RFR(JDK11/NIO) 8202285: (fs) Add a method to Files for comparing file contents

[Joe Wang]

Thanks John for the background and detailed information.

-Joe

On 4/30/2018 6:18 PM, John Rose wrote:
> On Apr 30, 2018, at 4:47 PM, Joe Wang <huizhe.wang at oracle.com 
> <mailto:huizhe.wang at oracle.com>> wrote:
>>
>> Are there real-life use cases? It may be useful for example to check 
>> if the files have the same header.
>
> After equality comparison, lexical comparison is a key use case.
> By allowing the user to interpret the data around the mismatch,
> the comparison can be made sensitive to things like locales.
>
> As Paul implies, finding a mismatch is the correct operation to build
> equality checks on top of, because (a) a mismatch has to be detected
> anyway to prove inequality, and (b) giving the location of the mismatch,
> instead of throwing it away, unlocks a variety of other operations.
>
> If you want real-life use cases, look at uses of /usr/bin/cmp in Unix
> shell scripts.  The cmp command is to Unix files what Paul's array
> mismatch methods are to Java arrays.  Here's a man page reference:
>
> https://docs.oracle.com/cd/E19683-01/816-0210/6m6nb7m6c/index.html
>
> As with the array mismatch methods, the cmp command allows the
> user to specify optional offsets within each file to start comparing, as
> well as an optional length to stop comparing after.
>
> See the file BufferMismatch.java for the (partial) application of these
> ideas to NIO buffers.
>
> I suppose the Java-flavored version of "cmp - file" would be a file
> comparator which would take a byte buffer as a second operand,
> and return an indication of the location of the mismatch.  Note that
> "cmp - file" compares a computed stream against a stored file.
>
> I think Paul and I have sketched a natural "sweet spot" for performing
> bitwise comparisons on stored data.  It's up to you how much to implement.
> I suggest that, if you don't feel inspired to do it all in one go, 
> that you
> leave room in the code for future expansions (maybe as with
> BufferMismatch), and perhaps file a follow-up RFE.
>
> — John
>

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