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From: Daniel Pitts <newsgroup.nospam@virtualinfinity.net>
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Subject: Re: Converting Sets
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NNTP-Posting-Date: Thu, 02 Feb 2012 19:02:01 UTC
Date: Thu, 02 Feb 2012 11:02:01 -0800
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On 2/2/12 5:32 AM, Roedy Green wrote:
> On Thu, 02 Feb 2012 12:47:58 +0100, Mayeul<mayeul.marguet@free.fr>
> wrote, quoted or indirectly quoted someone who said :
>
>> Conversion of a Set<X>  to a Set<? extends Y>, ensuring the Set contains
>> objects assignable to Y, but no new elements can be added to it:
>>
>> Set<X>  setOfX = obtainSetOfX();
>> Set<? extends Y>  setOfY = setOfX;
>
> This what I was looking for , that did not require an element by
> element copy.
> Ah, but this this you created is NOT a Set<Y>.  You cannot add
> arbirary Y to it, just more X. I feel queasy. How could the compiler
> keep track that setOfY could only contain X.
It does not. Instead, it prevents you from any add operation which 
requires the type.

In other words, setOfY becomes somewhat "read-only" for typed 
operations. You can still clear it, or sub-sets of it, but you can no 
longer add *any* object to it, whatever type.