How can I create cache map in this situation?

Question

For example, I've got some place in my code, that receives many files (many of them are identical) from disk and, further, unmarshalls them.

final File configurationFile = getConfigurationFile();
FileOutputStream fileOutputStream = new FileOutputStream(configurationFile);
Marshaller.marshal(configObject, fileOutputStream);

Obviously, I can create a special cache map for them to increase performance (in order not to unmarshall identical files again and again). For my case, HashMap implementation will be enough.

The question is: what key for that should I use?

configurationFile.hashCode() is very bad for this?

Thanks for all your answers!

How do you identify that two files are identical? By file name? If the answer is yes, file name should be the key. — Eran, Oct 01 '14 at 08:57
Anyway, using hashCode would be wrong, since different files may have the same hashCode. — Eran, Oct 01 '14 at 09:00

score 0 · Answer 1 · answered Oct 01 '14 at 08:59

You can also use hashset without actually worrying about key.

if(hashset.add(file)) {
   // do unmarshling;
} else {
   //do nothing
}

Hashset.add() method return true if an object can be added. If you try to add duplicate entry then it will return false since duplicacy is not allowed in sets.

icza · Answer 2 · 2014-10-01T09:13:12.887

0

...identical files again and again...

What is identical?

If the file content decides, you may use a hash of the file content (e.g. MD5, SHA1, SHA256) as the key.
If the file name must be identical, simply use the file name as the key.
If the file path, then use the full path of the file as the key (File.getCanonicalPath()).

edited Oct 01 '14 at 09:13

answered Oct 01 '14 at 09:01

icza

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score 0 · Accepted Answer · edited May 23 '17 at 11:47

Use the canonical path instead of the absolute path (explanation of the difference) and put it in a HashSet. Sets don't allow duplicated values. If you try to add a value that already exists, it will return false, otherwise true.

Example code (untested):

Set<String> filesMarshalled= new HashSet<>();
...
final File configurationFile = getConfigurationFile();
if (filesMarshalled.add(configurationFile.getCanonicalPath())) {
    //not marshalled yet
    FileOutputStream fileOutputStream = new FileOutputStream(configurationFile);
    Marshaller.marshal(configObject, fileOutputStream);
}

How can I create cache map in this situation?

3 Answers3