Working with files and directories in Java (2)

by Keld H. Hansen

A recap

In my first article about how to work with files and directories in Java we looked at these topics:

the "File" class from the package "java.util", that gives you information about the files and directories contained in a given directory
how to make some Java classes for walking through the tree structure of a directory (we used recursive method calls for this).
how to present the directory information in various formats: simple text lines, HTML, XML and transformed XML

The Java classes defined were:

MyFile - representing a file (in a directory)
MyDir - representing a directory
MyFileStructure - the workhorse for building and listing the directory structure
MyFileStructureXyz - a subclass of MyFileStructure used for a given output format

We used a simple directory structure for testing:

- Figure 1 -

(This figure is copy/pasted from a Microsoft FrontPage directory listing).

Our last attempt to produce our own HTML-view of this directory structure was not that impressive:

- Figure 2 -

And now: the new stuff

In this article we'll make a nice, graphical view like the one in figure 1. We'll also add the possibility to open or close the folders by clicking on them. Two solutions will be presented, both identical when seen in the browser, but the second will have superior response times. How this is done read on to find out.

Note that all the code for the examples--including graphics--are found in this zip-file.

Sorting the files and directories

First we'll close a hole we left open in the last article. The file- and directory-names that we get from the list-method in the File class seems to be sorted alphabetically, but actually they don't have to be. In the jdk-documentation we find this:

There is no guarantee that the name strings in the resulting array will appear in any specific order.
They are not, in particular, guaranteed to appear in alphabetical order.

My experience on Windows 98 and NT is that if only a few names are returned from "list", then they are sorted correctly. But there is no overall sorting when more names are returned, despite some sequences being obviously sorted. So we'll have to do some sorting of our own, and the simplest way to do this is to use the sort-features in the "Collections" class. If you're not familiar with the Collections framework that was introduced in Java 2, I highly recommend you study it, for example by reading Sun's own tutorial.

To have the names sorted we must:

let the MyFile and and MyDir classes implement the Comparable interface - e.g.
public class MyFile implements Comparable

add a compareTo-method in each of these classes - e.g. for MyFile:

  public int compareTo(Object o) {
    MyFile f = (MyFile)o;
    return name.compareToIgnoreCase(f.name);
  }

in the MyFileStructure class we finish the build-method by calling sort in the Collections class:

    . . .
    Collections.sort(mdir.getDirs());
    Collections.sort(mdir.getFiles());
    return mdir;
    . . .

That's all. If you would like to see the complete code then look in this zip-file.

Applet Index
(sorted alphabetically)
A B C D E F G H I J K
L M N O P Q R S T U
V W X Y Z #s

The Java Source
(applets w/source code)
A B C D E F G H I J K
L M N O P Q R S T U
V W X Y Z

How to Add Java Applets to Your Site

New on the Java Boutique:

New Review:

Time Management Made Easy with the Quartz Enterprise Job Scheduler
Why not just use the Java timer API? This open source scheduling API boasts simplicity, ease-of-integration, a well-rounded feature set, and it's free!

New Applet:

Reverse Complement
Reverse Complement is a simple applet that converts DNA or RNA sequences into three useful formats.