Re: nested generic HashMap problem

From:

Daniel Pitts <newsgroup.spamfilter@virtualinfinity.net>

Newsgroups:

comp.lang.java.programmer

Date:

Mon, 26 Apr 2010 16:47:09 -0700

Message-ID:

<4CpBn.298961$Dv7.44991@newsfe17.iad>

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<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, ?>>();

public <T> T get(Demo<T> demo, long id) {
return getMap(demo).get(id);
}

public <T> void add(Demo<T> demo) {
maps.put(demo.getBaseClass(), new HashMap<Long, T>());
}

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?

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.
--
Daniel Pitts' Tech Blog: <http://virtualinfinity.net/wordpress/>