Re: Generics Problem
Jim Garrison wrote:
I'm stuck trying to implement a recursive data structure (a simple
tree) using generics, where at each level of the tree the contained
object type may vary. For example, the object contained at the root
node of the tree is of type A; all of A's children contain objects of
type B. All children of any second-level object (i.e. all the
grandchildren of A) are of type C. Below is a simple reduction of
what I've implemented, along with the error I get. I understand WHY
the error occurs, but I'm stuck on how to modify the code to bypass
the error. I'm starting to think this isn't going to be possible with
generics as they are currently defined
It sounds like what you want is:
class BottomNode<C> { C value; }
class MiddleNode<B, C> { Collection<BottomNode<C>> children; B value; }
class TopNode<A, B, C> { Collection<MiddleNode<B, C> children; A value;}
class Tree<A, B, C> { Collection<TopNode<A, B, C>> nodes; }
interface TreeVisitor<A, B, C> {
void visit(TopNode<A, B, C> node);
void visit(MiddleNode<B, C> node);
void visit(BottomNode<C> node);
}
static <A, B, C> void preOrderTraversal(Tree<A, B, C> tree,
TreeVisitor<A, B, C> visitor) {
for (TopNode<A,B,C> top: tree.getChildren()) {
visitor.visit(top);
for (MiddleNode<B, C> middle: top.getChildren()) {
visitor.visit(middle);
for (Bottom<C> bottom: middle.getChildren()) {
visitor.visit(bottom);
}
}
}
}
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Daniel Pitts' Tech Blog: <http://virtualinfinity.net/wordpress/>