Re: setText() is not always updating Jtextfield and Jlabel items

From	Nigel Wade <nmw@ion.le.ac.uk>
Newsgroups	comp.lang.java.gui
Subject	Re: setText() is not always updating Jtextfield and Jlabel items
Date	2015-02-27 15:45 +0000
Message-ID	<clbhokF6emnU1@mid.individual.net> (permalink)
References	<20150226132317.6bbbc29a@efreet.linux>

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On 26/02/15 20:23, Kevin Nathan wrote:
> // Trying new "Motion" routine:
> while (clerkFuncs.scaleErr == -2) {
>      weight = clerkFuncs.readScale(tr);
>      showMotion(weight);
> }
>
>
> private void showMotion(long wt) {
>
>      noteField[1][weightRow].setText(Long.toString(wt));
>      // noteField[1][weightRow].setText("<html><font
>      color='red'>Motion</font></html>");
>
>      editField[1][weightRow].setText("Motion");
> }

This is the problem.

For setText() to work it needs to be invoked by the Event Dispatch Thread
(http://docs.oracle.com/javase/tutorial/uiswing/concurrency/index.html)

However, there is also the issue of allowing the EDT to do its work. If you block the thread in a loop then the EDT 
never gets a chance to do its work.

Your code above is wrong because it either
1) is not invoked on the EDT
2) is invoked on the EDT and therefore blocks the thread in the while() loop.

The normal way that Swing is used is either by event action, or by explicitly invoking a method on the EDT.
If your code is an action handler invoked on the EDT then it must perform an action and return to allow the EDT to get 
on with everything else it has to do. If your code is running on thread other than the EDT it should invoke the 
setText() method on the EDT by using SwingUtilities.invokeAndWait() or SwingUtilities.invokeLater(). setText() is one of 
those methods which "mostly" works if you use on a non-EDT thread, which may explain why your code appears to work for 
other uses of setText(). However, "mostly" is not always, and it's incorrect code because setText() is not thread safe, 
and it will likely cause you intermittent and non-reproducable problems when you least expect, or want, them in 
production use.

With the code you have, the while loop should not be running on the EDT. The showMotion() method should be. I don't know 
how you get to the path of the while loop, but it's probably going to have to be changed. To get the while loop to run 
the setText() method on the EDT you use something along the lines of:

  final long weight=wt;
  SwingUtilties.invokeAndWait(new Runnable() { public void run() {showMotion(weight); }} );

The other, and arguably more elegant solution, is to use a SwingWorker.
Your while loop would go in the doInBackground() method, and publish() the weights. The SwingWorker.process() method 
would essentially be your showMotion() method which processes the published weights.

The above link to the Java Swing tutorial should hopefully provide some insight.

Just remember the fundamental issue, Swing is event driven on the EDT.

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Thread

setText() is not always updating Jtextfield and Jlabel items Kevin Nathan <knathan@project54.com> - 2015-02-26 13:23 -0700
  Re: setText() is not always updating Jtextfield and Jlabel items Jeff Higgins <jeff@invalid.invalid> - 2015-02-26 19:43 -0500
    Re: setText() is not always updating Jtextfield and Jlabel items Kevin Nathan <knathan@project54.com> - 2015-02-27 08:21 -0700
  Re: setText() is not always updating Jtextfield and Jlabel items Knute Johnson <nospam@rabbitbrush.frazmtn.com> - 2015-02-26 20:05 -0800
    Re: setText() is not always updating Jtextfield and Jlabel items Kevin Nathan <knathan@project54.com> - 2015-02-27 08:24 -0700
  Re: setText() is not always updating Jtextfield and Jlabel items Nigel Wade <nmw@ion.le.ac.uk> - 2015-02-27 15:45 +0000
    Re: setText() is not always updating Jtextfield and Jlabel items Kevin Nathan <knathan@project54.com> - 2015-03-02 09:33 -0700

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