frame close operations (GUI)

[Lucas Mac]

i'm starting to play around with GUI's and i'm trying to write to a file. i've been using the following code in all of my GUI test programs:

chatFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

however, i want to be able to perform other operations on close, in particular, close the file i have written to...how do i do this?

[Craig Wood]

You can go back to the older way of using a WindowListener:

    chatFrame.addWindowListener(new WindowAdapter()
    {
        public void windowClosing(WindowEvent e)
        {
            // close files
            // clean up
            System.exit(0);
        }
    });

[Lucas Mac]

so, i made my class implement WindowListener, and wrote the function you mention, but i get this error:

myFunction is not abstract and does not override abstract method windowActivated(java.awt.event.WindowEvent) in java.awt.event.WindowListener
public class myFunction implements ActionListener, WindowListener {
^

the only why i don't get this error is if i include:

public void windowActivated(WindowEvent e) {}

but i have to include one for each of the WindowListener methods. why do i HAVE to override all the other functions just because i want to override windowClosing?

[Craig Wood]

Getting started with event listeners can seem confusing because of the many options available. A good resource to help you in learning java is the Creating a GUI with JFC/Swing trail in the
java tutorial.

Some general observations:
When you implement an interface you are required to implement every method defined in the interface. Extending an adapter class allows you to implement only the methods you are interested in. This is a convenience. See General Information about Writing Event Listeners for more, especially the section Event Adapters.

Options:
1 - For simple programs you can have your class implement an EventListener interface, eg, ActionListener. And of course we must implement all the methods defined in the interface.

class Startup implements ActionListener
{
    ...
    public void actionPerformed(ActionEvent e) { }

2 - If we need to keep a reference to a listener we can use either an outer class or a named inner class. There are two ways to do a named inner class:

class Explore
{
    public Explore()
    {
        JButton one = new JButton("one");
        JButton two = new JButton("two");
        ActionListener l = new ActionListener()
        {
            public void actionPerformed(ActionEvent e)
            {
                JButton button = (JButton)e.getSource();
            }
        };
        one.addActionListener(l);  // use the reference "l" to
        two.addActionListener(l);  // to refer to the listener
        JRadioButton[] buttons = new JRadioButton[4];
        RadioListener rl = new RadioListener();  // rl is the reference
        for(int j = 0; j < buttons.length; j++)
        {
            buttons[j] = new JRadioButton("option " + (j + 1));
            buttons[j].addActionListener(rl);
            ...
        }
    }
 
    private class RadioListener implements ActionListener
    {
        public void actionPerformed(ActionEvent e)
        {
            JRadioButton button = (JRadioButton)e.getSource();
        }
    }
}

3 - For a one–time use with no need to refer to the listener, ie, we don't need/want a reference to the listener, we have the anonymous inner class. This is what I used in the first reply above. And I used a WindowAdapter for the WindowListener. If I had specified a WindowListener instead I would have been required to implement the other methods within the listener.

    chatFrame.addWindowListener(new WindowListener()
    {
        public void windowActivated(WindowEvent e) { }
        ...

For more on inner class listeners see Inner Classes and Anonymous Inner Classes section on the last (link above) page.
[ June 16, 2004: Message edited by: Craig Wood ]

[Gregg Bolinger]

Moving to Swing...

[Eric Snell]

Originally posted by Craig Wood:
You can go back to the older way of using a WindowListener:

    chatFrame.addWindowListener(new WindowAdapter()
    {
        public void windowClosing(WindowEvent e)
        {
            // close files
            // clean up
            System.exit(0);
        }
    });

If you leave the default close operation as JFrame.EXIT_ON_CLOSE, you don't need the call to System.exit() in your windowClosing() method.

[Lucas Mac]

thank you Craig, your explanation helped immensely...