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From S├ębastien Brisard <>
Subject [math] Read-only RealVector
Date Thu, 11 Aug 2011 03:36:40 GMT
this is an idea I've had while thinking about Iterative Linear
Solvers, but I think it is much more general. I don't know whether
what I'm going to describe is a known pattern, or a very dirty trick I
should forget about. So any advice would be greatly appreciated.

For the sake of the argument, let us stick with iterative linear
solvers. Most of them modify in-place the current estimate of the
solution, x (which is a RealVector). In view of observing the state of
the solver during the iterations, I'd like to provide a method
RealVector getCurrentSolution()

As modifying the current solution outside the solver would ruin the
iterations, it is imperative to return a deep copy of x. This can take
time, if the vector is large. Also, it can be memory consuming. So I
was wondering: would it be possible to design a ReadOnlyRealVector
class, which would be final, have a unique constructor
ReadOnlyRealVector(RealVector v)
and extend AbstractRealVector

I would keep a reference to the underlying RealVector used to
construct the ReadOnlyRealVector, so that any call to a standard
RealVector method would be delegated to the same method in v. EXCEPT
when the method in question would normally modify the
ReadOnlyRealVector, in which case an UnsupportedException would be
thrown. For example

v = new RealVector();
vro = new ReadOnlyRealVector(v);; // Returns
vro.mapToSelf(f); // Throws an exception

Note that "Read-Only" does not mean "Immutable", since a modification
of v would modify vro. This would suit my requirements, but it should
be made clear in the Javadoc.
Is it clean code, or should I throw it away?
In the former case, would a ReadOnly tagging interface make sense?

Thanks for your comments/corrections,

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