What is the closest to ConcurrentHashMap and NavigableMap in Java?

[Jacek Furmankiewicz]

True.

While looking a D, I am just trying to focus on the parts which I 
know would be a showstopper for us on day one...and this 
particular issue is it.

I do like D a lot as well from what I've seen so far.

Regarding the GC, I've seen some slides on DConf about other 
garbage collectors available. Is there any resource/tutorial that 
shows how you can swap out the default GC for those alternative 
implementations?