Re: correct way of processing cache

Eric Sosman <esosman@ieee-dot-org.invalid>
Mon, 13 Dec 2010 21:31:47 -0500
On 12/13/2010 11:06 AM, mark jason wrote:

I need to check for a cache file that contains a MyCache object with
some precomputed values( based on data from image files in a folder).
The algorithm is as follows,
1.If a cache file exists, retrieve the MyCache object from cache file
and get a list of image file names from MyCache object,
and compare it with current List of filenames.
2.If both lists are same ,I can use the image data from MyCache
3.if lists are different ,compute the image based data,create new
MyCache object with this image based data and current list of
filenames. step 3 if no cache file exists.

I tried to implement this as follows.However,I am not sure if this is
the 'object oriented way' to do this.Can somebody advise?

     Just a few comments, "food for thought" more than "advice."

import java.util.List;

public class CacheChecker {
    private MyCache cachefile;

    public void checkCache(String folder){
        List<String> newFileNames = parseFolder(folder);

     It is strange that your cache represents its contents as a List,
when a Set would seem more natural. As Lists, [ "a.jpg", "b.jpg" ]
and [ "b.jpg", "a.jpg" ] are different, while as Sets they would be
the same, { "a.jpg" "b.jpg" }.

    private void calculateAndCreateNewCache() {
        //do calculations
        //create MyCache from calculated data and filenames list
        //write MyCache object to 'mycachefile'

     Perhaps you meant to imply this, but it's not entirely clear from
the commentary: If the old cache held { "a.jpg" "b.jpg" } and the new
one holds { "b.jpg" "c.jpg" }, there's no need to re-process "b.jpg".

     Also, I think that "on general principles" it would be better to
separate the calculation of the new cache from the writing thereof to
persistent storage. As Patricia Shanahan recently remarked on another
thread (I paraphrase), a method whose "natural" name is doOneAndTwo
would likely be better off as two methods with one purpose each.

     private MyCache getExistingCache(String folder)throws IOException,
        FileInputStream fin = new FileInputStream(folder+File.separator
        ObjectInputStream oin = new ObjectInputStream(fin);
        MyCache cache = (MyCache)oin.readObject();

     Closing oin also closes whatever it wraps, in this case fin. (And
closing fin also closes whatever invisible interior things it in turn
may wrap.) I think the double-close may have become harmless with the
introduction of the Closeable interface, but it makes my skin crawl

     It would be a very bad idea to close fin and *then* close oin ...

Eric Sosman

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