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From: supercalifragilisticexpialadiamaticonormalizeringelimatisticantations <supercalifragilisticexpialadiamaticonormalizeringelimatisticantations@averylongandannoyingdomainname.com>
Newsgroups: comp.lang.java.programmer
Subject: Re: Why "lock" functionality is introduced for all the objects?
Date: Sat, 02 Jul 2011 00:26:14 -0400
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On 01/07/2011 10:05 PM, Joshua Cranmer wrote:
> On 6/28/2011 10:12 PM,
> supercalifragilisticexpialadiamaticonormalizeringelimatisticantations
> wrote:
>> If the list wasn't mutable there'd be no problem casting a
>> List<Sub> to a List<Super>.
>
> And then I'd complain because my program would be spending more time
> copying the values between immutable queues than actually doing work. As
> long as the language has the potential for mutable collections (which
> most people want for performance reasons), you have the potential for
> generics casting issues.

Lists are, in my experience, typically constructed, then consumed; only 
infrequently is a mutable one maintained with recurring episodes of 
reading and writing over time. The common case could have been optimized 
with better support than the various Collections.unmodifiableFoo() 
methods provide. For example, if you could tag a list as not modifiable 
the compiler can both disallow writing through it and allow casting from 
UnmodifiableList<Sub> to UnmodifiableList<Super>. We kinda have that now 
in that we can cast List<Sub> to List<? extends Super> and then the 
compiler will indeed not let us add to it, but <? extends Super> is both 
awkward and not equal to Super. A lot of methods might be written to 
demand a List<Super> even if they won't modify the list, and will thus 
work with a List<? extends Super>. More generally it complicates 
generics. The fact of the matter is that <? extends X> is like of like 
"unmodifiable, and also <X>", at least for collections; a more clear way 
of (separately) expressing "unmodifiable" would have been nice.

So, basically, what I'm saying is that we should have had some notion of 
constness in Java. :)