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https://issues.apache.org/jira/browse/FTPSERVER-274?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12673824#action_12673824
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Steve Ulrich commented on FTPSERVER-274:
----------------------------------------
Unlike arrays, a generic definition doesn't allow subclasses.
My problem is a good example, I think:
For convenience reasons I had a method named "internalListFiles" which returned
an array of type MyFtpFile and the listFiles method just called
internalListFiles and returned the resulting array:
public FtpFile[] listFiles(){
return internalListFiles();
}
private MyFtpFile[] internalListFiles(){
....
}
Here, Java realises that MyFtpFile is a subclass of FtpFile and the arrays are
compatible. The new definition doesn't allow this:
public List<FtpFile> listFiles(){
return internalListFiles(); //Compiler Error: Incompatible types
}
private List<MyFtpFile> internalListFiles(){
....
}
There are two options I see: First would be to copy the List which is a
performance drawback, or implement the internalListFiles again, which could
lead to "copy and waste".
There's just one drawback of "? extends": You can't add any objects to that
list (because you actually don't know the exact type of this list anymore).
Which shouldn't be a problem here, since implementations could work with
covariant return type. (in my case: List<MyFtpFile> listFiles() )
See http://java.sun.com/docs/books/tutorial/java/generics/subtyping.html for a
more detailed explanation.
> Use Wildcard-Generics in API where possible and plausible
> ---------------------------------------------------------
>
> Key: FTPSERVER-274
> URL: https://issues.apache.org/jira/browse/FTPSERVER-274
> Project: FtpServer
> Issue Type: Improvement
> Affects Versions: 1.0.0-RC2
> Reporter: Steve Ulrich
> Fix For: 1.1
>
>
> Currently the API doesn't make use of wildcard generic types.
> Example FtpFile:
> public abstract List<FtpFile> listFiles();
> The used generic for the return type prevents users from returning lists with
> their own type (ie: List<MyFtpFile>).
> A change to:
> public abstract List<? extends FtpFile> listFiles();
> Would allow this without any costs (or I just can't see them *g*)
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