Question

I’m working on a project that requires GUI, but I have no experience with GUI, so everything I have been doing is guesswork in a sense.
I have a created an arraylist of objects that I have put into a JList, and now I am trying to change the text in a label based on what the user chooses. I get an error that says “Cannot refer to a non-final variable library inside an inner class defined in a different method”

The arraylist I am using is populated with objects that I can call strings from

How do I get this to work?

JList list = new JList(bookNames.toArray());

list.addListSelectionListener(new ListSelectionListener() {
    public void valueChanged(ListSelectionEvent event) {
        typeLabel.setText(library.get(list.getSelectedIndex()).getType());
    }
});

Answer 1

You cannot access the typeLabel variable inside your new ListSelectionListener(){…}.

JList list = new JList(bookNames.toArray());

list.addListSelectionListener(new ListSelectionListener() {
    public void valueChanged(ListSelectionEvent event) {
        // you cannot access typeLabel here
        typeLabel.setText(library.get(list.getSelectedIndex()).getType());
    }
});

A Quick fix would be to declare the typeLabel as final.
This means that you cannot reassign another value to typeLabel but that’s probably fine.

final typeLabel = whatever; // add the final modifier

final JList list = new JList(bookNames.toArray());

list.addListSelectionListener(new ListSelectionListener() {
    public void valueChanged(ListSelectionEvent event) {
        typeLabel.setText(library.get(list.getSelectedIndex()).getType());
    }
});

edit:
Also, list must be declared final.