for(int i=0;i<allNodes.size();i++)
{
            if(allNodes.get(i).isExternal())
                externalNodes.add(allNodes.get(i)); // error
            else
                internalNodes.add(allNodes.get(i)); //error
}

The problem is both the lines marked error say:

E:\webb\Personal\Java\NetBeans\DataRead\src\webb\tree
\UnrootedTree.java:38: cannot find symbol
symbol : method add(webb.tree.Node)
location: interface java.util.List<capture of ? extends
webb.tree.Node>
                externalNodes.add(allNodes.get(i));

What I don't understand is why allNodes.get(i) is of type Node rather
than type UnrootedNode and thus won't let me add it. Any help would be
apreciated...