Java(TM) Boutique - Java and JDOM: the perfect couple

Tutorials : Java and JDOM: the perfect couple :

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A web application to handle the DVD library

The XML file containing the DVD library

The simplest operations you can carry out on a document tree are create, update or delete an element. To show you how to do this I'll need a simple XML-structure, and the example from my SAX-article will do fine (an attribute "id" has been added to cover handling attributes as well):

 
<?xml version = "1.0" ?>
<collection>
  <dvd id="A">
    <title>Lord of the Rings: The Fellowship of the Ring</title>
    <length>178</length>
    <actor>Ian Holm</actor>
    <actor>Elijah Wood</actor>
    <actor>Ian McKellen</actor>
  </dvd>
  <dvd id="B">
    <title>The Matrix</title>
    <length>136</length>
    <actor>Keanu Reeves</actor>
    <actor>Laurence Fishburne</actor>
  </dvd>
  <dvd id="C">
    <title>Amadeus</title>
    <length>158</length>
    <actor>F. Murray Abraham</actor>
    <actor>Tom Hulce</actor>
    <actor>Elizabeth Berridge</actor>
  </dvd>
</collection>

The examples below will now show how to manipulate the DVD-elements in the structure. All DVDs are first stored in a List like this:

List dvds = doc.getRootElement().getChildren();

Creating a new dvd-Element in the tree

A new element is created like this:

Element dvd = new Element("dvd");

and the title and length can now be added:

dvd.addContent(new Element("title").setText(title));
dvd.addContent(new Element("length").setText(length));

"addContent" is an important method in the Element class. Even if I won't cover these subjects, it might be nice for you to know that addContent is also used to add XML-comments, Processing Instructions and other information to an Element.

To add the id-attribute we need an instance of the Attribute class:

dvd.setAttribute(new Attribute("id", id));

Adding actors is just like adding a title or length--we simply use addContents once for every actor. Let's wrap the code into a general method:

public void createDVD(String id, String title, String length, List actors) {
  Element dvd = new Element("dvd");
  dvd.setAttribute(new Attribute("id", id));
  dvd.addContent(new Element("title").setText(title));
  dvd.addContent(new Element("length").setText(length));
  for (int i = 0; i < actors.size(); i++) {
    dvd.addContent(new Element("actor").setText((String)actors.get(i)));
  }  
  dvds.add(dvd);
}

Note that JDOM now benefits from building on the List-Interface by using its add-method. There's no need for a special JDOM method for this purpose.

Updating a dvd-Element in the tree

Setting a new title or length is straightforward:

Element dvd = . . .
dvd.getChild("title").setText(title);
dvd.getChild("length").setText(length);

The value of the id-attribute is set like this:

dvd.getAttribute("id").setValue(id);

Updating the list of actors is most easily done by first removing all actors and then adding the updated list of actors. We'll wrap it up like this:

public void updateDVD(int index, String id, String title, String length, 
                      List actors) {
  Element dvd = (Element)dvds.get(index);
  dvd.getAttribute("id").setValue(id);
  dvd.getChild("title").setText(title);
  dvd.getChild("length").setText(length);
  dvd.removeChildren("actor");
  for (int i = 0; i < actors.size(); i++) {
    dvd.addContent(new Element("actor").setText((String)actors.get(i)));
  }  
}

In the code the dvd Element is located by its index in the List.

Deleting a dvd-Element

This is the easy part. Let's reveal the delete-method right away:

public void deleteDVD(int index) {
  dvds.remove(index);
}

Again we simply use one of the methods from the List interface for the operation.

To test handling of the DVDs I've made a DVDHandler class. that can be called from the command line. The main method includes various calls to the create, update, and delete methods.

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