... "I do know, based on the apps logic, when to call setText" ...
.
that means, for example, if a user is supposed to read 50 invoices and
she clicks "Next" to go through them, the user is actually cliking on
the "Next" (or "Previous") button, so you know which invoice she is at
and change the label accordingly via setText, but say if you want to
know how much time she will need to finish reading all invoices (based
on reasonable heuristics, like the speed at which she has been reading
them + all of those that still need to be read ...)
.
I think you will have to capture/reimplement some kind of "mouse over"
action responsible for then setting the ToolTipText ...
.
onetitfemme

.
Exactly! and thank you for "understanding" me ;-)
.
For those of you trying the same thing here is some demo code that
updates the tooltip based on the current time:
.
import javax.swing.*;
import javax.swing.event.*;
import javax.swing.border.*;

import java.awt.*;
import java.awt.event.*;

import java.util.*;
import java.text.*;

// __
public class ToolTipDemo00 extends JFrame{
JLabel JLbl;
SimpleDateFormat SDF;
// __
public ToolTipDemo00(){
 super();
 getContentPane().setLayout(new FlowLayout(FlowLayout.LEFT));
 String aPttrn = "yyyyMMddHHmmss";
 SDF = new SimpleDateFormat(aPttrn);
 String aC = ":" + getFrmtdTm() + ":";
 JLbl = new JLabel(aC){
// __ Implement JLabel's tool tips.
  public String getToolTipText(MouseEvent MEvnt){
return(getFrmtdTm()); }
 };
 JLbl.setToolTipText(getFrmtdTm());

JLbl.setBorder(BorderFactory.createEtchedBorder(EtchedBorder.RAISED));
 JLbl.setSize(100, 25);
 getContentPane().add(JLbl);
}
// __
public String getToolTipText() { return
ToolTipDemo00.this.getToolTipText(); }
// __
private String getFrmtdTm(){ return(SDF.format(new Date())); }
// __
public static void main(String[] args) {
 ToolTipDemo00 TTDm = new ToolTipDemo00();
 TTDm.addWindowListener(new WindowAdapter(){
  public void windowClosing(WindowEvent e){ System.exit(0); }
 });
 TTDm.setSize(200, 150);
 TTDm.setVisible(true);
}
}