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https://issues.apache.org/jira/browse/MATH-581?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13044638#comment-13044638
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Gilles commented on MATH-581:
-----------------------------

Didn't we agree that we should avoid defining an interface if there would be a 
single best way to implement it? [The rationale being that it would make it 
impossible to add, and a fortiori, remove) methods without breaking 
compatibility.]

I think that what we have here are perfect candidates for abstract classes.
All that is needed is:
{code}
public abstract class RealLinearOperator {
  // ...
}
{code}

{code}
public abstract class InvertibleRealLinearOperator extends RealLinearOperator {
  // ...
}
{code}


> Support for iterative linear solvers
> ------------------------------------
>
>                 Key: MATH-581
>                 URL: https://issues.apache.org/jira/browse/MATH-581
>             Project: Commons Math
>          Issue Type: New Feature
>    Affects Versions: 3.0, Nightly Builds
>            Reporter: Sébastien Brisard
>              Labels: iterative, linear, solver
>         Attachments: linearoperator.zip
>
>
> Dear all,
> this issue has already been discussed on the forum. The idea is to implement 
> the most popular linear iterative solvers (CG, SYMMLQ, etc...) in 
> commons-math. The beauty of these solvers is that they do not need direct 
> access to the coefficients of the matrix, only matrix-vector products are 
> necessary. This is goof, as sometimes it is inetficient to store the 
> coefficients of the matrix.
> So basically, before implementing the iterative solvers, we need to define an 
> interface slightly more general than a matrix, namely LinearOperator, with 
> only one basic operation: matrix-vector product.
> Here are a few interfaces and abstract classes that do that. Nothing fancy 
> yet, I just wanted to have you advice on the implementation before I commit 
> some solvers.
> I thought these classes could go in a package 
> org.apache.commons.math.linearoperator, but really, I haven't got a clue...
> Best regards,
> Sebastien

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