Re: Generics Problem
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Alex schreef:
On Mar 28, 11:06 am, Hendrik Maryns <hendrik_mar...@despammed.com>
wrote:
Alex schreef:
On Mar 27, 10:23 pm, Piotr Kobzda <p...@gazeta.pl> wrote:
Alex wrote:
Is it just not possible to do what I
want?
Will see...
Consider the Node declared as follows:
public abstract class Node<T extends Node<T>> {
...
with extensions (UnrootedNode, and RootedNode) declared like that:
public class UnrootedNode extends Node<UnrootedNode> {
...
And then, having the trees base class declared as:
public abstract class Tree<T extends Node<T>> {
protected List<T> allNodes;
protected List<T> internalNodes;
protected List<T> externalNodes;
...
check your code with a tree classes declared similarly to the
corresponding node classes, i.e. like that:
public class UnrootedTree extends Tree<UnrootedNode> {
...
And later then, let us know, how that meets your needs?
piotr
Hi,
Ok this "works" in so far as it gets rid off all the errors I was
getting. However I don't really understand the syntax you have used.
What does this line mean:
public abstract class Node<T extends Node<T>> ?
This sort of makes sense with the Tree declaration since it will be a
tree "parameterized" by a type of Node but I'm not really sure how to
make sense of it with the Node declaration...
The fact that the semantics of the declaration is totall vague indicates
to me that you should not use it/need it. Once again, please describe
more accurately what you want (see the link in my sig), so we can help
you. I still think my suggestion is what you need.
H.
Ok I will try to explain more accurately what I would like to do.
I wish to have UnrootedTree and RootedTree. UnrootedTree will consist
of UnrootedNode objects and RootedTree of RootedNode objects. However
they have something in common so making a Node type and a Tree type
and then subclassing them seemed sensible.
Indeed, that???s what inheritance is for, right?
Ok, so I want to tell the abstract Tree class about all the things
common to Trees. For example, the Tree knows about the Nodes in it so
I want a List of Nodes. In the UnrootedTree this is a List of
UnrootedNode and in the RootedTree this is a List of RootedNode.
In that case you have to tell the tree it contains some subclass of
Node, not Node itself.
Now, I would like to be able to have methods in the abstract Tree
class that are common to both subtypes such as numberOfNodes() or
something but I also want to be able to have specific methods in the
UnrootedTree and RootedTree that separately work on the list of Node
without having to cast them to UnrootedNode or RootedNode
respectively.
I hope that makes some sense...
It does. I sketched a class hierarchy that should work an compiles
below. It seems like you didn???t realise that it is possible to have a
class be generic and define an extension of it that isn???t, but rather
fills up the generic position, as (Un)RootedTree do.
class Node {
void node() {
System.out.println("This is a generic node method");
}
}
class UnRootedNode extends Node {
void unrooted() {
System.out.println("I have an unrooted method");
}
}
class RootedNode extends Node {
void rooted() {
System.out.println("I have a rooted method");
}
}
class Tree<T extends Node> {
List<T> myNodes;
}
class UnRootedTree extends Tree<UnRootedNode> {
UnRootedTree() {
System.out.println("I only contain UnRootedNodes");
myNodes = new ArrayList<UnRootedNode>();
myNodes.get(0).unrooted();
myNodes.get(0).node();
}
}
class RootedTree extends Tree<RootedNode> {
RootedTree() {
System.out.println("I only contain RootedNodes");
myNodes = new ArrayList<RootedNode>();
myNodes.get(0).rooted();
myNodes.get(0).node();
}
}
Note that you???ll get an NPE in the tree constructors, but it is just to
show the case.
HTH, H.
- --
Hendrik Maryns
http://tcl.sfs.uni-tuebingen.de/~hendrik/
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