Populating Trees and Lists in Java

I’d like to comment on what went into the new Font Chooser dialog for Kongzi Beta-5. The problem can be posed as follows:

Create a dialog box that takes, as input, existing data.
Complication: Selecting a value on some lists will change the contents of the others.

Obviously this sort of design pattern would have important usages; configuration boxes, persistant user profiles, and so forth. Or, for example, a specialized Font Chooser – which is the example I’ll discuss. Take a look at this screenshot from Kongzi Beta-4:

Kongzi Beta-4 Japanese Fonts

I remember how fiendishly difficult this was to code the first time around, so the second time around I was prepared. The problem would be that I had to follow a special order: populate the language box, select the language, then use that event to populate the font box. But then I had to interrupt the normal order of things and populate the font box before I selected the font from pre-existing data. I couldn’t think of a way to do that without cutting and pasting code or creating a very convoluted logic.

But worrying about that before I wrote any code would drive me insane. So I plunged in head first, fleshing out the dialog box in Netbeans, then adding several methods to find and select values in the list, such as “SelectLanguage(String)”. That way whenever I wanted to, I could populate the dialog box with pre-existing data, in this case, the language and font data that had been selected previously. This is important, of course, to provide a smooth user experience.

Then the nasty null pointer errors started. Basically because of the logic/flow problem I described above. The logic for populating from pre-existing data, and the logic to modify itself based on user input, was too difficult to reconcile. When I tried to populate language, the size box wouldn’t be populated yet, so I couldn’t preview sample text for the new language. If I tried to populate the size box first, this would also trigger a preview event which would crash. The workaround in Kongzi Beta-4 was to include a preview button as shown in the above screenshot. Then I was technically excused from having to do anything. But just because I put a button on it doesn’t mean it wasn’t a giant kludge. For Beta-5 I wanted something smoother. I wanted to use the same logic at all times, the same entry points, and I didn’t want a preview button. I wanted selection events to populate everything.

The key to my final solution was in realizing that when I sent commands to the lists, such as fontList.setSelectedIndex(i), I was going to trigger events which would send me back into the auto-population and auto-selection methods. This is how it had to be, to avoid cutting and pasting code, making a terrible mess. Yes, I wanted to follow “accepted practices” of reusing the same code which auto-populated the lists to accept events and propagate their own data to other lists. Doesn’t everyone?

The first idea I had was checking for null pointers and then skipping over those lines of code. I rejected this almost immediately because of the difficulty it would present to doing any sort of error checking. I would in essence be assuming that any time I encountered a null pointer, I knew why it was there; and that was simply untrue. To write proper code, I would then have to write a second check to determine if the null pointer was there because there was a bug, or simply because the dialog wasn’t fully populated yet.

Or, as an aside, populated properly. I saw the potential for sloppy logic to start creeping in, caused by my abuse of checking for null pointers. In horror, I imagined several cases where entire blocks of code would never execute because they would always be called at a time when null pointers were there. I imagined terrifying cases where a population function would be called four, five, or more times, each time getting sent back with a null pointer, until the dialog was finally ready to accept the input. I recoiled from checking for null pointers, and resolved to find a better way.”

After staring into the monitor for a while, I came up with the idea of using boolean flags to keep track of what procedure I was in. The solution worked like a charm. I created several class-wide variables such as “boolean populatingFontList”. Any time a method dialog was performing an operation on a list where the selection would change as an unintended consequence, I simply set “updatingFontList” or “updatingSelectLanguage” (or whatever) to true. Then, at the beginning of that function (and any other appropriate place) I would put in a clause which prevented anything bad from happening. Usually something like “if (updatingFonts) return;” at the beginning of a troublesome function.

populatingSelectLanguage = true;
populatingSelectLanguage = false;

Then when the program fires off it’s little events as a result of you removing whatever item was selected, you wont get an error because the font list isn’t populated yet. And so forth.


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: