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How to deliver object to a long-running SwingWorker from the EDT?

How can the EDT communicate to an executing SwingWorker? There a lot of ways for the SwingWorker to communicate information back to the EDT - like publish/process and property changes but no defined way (that I have seen) to communicate in the other direction. Seems like good old Java concurrent inter-thread communication would be the way to go via wait() and notify(). This doesn't work. I'll explain later. I actually got it to work but it uses an ugly polling mechanism. I feel like there should be a better way. Here is the process that I am trying to accomplish:
From the main Swing UI (EDT) a user starts a SwingWorker long-running task (the engine).
At some point the engine needs information from the EDT so it communicates this back to the EDT. this could be done through publish/process update of a visible UI component. Importantly, this step DOES NOT block the EDT because other things are also going on.
The engines blocks waiting for an answer.
At some point the user notices the visual indication and provides the required information via some UI (EDT) functionality - like pressing a Swing button.
The EDT updates an object on the engine. Then "wakes up" the engine.
The engine references the updated object and continues to process.
The problem I have with wait()/notify() is that in step 3 any invocation of wait() in doInBackground() causes the done() method to be immediately fired and the SwingWorker to be terminated.
I was able to get the above process to work by using an ugly sleep() loop in doInBackground():
for (;;)
{
Thread.sleep(10);
if (fromEDT != null)
{
// Process the update from the EDT
System.out.println("From EDT: " + fromEDT);
fromEDT = null;
break;
}
}
What this really is that in step 5 the engine wakes itself up and checks for updates from the EDT.
Is this the best way to do this? I kind of doubt it.

The following is an mre demonstrating a SwingWorker paused and waiting for user's input:
import java.awt.*;
import java.util.List;
import javax.swing.*;
public class SwingWorkerWaitDemo {
public static void creategui(){
JFrame f = new JFrame("SwingWorker wait Demo");
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
f.setLocationRelativeTo(null);
f.add(new MainPanel());
f.pack();
f.setVisible(true);
}
public static void main(String[] args) {
creategui();
}
}
class MainPanel extends JPanel {
private static final String BLANK = " ";
private MyWorker swingWorker;
private final JLabel output, msg;
private final JButton start, stop, respond;
MainPanel() {
setLayout(new BorderLayout(2, 2));
start = new JButton("Start");
start.addActionListener(e->start());
stop = new JButton("Stop");
stop.setEnabled(false);
stop.addActionListener(e->stop());
JPanel ssPane = new JPanel(new FlowLayout(FlowLayout.CENTER));
ssPane.add(start); ssPane.add(stop);
add(ssPane, BorderLayout.PAGE_START);
output = new JLabel(BLANK);
JPanel outputPane = new JPanel(new FlowLayout(FlowLayout.CENTER));
outputPane.add(output);
add(outputPane, BorderLayout.CENTER);
msg = new JLabel(BLANK);
respond = new JButton("Respond");
respond.addActionListener(e->respond());
respond.setEnabled(false);
JPanel responsePane = new JPanel();
responsePane.add(msg); responsePane.add(respond);
add(responsePane, BorderLayout.PAGE_END);
}
#Override
public Dimension getPreferredSize(){
return new Dimension(400, 200);
}
private void start() {
start.setEnabled(false);
stop.setEnabled(true);
swingWorker = new MyWorker();
swingWorker.execute();
}
private void stop() {
stop.setEnabled(false);
swingWorker.setStop(true);
}
private void message(String s){
msg.setText(s);
}
private void clearMessage(){
msg.setText(BLANK);
}
private void askForUserResponse(){
respond.setEnabled(true);
message("Please respond " );
}
private void respond(){
clearMessage();
respond.setEnabled(false);
swingWorker.setPause(false);
}
class MyWorker extends SwingWorker<Integer, Integer> {
private boolean stop = false;
private volatile boolean pause = false;
#Override
protected Integer doInBackground() throws Exception {
int counter = 0;
while(! stop){
publish(counter++);
if(counter%10 == 0) {
pause = true;
askForUserResponse();
while(pause){ /*wait*/ }
}
Thread.sleep(500);
}
return counter;
}
#Override
protected void process(List<Integer> chunks) {
for (int i : chunks) {
output.setText(String.valueOf(i));
}
}
#Override
protected void done() {
message("All done");
}
void setStop(boolean stop) {
this.stop = stop;
}
void setPause(boolean pause) {
this.pause = pause;
}
}
}

Swing going from menu to actual gameplay

So I am making a space invaders clone. Originally I had no problem getting my game to work with a simple main class that created the frame, created the gameplay and started the thread.
But then I tried to implement a start menu and it all went to crap. The menu appears with success but the gameplay does not appear when I press start.
I am running out of ideas and I am completely stumped. I am somewhat new as well to SO, so if there is anything I left out, I appreciate any help.
Here is the original with no menu that worked fine:
public static void main(String[] args) {
JFrame frame = new JFrame("SpaceRaiders");
frame.setSize(600, 600);
frame.setLocationRelativeTo(null);
frame.setResizable(false);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
Gameplay gameplay = new Gameplay();
frame.add(gameplay);
frame.setVisible(true);
Thread t1 = new Thread(gameplay);
t1.start();
}
However, the moment I tried to implement a menu to then play the game, I am running into all sorts of trouble. I created a UI class as well as an actual "game" class like so:
public class UI {
JFrame frame, f2;
JPanel titrePanel, startButtonPanel, loadButtonPanel, p2;
JLabel nomJeu;
JButton startButton, loadButton;
Font fontTitre, fontStart;
Gameplay gameplay;
public void createUI(ChoixJeu cj) {
frame = new JFrame("SpaceRaiders");
frame.setSize(600, 600);
frame.setLocationRelativeTo(null);
frame.setResizable(false);
frame.setLayout(null);
frame.getContentPane().setBackground(Color.black);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
//------------------ECRAN MENU---------------------
//Titre
titrePanel = new JPanel();
titrePanel.setBounds(100, 100, 400, 100);
titrePanel.setBackground(Color.BLUE);
Font fontTitre = new Font("Times New Roman", Font.BOLD, 50);
Font fontStart = new Font("Times New Roman", Font.PLAIN, 20);
nomJeu = new JLabel("SpaceRaiders");
nomJeu.setForeground(Color.white);
nomJeu.setFont(fontTitre);
titrePanel.add(nomJeu);
//Start button
startButtonPanel = new JPanel();
startButtonPanel.setBounds(200, 400, 200, 40);
startButtonPanel.setBackground(Color.BLACK);
startButton = new JButton("START");
startButton.setBackground(Color.BLACK);
startButton.setForeground(Color.WHITE);
startButton.setFont(fontStart);
startButton.setFocusPainted(false);
startButton.addActionListener(cj);
startButton.setActionCommand("start");
startButtonPanel.add(startButton);
//Load Button
loadButtonPanel = new JPanel();
loadButtonPanel.setBounds(200, 440, 200, 100);
loadButtonPanel.setBackground(Color.BLACK);
loadButton = new JButton("LOAD");
loadButton.setBackground(Color.BLACK);
loadButton.setForeground(Color.WHITE);
loadButton.setFont(fontStart);
loadButton.setFocusPainted(false);
titrePanel.add(nomJeu);
loadButtonPanel.add(loadButton);
frame.add(startButtonPanel);
frame.add(titrePanel);
//------------------ECRAN MENU FIN---------------------
frame.setVisible(true);
}
And the game class...
public class Jeu {
ChoixJeu cj = new ChoixJeu();
UI ui = new UI();
Ecrans e = new Ecrans(ui);
Gameplay gp;
public static void main(String[] args) {
new Jeu();
}
public Jeu() {
ui.createUI(cj);
Gameplay gameplay = new Gameplay();
this.gp = gameplay;
}
public class ChoixJeu implements ActionListener {
#Override
public void actionPerformed(ActionEvent ae) {
String yourChoice = ae.getActionCommand();
switch (yourChoice) {
case "start":
e.montrerEcranJeu();
new Thread(gp).start();
ui.frame.add(gp);
break;
default:
break;
}
}
}
}
I also tried to make a class/method that hides the menu panels
public void montrerEcranJeu() {
//Cache Menu
ui.titrePanel.setVisible(false);
ui.startButtonPanel.setVisible(false);
//Montre Jeu
// ui.frame.add(gameplay);
}
And just in case the Gameplay class. The run() method is at the bottom
public class Gameplay extends JPanel implements KeyListener, ActionListener, Runnable {
private Ship player = new Ship(new Point(200, 555));
Timer t = new Timer(5, this);
private ArrayList<Laser> lasers = new ArrayList<Laser>();
private int laserNb;
private boolean readytofire;
private boolean shot = false;
private ArrayList<Invader> invaders = new ArrayList<Invader>();
private boolean pause;
public Gameplay() {
super();
t.start();
addKeyListener(this);
setFocusable(true);
setFocusTraversalKeysEnabled(false);
for (int j = 0; j < 80; j += 20) {
for (int i = 0; i < 20; i++) {
invaders.add(new Invader(5 + i * 30, j));
}
}
}
public boolean addLaser(Laser a) {
lasers.add(a);
return true;
}
public boolean addPlayer(Ship p) {
this.player = p;
return true;
}
#Override
public void keyTyped(KeyEvent ke) {
}
public void keyPressed(KeyEvent e) {
if (KeyEvent.VK_RIGHT == e.getKeyCode()) {
moveRight();
}
if (KeyEvent.VK_LEFT == e.getKeyCode()) {
moveLeft();
}
if (KeyEvent.VK_SPACE == e.getKeyCode()) {
shoot();
System.out.println("Space Action from Gameplay is working");
}
}
#Override
public void keyReleased(KeyEvent e) {
}
#Override
public void actionPerformed(ActionEvent ae) {
repaint();
}
public void moveRight() {
if (player.getCentre().getX() >= 580) {
player.setX(580);
} else {
double movement = player.getCentre().getX();
movement += 10;
player.setX(movement);
}
this.repaint();
}
public void moveLeft() {
if (player.getCentre().getX() <= 20) {
player.setX(20);
} else {
double movement = player.getCentre().getX();
movement -= 10;
player.setX(movement);
}
this.repaint();
}
public void shoot() {
shot = true;
if (readytofire) {
Point top = new Point(player.getTopX(), player.getTopY());
Laser laser = new Laser(top);
addLaser(laser);
}
}
public void moveShot() {
if (shot) {
for (Laser l : lasers) {
l.setY(l.getTopLeft().getY() - 1);
}
}
}
#Override
public void paint(Graphics g) {
setBackground(Color.black);
super.paint(g);
player.draw(g);
for (Laser l : lasers) {
l.draw(g);
}
for (Invader i : invaders) {
i.draw(g);
}
}
// public void paintComponent (Graphics g){
// Controle Thread
public void run() {
while (true) {
moveShot();
for (Invader i : invaders) {
i.moveAndUpdate();
}
// for (Invader i : invaders) {
// if (){
// System.out.println("YOU ARE DEAD!");
// }
// }
try {
Thread.sleep(10);
readytofire = true;
} catch (InterruptedException ex) {
Logger.getLogger(Gameplay.class.getName()).log(Level.SEVERE, null, ex);
}
}
}
}

So, using null layouts is the beginning of your problems. I might recommend using CardLayout which is designed to help you dynamically switch between views. See How to Use CardLayout for more details. I'd also suggest taking the time to read through Laying Out Components Within a Container and finding one or more appropriate layouts to support your menu.
You're also making a lot of fundamental mistakes. Swing is not thread safe, so you should avoid updating the UI (or something the UI depends on) from outside the context of the EDT - see Concurrency in Swing for more information and How to Use Swing Timers for a possible solution.
As a general recommendation, you should avoid overriding paint and, in the case of classes which extend from JComponent, prefer paintComponent instead. You should also avoid call methods which might change the state of the component during a paint cycle, this can increase the number of repaint requests and degrade the performance of your program (ie, don't call setBackground inside paint).
Have a look at Performing Custom Painting and Painting in AWT and Swing for more details about how the paint system works and how best you can work with it.
You should also avoid KeyListener, this is likely to cause you issues when you introduce other, focusable, components into the picture. Instead, you should favour the Key bindings API instead
I've read through [insert link or tutorial], but it still doesn't help...
And forgive me if this doesn't happen all the time.
The point of providing you the tutorial links is to encourage you to learn something;
Learn where to find answers to your questions
Learn how the APIs work
Expand your knowledge and understanding of how the APIs work
Having said that, they're not always "obvious" as to the solution. What I do when I'm in this situation is start with one or more new projects, dedicated to just working on that aspect of the API I'm trying to understand. For here I can explore the concepts in isolation and when I "think" I understand them, try and implement them into the project I'm working on. This might take a number of iterations, but once it works, I have gained a much deeper understanding and appreciation of the API then I would have gained from a simple "copy-n-paste" solution

Java Swing: How do I wake up the main thread from the event-dispatch thread?

I want to cause the "main thread" (the thread started which runs main()) to do some work from the actionPerformed() method of a button's ActionListener, but I do not know how to achieve this.
A little more context:
I am currently programming a 2D game using Swing (a flavour of Tetris).
When the application starts, a window opens which displays the main menu of the game.
The user is presented several possibilities, one of them is to start the game by pushing a "Start" button, which causes the game panel to be displayed and triggers the main loop of the game.
To be able to switch between the two panels (that of the main menu and that of the game), I am using a CardLayout manager, then I can display one panel by calling show().
The idea is that I would like my start button to have a listener that looks like this:
public class StartListener implements ActionListener {
StartListener() {}
public void actionPerformed(ActionEvent e) {
displayGamePanel();
startGame();
}
}
but this does not work because actionPerformed() is called from the event-dispatch thread, so the call to startGame() (which triggers the main loop: game logic update + repaint() call at each frame) blocks the whole thread.
The way I am handling this right now is that actionPerformed() just changes a boolean flag value: public void actionPerformed(ActionEvent e) {
startPushed = true;
}
which is then eventually checked by the main thread:
while (true) {
while (!g.startPushed) {
try {
Thread.sleep(100);
} catch (Exception e) {}
}
g.startPushed = false;
g.startGame();
}
But I find this solution to be very inelegant.
I have read the Concurrency in Swing lesson but I am still confused (should I implement a Worker Thread – isn't that a little overkill?). I haven't done any actual multithreading work yet so I am a little lost.
Isn't there a way to tell the main thread (which would be sleeping indefinitely, waiting for a user action) "ok, wake up now and do this (display the game panel and start the game)"?.
Thanks for your help.
EDIT:
Just to be clear, this is what my game loop looks like:
long lastLoopTime = System.currentTimeMillis();
long dTime;
int delay = 10;
while (running) {
// compute the time that has gone since the last frame
dTime = System.currentTimeMillis() - lastLoopTime;
lastLoopTime = System.currentTimeMillis();
// UPDATE STATE
updateState(dTime);
//...
// UPDATE GRAPHICS
// thread-safe: repaint() will run on the EDT
frame.repaint()
// Pause for a bit
try {
Thread.sleep(delay);
} catch (Exception e) {}
}

This doesn't make sense:
but this does not work because actionPerformed() is called from the event-dispatch thread, so the call to startGame() (which triggers the main loop: game logic update + repaint() call at each frame) blocks the whole thread.
Since your game loop should not block the EDT. Are you using a Swing Timer or a background thread for your game loop? If not, do so.
Regarding:
while (true) {
while (!g.startPushed) {
try {
Thread.sleep(100);
} catch (Exception e) {}
}
g.startPushed = false;
g.startGame();
}
Don't do this either, but instead use listeners for this sort of thing.
e.g.,
import java.awt.CardLayout;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;
public class GameState extends JPanel {
private CardLayout cardlayout = new CardLayout();
private GamePanel gamePanel = new GamePanel();
private StartPanel startpanel = new StartPanel(this, gamePanel);
public GameState() {
setLayout(cardlayout);
add(startpanel, StartPanel.DISPLAY_STRING);
add(gamePanel, GamePanel.DISPLAY_STRING);
}
public void showComponent(String displayString) {
cardlayout.show(this, displayString);
}
private static void createAndShowGui() {
GameState mainPanel = new GameState();
JFrame frame = new JFrame("GameState");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.getContentPane().add(mainPanel);
frame.pack();
frame.setLocationByPlatform(true);
frame.setVisible(true);
}
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
public void run() {
createAndShowGui();
}
});
}
}
class StartPanel extends JPanel {
public static final String DISPLAY_STRING = "Start Panel";
public StartPanel(final GameState gameState, final GamePanel gamePanel) {
add(new JButton(new AbstractAction("Start") {
#Override
public void actionPerformed(ActionEvent e) {
gameState.showComponent(GamePanel.DISPLAY_STRING);
gamePanel.startAnimation();
}
}));
}
}
class GamePanel extends JPanel {
public static final String DISPLAY_STRING = "Game Panel";
private static final int PREF_W = 500;
private static final int PREF_H = 400;
private static final int RECT_WIDTH = 10;
private int x;
private int y;
public void startAnimation() {
x = 0;
y = 0;
int timerDelay = 10;
new Timer(timerDelay , new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
x++;
y++;
repaint();
}
}).start();
}
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
g.fillRect(x, y, RECT_WIDTH, RECT_WIDTH);
}
#Override
public Dimension getPreferredSize() {
return new Dimension(PREF_W, PREF_H);
}
}

you should be using a SwingWorker this will execute the code in doInBackground() in a background thread and the code in done() in the EDT after doInBackground() stops

The easiest way: use a CountDownLatch. You set it to 1, make it available in the Swing code by any means appropriate, and in the main thread you await it.

You can consider showing a modal dialog with the game panel using SwingUtilities.invokeAndWait() so that when the dialog is closed the control returns back to main thread.

You can make all code except the EDT run on single thread execution service and then just post runnables whenever you need some code executed.

Pausing a timerTask

I have hit another wall. After getting my key input working, I have been racking my brains for hours, i want to create a pause function, so that if the same key is pressed again the timertask stops running (i.e the game is paused)
JPanel component = (JPanel)frame.getContentPane();
component.getInputMap().put(KeyStroke.getKeyStroke("SPACE"), "space");
component.getActionMap().put("space", (new AbstractAction(){
public void actionPerformed(ActionEvent e){
Timer timer = new Timer();
timer.scheduleAtFixedRate(new TimerTask(){
public void run(){
grid.stepGame();
}
},250, 250);
}}));
}
The problem is i cant use a global boolean isRunning var and switch it each time the key is pressed because the timerTask method in a nested class (so the boolean isRunning would have to be declared final to be accessed...). Any ideas on how to detect if the key is pressed again or if the game is already running so i can pause/cancel my timerTask.
Many Thanks Sam

Since this is a Swing game, you should be using a javax.swing.Timer or Swing Timer and not a java.util.Timer. By using a Swing Timer, you guarantee that the code being called intermittently is called on the EDT, a key issue for Swing apps, and it also has a stop method that pauses the Timer. You can also give your anonymous AbstractAction class a private boolean field to check if the key is being pressed for the first time or not.
Also, kudos and 1+ for using Key Bindings instead of a KeyListener.
e.g.,
JPanel component = (JPanel) frame.getContentPane();
component.getInputMap().put(KeyStroke.getKeyStroke("SPACE"), "space");
component.getActionMap().put("space", (new AbstractAction() {
private boolean firstPress = true;
private int timerDelay = 250;
private javax.swing.Timer keyTimer = new javax.swing.Timer(timerDelay , new ActionListener() {
// Swing Timer's actionPerformed
public void actionPerformed(ActionEvent e) {
grid.stepGame();
}
});
// key binding AbstractAction's actionPerformed
public void actionPerformed(ActionEvent e) {
if (firstPress) {
keyTimer.start();
} else {
keyTimer.stop();
}
firstPress = !firstPress;
}
}));
Another useful option is to perform a repeating task on key press and stop it on key release, and this can be done easily by getting the keystrokes for on press and on release:
KeyStroke.getKeyStroke(KeyEvent.VK_SPACE, 0, true) // for key release
KeyStroke.getKeyStroke(KeyEvent.VK_SPACE, 0, false) // for key press
For example:
import java.awt.event.*;
import javax.swing.*;
public class SwingTimerEg2 {
private JFrame frame;
private Grid2 grid = new Grid2(this);
private JTextArea textarea = new JTextArea(20, 20);
private int stepCount = 0;
public SwingTimerEg2() {
frame = new JFrame();
textarea.setEditable(false);
frame.add(new JScrollPane(textarea, JScrollPane.VERTICAL_SCROLLBAR_ALWAYS,
JScrollPane.HORIZONTAL_SCROLLBAR_NEVER));
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
setUpKeyBinding();
}
void setUpKeyBinding() {
final int timerDelay = 250;
final Timer keyTimer = new Timer(timerDelay, new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
grid.stepGame();
}
});
JPanel component = (JPanel) frame.getContentPane();
final int condition = JComponent.WHEN_IN_FOCUSED_WINDOW;
final String spaceDown = "space down";
final String spaceUp = "space up";
component.getInputMap(condition).put(KeyStroke.getKeyStroke(KeyEvent.VK_SPACE, 0, false), spaceDown);
component.getActionMap().put(spaceDown, (new AbstractAction() {
public void actionPerformed(ActionEvent e) {
keyTimer.start();
}
}));
component.getInputMap(condition).put(KeyStroke.getKeyStroke(KeyEvent.VK_SPACE, 0, true), spaceUp);
component.getActionMap().put(spaceUp, (new AbstractAction() {
public void actionPerformed(ActionEvent e) {
keyTimer.stop();
}
}));
}
public void doSomething() {
textarea.append(String.format("Zap %d!!!%n", stepCount));
stepCount ++;
}
private static void createAndShowGui() {
new SwingTimerEg2();
}
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
public void run() {
createAndShowGui();
}
});
}
}
class Grid2 {
private SwingTimerEg2 stEg;
public Grid2(SwingTimerEg2 stEg) {
this.stEg = stEg;
}
void stepGame() {
stEg.doSomething();
}
}

Easiest and dirty solution:
final boolean[] isRunning = new boolean[1];
You don't want to do that—but it works assuming proper synchronization around.
What would be better is
final AtomicBoolean isRunning = new AtomicBoolean();
What would be even better is to review the design once again: global state usually means, "global problems"

The final qualifier requirement can easily be avoided -- replace your inner method (which has the final requirement) with a call to a class method.

No you got the wrong idea about WHY you need final for anonymous classes! Final is only needed for local variables (well more exactly any variable that might have a live time shorter than the given object).
Hence a static variable in a class is perfectly fine and will work perfectly!
Edit: example:
public class Main {
interface Test {
void call();
}
public static volatile boolean running = true;
public static void main(String[] args) {
Test t = new Test() {
#Override
public void call() {
System.out.println(Main.running);
}
};
t.call();
running = false;
t.call();
}
}

Keep a reference to the Timer somewhere, say in your game class.
When the game is paused cancel the Timer.
This will cancel any currently scheduled tasks.
Then when the game is unpaused schedule the timer again as you have done above.
public class Game {
private Timer timer;
public void pause() {
if (timer != null) {
timer.pause();
}
}
public void startOrResumeGame() {
if (timer == null) {
timer = new Timer();
} else {
// Just in case the game was already running.
timer.cancel();
}
timer.scheduleAtFixedRate(new TimerTask() {
public void run() {
grid.stepGame();
}
}, 250, 250);
}
}

Showing text in JTextArea while calculating

An application I am writing consists, among others, a JButton and a JTextArea. A click on the button leads to a long calculation, resulting in a text shown in the JTextArea. Even though the calculation is long, I can have middle-results on the go (think, for example, of an application which approximates pi up to 100 digits - every few seconds I could write another digit). The problem is, that even if I write (being in the ActionListener class because the button invoked the calculation) to set the text of the JTextArea to something, it isn't shown while the calculation is done, and I can only see the end result, after the calculation is over.
Why is it so, and how can I fix it?
Thank you in advance.

Your problem is that you're doing a long calculation in the main Swing thread, the EDT, and this will freeze your entire GUI until the process has completed itself. A solution is to use a background thread for your calculation, and an easy way to do this it to use a SwingWorker to create a thread background to the main Swing thread, the EDT, and publish/process the interim results into the JTextArea. For more on SwingWorkers and the EDT, please look here: Concurrency in Swing
Also, if you provide a decent sscce we can probably give you a more detailed response perhaps even with sample code.
An example SSCCE:
import java.awt.event.*;
import java.text.DecimalFormat;
import java.util.List;
import javax.swing.*;
public class InterimCalc {
private JPanel mainPanel = new JPanel();
private JTextField resultField = new JTextField(10);
private JButton doItBtn = new JButton("Do It!");
private DecimalFormat dblFormat = new DecimalFormat("0.0000000000");
private SwingWorker<Void, Double> mySwingWorker = null;
public InterimCalc() {
mainPanel.add(doItBtn);
mainPanel.add(resultField);
displayResult(0.0);
doItBtn.addActionListener(new DoItListener());
}
public void displayResult(double result) {
resultField.setText(dblFormat.format(result));
}
public JPanel getMainPanel() {
return mainPanel;
}
private class DoItListener implements ActionListener {
public void actionPerformed(ActionEvent e) {
if (mySwingWorker != null && !mySwingWorker.isDone()) {
mySwingWorker.cancel(true);
}
displayResult(0.0);
mySwingWorker = new MySwingWorker();
mySwingWorker.execute();
}
}
private class MySwingWorker extends SwingWorker<Void, Double> {
private static final int INTERIM_LENGTH = 10000; // how many loops to do before displaying
#Override
protected Void doInBackground() throws Exception {
boolean keepGoing = true;
long index = 1L;
double value = 0.0;
while (keepGoing) {
for (int i = 0; i < INTERIM_LENGTH; i++) {
int multiplier = (index % 2 == 0) ? -1 : 1;
value += (double)multiplier / (index);
index++;
}
publish(value);
}
return null;
}
#Override
protected void process(List<Double> chunks) {
for (Double dbl : chunks) {
displayResult(dbl);
}
}
}
private static void createAndShowUI() {
JFrame frame = new JFrame("Decay Const");
frame.getContentPane().add(new InterimCalc().getMainPanel());
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
public static void main(String[] args) {
java.awt.EventQueue.invokeLater(new Runnable() {
public void run() {
createAndShowUI();
}
});
}
}

you may also want to display some sort of spinning gif or "progress bar" to show that the answer is being calculated; feedback to the user is good.
(once you are using a swingworker, then the gui won't freeze and the gui can do its own thing while the calculation is taking place)