Re: bounded generic problem
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Eric Smith schreef:
I've written a class SPF to implement Dijkstra's Shortest Path First
algorithm using bounded generics to allow the user to supply his own
objects for vertices and edges of the graph, where those objects
must implement interfaces SPFVertex and SPFEdge. I have it working,
but I've got one place where the compiler complains about not finding
the right method profile. If I insert an explicit cast, then it
compiles with a warning about the cast, but executes correctly.
Here are the interfaces and the class stripped down to the bare
minimum that will exhibit the problem:
interface SPFEdge
{
public long getCost ();
}
interface SPFVertex<E extends SPFEdge>
extends Iterable<E>
{
public SPFVertex traverse (E edge);
Did you mean SPFVertex<E> here?
}
public class SPF<V extends SPFVertex<E>,
E extends SPFEdge>
{
private class State
{
V vertex;
public State (V vertex)
{
this.vertex = vertex;
}
}
public void traverseEdge (State state, E edge)
{
State newState = new State (
state.vertex.traverse (edge)
);
}
}
The compiler complains that it can't find the constructor for State:
% javac *.java
SPF.java:15: cannot find symbol
symbol : constructor State(SPFVertex)
location: class SPF<V,E>.State
State newState = new State (
^
1 error
You will get a warning about the traverse line as well.
The problem seems to be that the SPFVertex traverse() method
returns an SPFVirtex, which should be a V in class SPF, but for
some reason the compiler doesn't think it matches.
Well, of course it doesn???t: traverse returns an SPFVertex<E>, but State
expects a subclass of it. It???s as if you would pass a Number to a
method which expects a Double. Not the same thing!
If I put in an explicit cast to V:
State newState = new State (
(V) (state.vertex.traverse (edge))
);
then the compiler gives the warning:
% javac -Xlint:unchecked *.java
SPF.java:17: warning: [unchecked] unchecked cast
found : SPFVertex<E>
required: V
(V) (state.vertex.traverse (edge))
^
1 warning
Well, yes, you could be casting a MyVertex<MyEdge> to a HisVertex<HisEdge>.
I tried changing the return type of traverse to SPFVertex<E>, but
that didn't eliminate the error.
You should do that anyway. Once you use generics, use them consistently!
Can anyone explain why the result of the traverse() method is
not being matched with V in class SPF?
I hope I did.
Now about a solution: I don???t see one at first. Do you really need
State to have a V? I.e., couldn???t you change State such that it stores
an SPFVertex<E>?
You could make Edges vertex-aware instead. But I didn???t get a nice
design out of that as well.
HTH, H.
- --
Hendrik Maryns
http://tcl.sfs.uni-tuebingen.de/~hendrik/
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