Collections and Generics: nested interfaces possible?

[Ulf Dittmer]

I'm trying to construct a nested data structure using collections and generics, and am having trouble using interfaces instead of implementations at the deeper levels. What I'd like to do is this:

Map<A, Map<B, C>>

but the compiler forces me to use a concrete implementation for the nested Map:

Map<A, HashMap<B, C>>

This works fine, but is there a way to use the interface instead of the implementation, in case I want to use a different one later? There's the "? extends X" syntax for subclasses, but I don't see something similar for interfaces.

Taking the question further, is there an introduction into these finer points of generics? The Sun Java Tutorial doesn't seem to go that far.

[Rob Spoor]

What compiler are you using? Because the following code compiles perfectly in Java 1.6:

Map<Integer,Map<String,Long>> maps = new HashMap<Integer,Map<String,Long>>();
Map<String,Long> map = new HashMap<String,Long>();
map.put("1", 1L);
maps.put(1, map);

Even if I declare map of being a HashMap instead of a Map it compiles.

[Jim Yingst]

Hm, I have no problem compiling and executing this line:

Map<String, Map<String, String>> map = new HashMap<String, Map<String, String>>();

Do you get a different result? I'm using JDK 6 update 3 - how about you?

[Ulf Dittmer]

I'm using Java 5, which does indeed compile the code you posted.

After some more fiddling with the types, it now seems to work the way I intended it to in the first place.

Thanks for your help. Looks like the problem was actually an InsufficientCaffeineError on my part.