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

[John Rose]

On Apr 30, 2018, at 4:47 PM, Joe Wang <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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