Friday, May 15, 2009

JTextArea and javax.swing.text.Document

I was playing with a JTextArea in my Swing project today. My underlying data is actually quite simple (a list of names and quotes), and my JTextArea is read-only. Therefore, I decided to try to make my own class that implements javax.swing.text.Document, but whose internal structure more closely models my real... model.

I had a hard time getting started. From poking around in Document's Javadocs and in related Java source code, it appears that Document is a very general concept. It seems to have originally been intended to model an SGML document's structure - or something like that. The Document interface lets you navigate a tree structure that represents the document. However, when it comes to modeling data for a JTextArea, things are much simpler. Instead, it appears that JTextComponent (the base class of JTextArea) assumes that the Document models a list of lines of text. That is to say, it seems to expect that the Document has one root element which contains a separate element for each line. This is most obvious from JTextArea's getLineCount() method:

public int getLineCount() {
Element map = getDocument().getDefaultRootElement();
return map.getElementCount();

Now, the problem with all of this is that Document is a pretty complicated interface. On top of that, since SGML is a tree-oriented structure, it is not quite intuitive to map the concepts of "stream of text" to "tree-shaped structure". More problematically, I didn't see anything in the documentation to even begin to shed some light on this unspoken relationship. I stumbled upon it while examining stack traces from my first cut. I vaguely recall this being much easier in the little bit of Cocoa that I dabbled in, but I can't be sure. Maybe I'm just crazy - there are certainly a number of other concrete, Document-derived classes out there.

So remember, if you want to implement Document, make sure to model your document as a list of lines of text.