Using final 1-element array for anonymous inner class

Question

I stumbled across this trick for getting a value from an anonymous inner class to a variable which is declared in the outer class. It works, but it feels like a dirty hack:

private int showDialog()
{
    final int[] myValue = new int[1];

    JPanel panel = new JPanel();
    final JDialog dialog = new JDialog(mainWindow, "Hit the button", true);
    dialog.setDefaultCloseOperation( WindowConstants.DO_NOTHING_ON_CLOSE );

    JButton button = new JButton("Hit me!");
    button.addActionListener(new ActionListener()
    {
        @Override
        public void actionPerformed(ActionEvent e)
        {
            myValue[0] = 42;
            dialog.setVisible(false);
        }
    });

    panel.add(button);
    dialog.add(panel);
    dialog.pack();
    dialog.setVisible(true);

    return myValue[0];
}

(Yes, I realize this example could be replaced with a simple JOptionPane, but my actual dialogs are much more complicated.) The inner function insists that all variables it interacts with be final, but I can't declare the myValue as final because the inner function needs to assign it a value. Declaring it as a 1-element array gets around this problem, but seems like it might be a Bad Thing^TM somehow. I'm wondering if a.) this is common practice or b.) there's any serious problems that could result from doing this.

Answer 1

If the code is legible, which it is, I wouldn't say doing it that way is terrible.

An alternative is to have the JButton call a function in the class that has showDialog (which is allowed). The function could set an instance variable that will be returned. But that seems less legible to me, so I'd actually prefer your method.

Unless you are making a deeply hierarchical UI framework, sometimes these little hacks are exactly the sort of thing you should do.

If you are concerned, you can do basically the same thing with a private inner class:

private class DialogReturnValue {
    public int value;
}

private int showDialog()
{
    final DialogReturnValue myValue = new DialogReturnValue();

    JPanel panel = new JPanel();
    final JDialog dialog = new JDialog(mainWindow, "Hit the button", true);
    dialog.setDefaultCloseOperation( WindowConstants.DO_NOTHING_ON_CLOSE );

    JButton button = new JButton("Hit me!");
    button.addActionListener(new ActionListener()
    {
        @Override
        public void actionPerformed(ActionEvent e)
        {
            myValue.value = 42;
            dialog.setVisible(false);
        }
    });

    panel.add(button);
    dialog.add(panel);
    dialog.pack();
    dialog.setVisible(true);

    return myValue.value;
}

And there's also ActionListeners to look at (which may well be the "right" approach).

Answer 2

Using AtomicInteger or AtomicReference can make it little better. It is actually a common practice, but you can make it cleaner by introducing the actual class that implements ActionListener and provides the value through a getter.