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Re: [really patch] Re: HashMap putAll/putAllInternal bug

From: Bryce McKinlay
Subject: Re: [really patch] Re: HashMap putAll/putAllInternal bug
Date: Tue, 14 Oct 2003 16:04:23 +1300

On Oct 11, 2003, at 4:00 AM, Stuart Ballard wrote:

Bryce McKinlay wrote:
The collections classes, on the other hand, are actually quite well specified in my opinion (having architected many of the current collections implementations used in classpath). I don't believe that anyone should need to write collections code that relies on unspecified details. Besides, matching every single one of those details would be close to impossible without copying Sun's source code virtually line for line. I think its better to stop now, while we have a very stable and bug-free set of collections classes, or we will forever be chasing silly, unspecified details like this, and potentially introducing real regressions and reducing performance in the process.

I'm curious as to whether you can come up with an architecture for the type of Map I need that includes an efficient and correct implementation of size() - that is, an architecture such that the prior implementation of putAll() would both work correctly and be not significantly less efficient than calling hasNext().

My requirement is for a map which holds references to two other maps, "front" and "back". The keySet() of my map is the union of front.keySet() and back.keySet(). The value corresponding to a given key is 'front.containsKey(key) ? front.get(key) : back.get(key)'. Both front and back can be accessed directly through other code and may change in any way without my map being notified. How can I implement size() without a full iteration?

If you need this to be fast, perhaps the best way (to guarantee good performance on any implementation) would be to define your own method:

HashMap asHashMap()
  HashMap hash = new HashMap(estimatedSize);
  return hash;


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