This is to demonstrate how a comparator is built and put to work. Try adding these test files to your data folder (use import), then try modifying one of the files:



import java.io.File;
import java.util.ArrayList;


class LastModifiedFileComparator implements Comparator{
public int compare(Object o1, Object o2) {
               Long i = ((File)o1).lastModified() -  ((File)o2).lastModified();
return i.intValue();
}
}
File f1 = new File(dataPath("hello.txt"));
File f2 = new File(dataPath("world.txt"));
File f3 = new File(dataPath("cliche.txt"));

ArrayList files = new ArrayList();

files.add(f1);
files.add(f2);
files.add(f3);


for(int i = 0; i < files.size(); i++){
 File x = (File)files.get(i);
 println(x.getName());
println("" + x.lastModified());
}

Collections.sort(files, new LastModifiedFileComparator() );

for(int i = 0; i < files.size(); i++){
 File x = (File)files.get(i);
 println(x.getName());
println("" + x.lastModified());
}

I can see how it is confusing.

A File object is a *representation* of a file, so when you create a File object like:

File f = new File(  <some path> );

You really aren't creating a new file.  In English, you are saying "I would like to create a representative object of a file in this location (indicated by file path name, like "/User/user1/sometext.txt").  If I want to do anything to this file, I will refer to this representation."


That's why you need to actually import a file into the "Data" directory of your sketch before you can manipulate it.



So, if you want to sort a file that already exist elsewhere, try replacing:  

File f1 = new File(dataPath("hello.txt"));


with

File f1 = new File("  ....some path name .... ");


Where some path name is the file path of your file.

For an explanation of how file paths work, see:
http://en.wikipedia.org/wiki/Path_(computing)