ActionListener not linking lists across methods

Question

I'm very new to programming in general, but ive read half way through this book "Head first into java". It displays an example which i tried to write down and compile but i cant get past the compiler. My valueChanged method wont link the list from go() to "String selection = (String) list.getSelectedValue(); the "list" is underlined with a red line and i cant seem to get the correct syntax to link the main syntax with my listeners. Any help would be appriciated. `

package Test1;

import javax.swing.*;
import javax.swing.event.ListSelectionEvent;
import javax.swing.event.ListSelectionListener;
import java.awt.*;

public class GUi extends JPanel implements ListSelectionListener {

    public static void main(String [] args){
        GUi gui = new GUi();
        gui.go();


    }

    public void go() {
        JFrame frame = new JFrame();
        JPanel panel = new JPanel();
        panel.setBackground(Color.DARK_GRAY);
        panel.setLayout(new BoxLayout(panel, BoxLayout.Y_AXIS));
        JList list = new JList();
        String[] listEntries = {"alpha", "beta", "gamma", "delta", "epsilon", "zeta", "eta", "theta"};

        list = new JList(listEntries);

        JScrollPane scroller = new JScrollPane(list);
        scroller.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
        scroller.setVerticalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_NEVER);
        panel.add(scroller);

        list.setVisibleRowCount(4);
        list.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
        list.addListSelectionListener(this);

        frame.setSize(500, 500);
        frame.setVisible(true);
    }

    public void valueChanged(ListSelectionEvent lse) {
        if (!lse.getValueIsAdjusting()) {
            String selection = (String) list.getSelectedValue();
            System.out.println(selection);
        }
    }

}

`

You need to learn about scope. See: http://stackoverflow.com/questions/25706216/what-does-a-cannot-find-symbol-compilation-error-mean — azurefrog, Jul 13 '16 at 21:05
@azurefrog alright. but how do i define a list which can be used in two methods at once such as go() and valueChanged() ? — Benjamin Rasmussen, Jul 13 '16 at 21:21
This error suggests that you might be putting the cart before the horse as usually it's recommended to learn the basics of a language, such as variable scoping, before trying to tackle the advanced topics, such as GUI programming. Going through the introductory chapters of a Java text book or the intro tutorials of a Java tutorial would likely be time well spent and would help you in your future GUI programming. — Hovercraft Full Of Eels, Jul 13 '16 at 22:23
@HovercraftFullOfEels I always cringe when newbies tag something with Swing yet are asking basic OO/attribute access questions. 'Putting the cart before the horse' certainly sums it up! — Andrew Thompson, Jul 14 '16 at 02:46

score 0 · Answer 1 · answered Jul 13 '16 at 21:38

just add the list as a class field so it's visible to both method like this way

import javax.swing.*;
import javax.swing.event.ListSelectionEvent;
import javax.swing.event.ListSelectionListener;
import java.awt.*;

public class GUi extends JPanel implements ListSelectionListener {

    JList list;
    public static void main(String [] args){
        GUi gui = new GUi();
        gui.go();


    }

    public void go() {
        JFrame frame = new JFrame();
        JPanel panel = new JPanel();
        panel.setBackground(Color.DARK_GRAY);
        panel.setLayout(new BoxLayout(panel, BoxLayout.Y_AXIS));
        String[] listEntries = {"alpha", "beta", "gamma", "delta", "epsilon", "zeta", "eta", "theta"};

        list = new JList(listEntries);

        JScrollPane scroller = new JScrollPane(list);
        scroller.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
        scroller.setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_NEVER);
        panel.add(scroller);

        list.setVisibleRowCount(4);
        list.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
        list.addListSelectionListener(this);

        frame.add(panel);
        frame.setSize(500, 500);
        frame.setVisible(true);
    }

    public void valueChanged(ListSelectionEvent lse) {
        if (!lse.getValueIsAdjusting()) {
            String selection = (String) list.getSelectedValue();
            System.out.println(selection);
        }
    }

}

That makes alot of sense, i tired to declare list in the main method but it still failed. Putting it right over the main method certainly did the trick! Thanks alot :) — Benjamin Rasmussen, Jul 13 '16 at 22:15

ActionListener not linking lists across methods

1 Answers1