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From Stefan Bodewig <bode...@apache.org>
Subject Re: [COMPRESS] Add getArchiveType() method to [Archive|Compressor]InputStream classes?
Date Fri, 14 May 2010 09:37:25 GMT
On 2010-05-14, sebb <sebbaz@gmail.com> wrote:

> On 14/05/2010, Stefan Bodewig <bodewig@apache.org> wrote:
>> On 2010-05-12, sebb <sebbaz@gmail.com> wrote:

>>> Can we restart this?

>>> Compress currently has Archiver and Compressor InputStreamFactory
>>> classes which have the following signatures:

>>> public ArchiveInputStream createArchiveInputStream(
>>>             final String archiverName, final InputStream in)

>>>  public CompressorInputStream createCompressorInputStream(final String name,
>>>             final InputStream in)

>>> The archiverName or name parameters are used to determine which stream
>>> class to create, and should use one of the provided public constants.

>>> I just want to provide the inverse conversion, i.e. tag the created
>>> class with the key that was used to create it, so that this can be
>>> easily determined later e.g. if the autodetect mode is used.

>> In theory the inverse is not unique - the same implementation could be
>>  used for several names.  It may become true in practice if we'd choose
>>  to use different names for variants of archive types like pax, posixtar
>>  and gnutar for tar.

>>  I think it depends on what you want the method that returns the archive
>>  type/name (s) to reflect.

>>   * The archive names you could pass into the create methods to obtain an
>>    instance of the same stream class?  Then the method actually should be
>>    inside the factory rather than the stream class since only the
>>    factory can know that for sure.

> Yes, but the factory can pass in the name to the created stream.

What if I don't create the stream via the factory?

To make this work all implementations would need a new constructor with
an additional argument - that wouldn't make any sense for users who
bypass the factory.

If this is the purpose of the method I'd prefer to see it inside the
factory, some getArchiverNames(Class) or
getArchiverNames(ArchiveInputStream) that returned an immutable list of
strings.

We could still put a method into ArchiveInputStream that queried the
factory passing in itself.

>>   * The format of the current stream?  In that case there can't be any
>>    reasonable default (other than asking the factory and potentially
>>    getting back a list of names rather than a single one).

> That would not be necessary if every IS that is created is given its
> name via the ctor.

Assume one we have multiple names for one implementation.  For example
old_ascii, old_binary, new_portable and new_portable_crc for cpio since
these really are different formats.  If we use autodetection - which is
why you want the method in the first place IIUC - then which name do you
expect the factory to pass into said constructor?

Only the implementation knows the real format it has detected.

So if this is the purpose of the new method then it has to live inside
the implementation and I don't see which default implementation could
make sense.

>>> It might also be useful for the classes to provide access to their
>>> associated mime type(s) and "usual" file extension(s). These should
>>> both be read-only Lists.

>> OK.

>>  This isn't anything the factory could do (unless we broaden its
>>  contract) nor the base class could provide a reasonable default for.

> The base class could define an abstract method.
> The only possible defaults would be empty Lists.
> These would only be useful in the sense that subclasses would not need
> to implement the methods.

That's what I was trying to say - the default of an empty list is not
reasonable ("it simply doesn't make sense" in case I'm just using the
wrong English word for what I mean).

Stefan

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