Re: nested generic HashMap problem
Lew wrote:
Chris Riesbeck wrote:
I've looked at the Java generics tutorial, Langer's FAQ, and similar
online pages, but I'm still don't see what, if anything, I can do to
make the last line (marked with the comment) compile, other than do a
typecast and suppress the unchecked warning. Am I asking the
impossible or missing the obvious?
...
public class Demo<T> {
>> ...
Class<T> getBaseClass() { return base; }
T get(long id) { return cache.get(this, id); }
}
class Cache {
private HashMap<Class<?>, HashMap<Long, ?>> maps
= new HashMap<Class<?>, HashMap<Long, ?>>();
...
private <T> HashMap<Long, T> getMap(Demo<T> demo) {
return maps.get(demo.getBaseClass()); // incompatible types
}
}
No way while you have the wildcards there. Suppressing unchecked
warnings will only expose you to ClassCastException.
Did you also read the free chapter on generics from Josh Bloch's
/Effective Java/ available at java.sun.com?
Part of the above was meant to follow as best I could his pattern for
type-safe heterogeneous containers.
As defined, your 'Cache#maps' variable cannot even guarantee that the
base type of the 'Class' key matches the type of the value map's value.
>
Furthermore, you define all that in terms of concrete classes instead of
interfaces. Oops.
I agree. This was the shortest compilable example I could come up with
that still had the interactions I needed to support.
You might have better luck putting an upper bound (e.g., 'Foo') on the
type of 'T'
>
> [...snip...]
Whenever I find tricky generics questions like these, I find it pays to
really think very hard about what to assert about the types. Once I
figure that out the generics are a simple reflection of that analysis.
T can be anything. What I can assert is that if the key is Demo<T> then
the nested map value is Map<T, Long>, where T = the return type of
Demo<T> getBaseClass(). I can't figure out if there's a way to write
that relationship in the type declaration.
I really appreciate the quick response, Lew. Thanks