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.

1 comment:

Alex R. said...

Hello, Daniel!

Thanks for your post, it pushed me forward to understanding of the internals of data models behind the Swing text components.

The article The Element Interface clearly explains the "stream of text" to "tree-shaped structure" mapping. It ended my painful searches for simple explanation of javax.swing.text.Document (and its derivers) structure.