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From Phil Steitz <phil.ste...@gmail.com>
Subject Re: [math] Read-only RealVector
Date Thu, 11 Aug 2011 20:01:35 GMT
On 8/11/11 4:22 AM, Gilles Sadowski wrote:
> Hello S├ębastien.
>
>>> Well, in fact I would very much like to have immutable vectors too.
>>> Immutability is really a way to simplify implementations. Surprisingly it
>>> sometimes also decrease time and memory consumption, because defensive
>>> copies littering user code can be avoided.
>>>
>> Luc, I have a silly question. Why do you think immutable vectors would
>> prevent defensive copy ? Creating an immutable vector from an existing
>> mutable vector would require a defensive copy, wouldn't it?
>> Thats the reason why I'm talking about read-only vectors, and not
>> immutable vectors.
>> The solver would keep a reference to the underlying (mutable) vector,
>> so would be able to modify (indirectly) the read-only vector.
>> The observer would only have a reference to the read-only vector, so
>> would not be able to modify the underlying vector.
> I think that you were right to stress the difference between immutable and
> read-only.
> If I'm not mistaken, in JDK parlance they call the latter "unmodifiable"
> (cf. the methods "unmodifiableCollection" and siblings in the standard class
> "java.util.Collections").
>
> The idea which you referred to at the beginning of this thread is exactly
> what they do: The wrapping class throws "UnsupportedOperationException" from
> any "mutating" method.
>
> What Luc said is that when you pass an (already) immutable object, you do not
> have to make a copy; a reference is as safe.
>
> I'm not sure whether that would still be true with an unmodifiable
> reference. I.e. couldn't it be cast to its underlying modifiable reference?
> However that would be on-purpose nasty code, not a mistake on the part of
> the checker code.
>
> So, a utility method like
>   public static RealVector unmodifiableRealVector(RealVector v)
> in class "AbstractRealVector", would fit your need, IMO.

+1 - looks to me like what is needed is an unmodifiable view,
exactly like what the UnmodifiableCollections provide.   What is
returned in the methods analogous to the above in
Collections.unModifiableXxx are instances of private static inner
classes that decorate the actual parameter and delegate, throwing
UnsupportedOperationExceptions for mutators.  See either
[collections] or the Harmony code [1] for examples.

Phil

[1] http://s.apache.org/XJ4
>
>
> Best,
> Gilles
>
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