I wouldn't say that MDI is an "older" style:
true multi-window apps are as old as Smalltalk.

There is a tradeoff here,
and people have different preferences.
If you use only one app at a time,
but have multiple live docs within that app,
then a full-screen MDI is a nice way to go.
If you use one app at a time,
but only have one live doc at a time,
then MDI is a waste of screen space.
If you have multiple apps running at once,
and use them to arrange, compare, and transfer information
between windows in ways that no single application would allow,
then MDI is entirely counterproductive.

I've seen a lot of people who full-screen all their MDI apps,
and then painstakingly switch between apps
when they need to evaluate information from more than one.
It's an ineffective way of working,
but many people think it's the way things are supposed to be.
Breaking them of this habit can be hard.

A nice feature of a non-MDI app
is to be able to gather all of that app's windows together
in some nicely ranked arrangement on demand,
usually via some global menu item.
This allows you to clean up desktop clutter instantly,
which is the only thing that MDIs really have in their favor.

Steve

What are you missing? There isn't much more than placing JInternalFrames
on a JDesktop.

Microsoft, for a long time the greatest proponent of MDI applications,
gave up on them and changed their GUI design rules some years ago.

On other platforms they were never popular at all. It was basically the
Microsoft world who made them so popular.

Screen space is a scare resource. The big fat (and IMHO superfluous)
main windows of MDI applications take up so much of this scare resource
without delivering an adequate value.

And why should that matter? I always hear this from MDI fans. When do
you have the need to figure out (at a glance) "oh, all these windows
belong to application XYZ!". What you usually do is to look at a
particular window and check which file/data you are processing with that
window.

And even if you really need such a feature from time to time, is it
really worth to sacrify so much screen space all the time for the
occassional "tell at a glance"?

There is not "THE" user. For the same reason users almost blindly click
on OK buttons if you present them one (it is a habit), there is a
category of users who absolutely want MDI. They are used to it and don't
want to change at all. You can sometimes also observer that MDI fan like
the "information throttling" caused by MDI. MDI limits the amount of
information you can have on the screen at once, because there isn't much
more room to see data from another application.

But if you take Unix users for example, they almost universally hate MDI
applications. Historically there were never many MDI applications for
Unix and any vendor trying to deliver an MDI application for Unix today
has a hard time. I have more than once seen that an application already
failed the first round in some application selection process because
just seconds after it started it came up with a big fat MDI window - of
course taking up the whole 21" screen.

I have seen people arguing that management types usually prefer MDI,
while technical peoples prefer SDI. I haven't observed this, but my GUI
applications were usually for technical people, and I never had a
request to change them to MDI.

/Thomas

Using Swing, it isn't too much work to offer the user both modes, as well as
the
ability to move individual windows between the virtual desktop and system
desktop.