Re: nested generic HashMap problem
Daniel Pitts wrote:
On 4/26/2010 2:54 PM, 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?
import java.util.HashMap;
public class Demo<T> {
private Class<T> base;
private Cache cache;
Demo(Class<T> b, Cache c) { base = b; cache = c; }
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
}
}
Perhaps you should move the Map<Long, T> from Cache directly into Demo?
Which is actually where it started long ago, in the full set of classes,
before Factory came along. And probably the best way to go after all.
public class Demo<T> {
private Class<T> base;
private Map<Long, T> cache;
Demo(Class<T> b, Map<Long, T> c) { base = b; cache = c; }
Class<T> getBaseClass() { return base; }
T get(long id) { return cache.get(id); }
}
As a side note, I would make base and cache final.
I agree. It's a habit I still haven't gotten into, even though I const
like crazy in C++.
Thanks