Detailed Java Generics

From:
jsc59@cornell.edu
Newsgroups:
comp.lang.java.help
Date:
11 Jul 2006 11:45:16 -0700
Message-ID:
<1152643516.449547.158100@b28g2000cwb.googlegroups.com>
I have a very detailed/complicated java generics question. Consider
the following code

public abstract class AEdge {
    ANode fr, to;
}

public abstract class ANode { }

public class CEdge extends AEdge { }

public class CNode extends ANode { }

public class MyEdgeDatabase<N extends ANode, E extends AEdge> {
    Set<E> edges;

    public abstract class ForEachEdgeFrom {
        public ForEachEdgeFrom(N n) {
            for(E e : edges) if (e.fr == n) run(e);
        }
        public abstract void run(E e);
    }
}

public class Test {
    MyEdgeDatabase<CNode, CEdge> edgeDB
        = new MyEdgeDatabase<CNode, CEdge>();
    CNode n = new CNode();

    public void doIt() {
        edgeDB.new ForEachEdgeFrom(n){
            public void run(CEdge e) {
                System.out.println(e);
            }
        };
    }
}

The doIt() method will not compile. It gives the following error:

"No enclosing instance of type MyEdgeDatabase<N,E> is accessible. Must
qualify the allocation with an enclosing instance of type
MyEdgeDatabase<N,E> (e.g. x.new A() where x is an instance of
MyEdgeDatabase<N,E>)."

What does this mean and how to I fix it? Please don't suggest other
ways of iterating through edges in MyEdgeDatabase (we have reason to do
it this way).

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