Java: Comparators

// File: arrays/filelist/Filelistsort.java
// Purpose: List contents of currrent diectory (.)
//          Demonstrates use of Comparators.
//          Java 5: Generics and for each loop.
// Author: Fred Swartz
// Date  : 2005-Feb-22

import java.util.Arrays;
import java.util.Comparator;
import java.io.*;

public class Filelistsort {

    //======================================================= main
    public static void main(String[] args) {
        //... Create some comparators that we'll use for sort.
        Comparator<File> fileComp = new FileComparator();
        Comparator<File> longComp = new LongNamesFirstComparator();

        //... Create a File object for the current directory.
        File dir = new File(".");
        File[] children = dir.listFiles();

        //.. Print out the names in one order.
        System.out.println("Files by directory, then alphabetical");
        Arrays.sort(children, fileComp); // Sort using a comparator.
        for (File oneEntry : children) {
            System.out.println("   " + oneEntry.getName());
        }

        //.. Print out the names in another order.
        System.out.println("Files by length of name (long first)");
        Arrays.sort(children, longComp); // Sort using a comparator.
        for (File oneEntry : children) {
            System.out.println("   " + oneEntry.getName());
        }
    }
}


//////////////////////////////////////////////////// FileComparator
// This comparator will compare two File objects and
// return -1, 0, or +1 depending on their relative order.
class FileComparator implements Comparator<File> {

    // Comparator interface requires defining compare method.
    public int compare(File filea, File fileb) {
        //... Sort directories before files,
        //    otherwise alphabetical ignoring case.
        if (filea.isDirectory() && !fileb.isDirectory()) {
            return -1;

        } else if (!filea.isDirectory() && fileb.isDirectory()) {
            return 1;

        } else {
            return filea.getName().compareToIgnoreCase(fileb.getName());
        }
    }
}


/////////////////////////////////////////// LongNamesFirstComparator
// This comparator will compare two File objects and
// return -1, 0, or +1 depending on their name sizes
class LongNamesFirstComparator implements Comparator<File> {

    // Comparator interface requires defining compare method.
    public int compare(File filea, File fileb) {
        int comp = fileb.getName().length() - filea.getName().length();
        if (comp != 0) {
            //... If different lengths, we're done.
            return comp;
        } else {
            //... If equal lengths, sort alphabetically.
            return filea.getName().compareToIgnoreCase(fileb.getName());
        }
    }
}