Catching Exceptions in GUI Code - Java Tutorials

Catching Exceptions in GUI Code - Java Tutorials

Catching Exceptions in GUI Code - Java Tutorials

Catching uncaught exception in GUI

In this section, we will discuss how to catch uncaught exceptions in GUI.

Lets see the given below code to identify the uncaught exception :

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;

public class Gui extends JFrame {
private static final int[] DAYS_PER_MONTH =
{31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
public Gui() {
super("GUI Example");
final JTextArea text = new JTextArea(14, 30);
getContentPane().add(new JScrollPane(text));
getContentPane().add(new JButton(new AbstractAction("Calculate") {
public void actionPerformed(ActionEvent e) {
for (int i=0; i<=DAYS_PER_MONTH.length; i++) {
text.append("Month " + (i+1) + ": " +
DAYS_PER_MONTH[i] + "\n");
}), BorderLayout.NORTH);
public static void main(String[] args) {
Gui gui = new Gui();

First we compile and run this code using javaw.exe as javaw Gui

When you press the button, everything works ok except the button stay pressed.

When you run the same code using java.exe you will get the following exception displayed on the screen:

java.lang.ArrayIndexOutOfBoundsException: 12
at Gui$1.actionPerformed(
at javax.swing.AbstractButton.fireActionPerformed
at javax.swing.AbstractButton$ForwardActionEvents.actionPerformed
at javax.swing.DefaultButtonModel.fireActionPerformed
at javax.swing.DefaultButtonModel.setPressed
at javax.swing.plaf.basic.BasicButtonListener$ReleasedAction.actionPerformed
at javax.swing.SwingUtilities.notifyAction
at javax.swing.JComponent.processKeyBinding
at javax.swing.JComponent.processKeyBindings
at javax.swing.JComponent.processKeyEvent
at java.awt.Component.processEvent
at java.awt.Container.processEvent
at java.awt.Component.dispatchEventImpl
at java.awt.Container.dispatchEventImpl
at java.awt.Component.dispatchEvent
at java.awt.KeyboardFocusManager.redispatchEvent
at java.awt.DefaultKeyboardFocusManager.dispatchKeyEvent
at java.awt.DefaultKeyboardFocusManager.preDispatchKeyEvent
at java.awt.DefaultKeyboardFocusManager.typeAheadAssertions
at java.awt.DefaultKeyboardFocusManager.dispatchEvent
at java.awt.Component.dispatchEventImpl
at java.awt.Container.dispatchEventImpl
at java.awt.Window.dispatchEventImpl
at java.awt.Component.dispatchEvent
at java.awt.EventQueue.dispatchEvent
at java.awt.EventDispatchThread.pumpOneEventForHierarchy
at java.awt.EventDispatchThread.pumpEventsForHierarchy
at java.awt.EventDispatchThread.pumpEvents
at java.awt.EventDispatchThread.pumpEvents

After printing the above exceptions at console the java event dispatch thread dies after this.


The Solution of the above problem is ThreadGroup. A thread may corresponds to a Thread Group. The ThreadGroup has a function called as uncaughtException(Thread t, Throwable e) This method can be override and we call it whenever an uncaught exception occur. The event dispatch thread still dies, but at least it will let us know about it.

import javax.swing.*;
import java.awt.*;

public class ExceptionGroup extends ThreadGroup {
public ExceptionGroup() {
public void uncaughtException(Thread t, Throwable e) {
e.toString(), "Exception Occurred", JOptionPane.ERROR_MESSAGE);
* I hate ownerless dialogs. With this method, we can find the
* currently visible frame and attach the dialog to that, instead
* of always attaching it to null.
private Frame findActiveFrame() {
Frame[] frames = JFrame.getFrames();
for (int i = 0; i < frames.length; i++) {
Frame frame = frames[i];
if (frame.isVisible()) {
return frame;
return null;

All we now need to do is start the Gui from within a Thread that belongs to this ExceptionGroup, and we will catch all uncaught exceptions that are caused by the event dispatch thread:

import javax.swing.*;

public class BetterGui {
public static void main(String[] args) {
ThreadGroup exceptionThreadGroup = new ExceptionGroup();
new Thread(exceptionThreadGroup, "Init thread") {
public void run() {
Gui gui = new Gui();


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