Question

Writing a program for a project. I need to use a ListSelectionListener in order to grab the current selection from a JList in order to determine the current card for the CardLayout utilized by the other panel. I’d like to be able to at the very least have a String with the name of the selection be created by the Listener, if not outright using the CardLayout class’s show method to change the current card from within the Listener. What would be the simplest way to do this that would ultimately result in the current card being changed? Thank you so much for any assistance!

Source:

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

public class ClientApp extends JFrame
{
    public static void main(String[] args)
    {
        new ClientApp();
    }


    public ClientApp()
    {
        this.setSize(750,380);
        this.setTitle("Honeydukes Muggle Ordering System");
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JPanel infoPanel = new JPanel(new CardLayout());
        JPanel invntryPanel = new JPanel();


        //Creating the array for the invntryPanel Jlist

        String[] candy = {"Acid Pops", "Bat's Blood Soup",
                          "Bertie Bott's Every Flavour Beans",
                          "Blood-flavoured Lollipops",
                          "Cauldron Cakes", "Charm Choc",
                          "Chocoballs", "Chocolate Cauldrons",
                          "Chocolate Frogs", "Chocolate Skeletons",
                          "Chocolate Wands", "Choco-Loco", "Cockroach Clusters",
                          "Nougat", "Crystallised Pineapple",
                          "Drooble's Best Blowing Gum", "Exploding Bonbons",
                          "Toffees", "Fizzing Whizzbees",
                          "Fudge Flies", "Ice Mice",
                          "Jelly Slugs", "Liquourice Wands",
                          "Pepper Imps", "Peppermint Toads",
                          "Pink Coconut Ice", "Pixie Puffs",
                          "Pumpkin Fizz", "Salt Water Taffy",
                          "Shock-o-Choc", "Skeletal Sweets",
                          "Splindle's Lick'O'Rish Spiders",
                          "Sugar Quills", "Sugared Butterfly Wings",
                          "Toothflossing Stringmints", "Tooth-Splintering Strongmints",
                          "Treacle Fudge", "Chocolates", "Wizochoc"};
        JList candyList = new JList(candy);
        candyList.setVisibleRowCount(18);
        candyList.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);

        ListSelectionListener sl = new ListSelectionListener() {
           public void valueChanged(ListSelectionEvent e) {
            if (e.getValueIsAdjusting() == false) {
                CardLayout.show(infoPanel, (String)candyList.getSelectedValue());
            }
           }
        };

        candyList.addListSelectionListener(sl);

        //Creating a scrollpane for the JList
        JScrollPane scroll = new JScrollPane(candyList, JScrollPane.VERTICAL_SCROLLBAR_ALWAYS,
                                             JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED);
        invntryPanel.add(scroll);


        //Creating the cards
        JPanel startCard = new JPanel(new BorderLayout());
        JPanel acidPopsCard = new JPanel();
        JPanel batsBloodSoupCard = new JPanel();
        JPanel bertieBottsCard = new JPanel();
        JPanel bloodPopsCard = new JPanel();
        JPanel cauldronCakesCard = new JPanel();
        JPanel charmChocCard = new JPanel();
        JPanel chocoballsCard = new JPanel();
        JPanel chocCauldronsCard = new JPanel();
        JPanel chocFrogsCard = new JPanel();
        JPanel chocSkeleCard = new JPanel();
        JPanel chocWands = new JPanel();
        JPanel chocolocoCard = new JPanel();
        JPanel roachClustersCard = new JPanel();
        JPanel nougatCard = new JPanel();
        JPanel crystalPineappleCard = new JPanel();
        JPanel droobleGumCard = new JPanel();
        JPanel explodeBonbonsCard = new JPanel();
        JPanel toffeesCard = new JPanel();
        JPanel fizzWhizCard = new JPanel();
        JPanel fudgeFliesCard = new JPanel();
        JPanel iceMiceCard = new JPanel();
        JPanel jellySlugsCard = new JPanel();
        JPanel liquorWandsCard = new JPanel();
        JPanel pepImpsCard = new JPanel();
        JPanel pepToadsCard = new JPanel();
        JPanel pinkCocoIceCard = new JPanel();
        JPanel pixiePuffsCard = new JPanel();
        JPanel pumpkFizzCard = new JPanel();
        JPanel saltTaffeyCard = new JPanel();
        JPanel shockChocCard = new JPanel();
        JPanel skeleSweetsCard = new JPanel();
        JPanel spindleSpidersCard = new JPanel();
        JPanel sugarQuillsCard = new JPanel();
        JPanel sugarWingsCard = new JPanel();
        JPanel flossMintsCard = new JPanel();
        JPanel splintMintsCard = new JPanel();
        JPanel treacleFudgeCard = new JPanel();
        JPanel chocolatesCard = new JPanel();
        JPanel wizochocCard = new JPanel();

        //Adding the cards to the infoPanel
        infoPanel.add(startCard, "Start");
        infoPanel.add(acidPopsCard, "Acid Pops");
        infoPanel.add(batsBloodSoupCard, "Bat's Blood Soup");
        infoPanel.add(bertieBottsCard, "Bertie Bott's Every Flavour Beans");
        infoPanel.add(bloodPopsCard, "Blood-flavoured Lollipops");
        infoPanel.add(cauldronCakesCard, "Cauldron Cakes");
        infoPanel.add(charmChocCard, "Charm Choc");
        infoPanel.add(chocoballsCard, "Chocoballs");
        infoPanel.add(chocCauldronsCard, "Chocolate Cauldrons");
        infoPanel.add(chocFrogsCard, "Chocolate Frogs");
        infoPanel.add(chocSkeleCard, "Chocolate Skeletons");
        infoPanel.add(chocWands, "Chocolate Wands");
        infoPanel.add(chocolocoCard, "Choco-Loco");
        infoPanel.add(roachClustersCard, "Cockroach Clusters");
        infoPanel.add(nougatCard, "Nougat");
        infoPanel.add(crystalPineappleCard, "Crystallised Pineapple");
        infoPanel.add(droobleGumCard, "Drooble's Best Blowing Gum");
        infoPanel.add(explodeBonbonsCard, "Exploding Bonbons");
        infoPanel.add(toffeesCard, "Toffees");
        infoPanel.add(fizzWhizCard, "Fizzing Whizzbees");
        infoPanel.add(fudgeFliesCard, "Fudge Flies");
        infoPanel.add(iceMiceCard, "Ice Mice");
        infoPanel.add(jellySlugsCard, "Jelly Slugs");
        infoPanel.add(liquorWandsCard, "Liquourice Wands");
        infoPanel.add(pepImpsCard, "Pepper Imps");
        infoPanel.add(pepToadsCard, "Peppermint Toads");
        infoPanel.add(pinkCocoIceCard, "Pink Coconut Ice");
        infoPanel.add(pixiePuffsCard, "Pixie Puffs");
        infoPanel.add(pumpkFizzCard, "Pumpkin Fizz");
        infoPanel.add(saltTaffeyCard, "Salt Water Taffy");
        infoPanel.add(shockChocCard, "Shock-o-Choc");
        infoPanel.add(skeleSweetsCard, "Skeletal Sweets");
        infoPanel.add(spindleSpidersCard, "Splindle's Lick'O'Rish Spiders");
        infoPanel.add(sugarQuillsCard, "Sugar Quills");
        infoPanel.add(sugarWingsCard, "Sugared Butterfly Wings");
        infoPanel.add(flossMintsCard, "Toothflossing Stringmints");
        infoPanel.add(splintMintsCard, "Tooth-Splintering Strongmints");
        infoPanel.add(treacleFudgeCard, "Treacle Fudge");
        infoPanel.add(chocolatesCard, "Chocolates");
        infoPanel.add(wizochocCard, "Wizochoc");

        //startCard building
        JLabel startLbl = new JLabel("<html><center>Welcome to the Honeydukes Muggle Ordering System!<br />Please select from one of our products to the left to begin!</center></html>");
        startCard.add(startLbl, BorderLayout.CENTER);

        this.add(invntryPanel, BorderLayout.LINE_START);
        this.add(infoPanel, BorderLayout.CENTER);
        this.setVisible(true);
    }
}

Errors:

ClientApp.java:54: error: local variable infoPanel is accessed from within inner
 class; needs to be declared final
                CardLayout.show(infoPanel, (String)candyList.getSelectedValue())
;
                                ^
ClientApp.java:54: error: local variable candyList is accessed from within inner
 class; needs to be declared final
                CardLayout.show(infoPanel, (String)candyList.getSelectedValue())
;
                                                   ^
ClientApp.java:54: error: non-static method show(Container,String) cannot be ref
erenced from a static context
                CardLayout.show(infoPanel, (String)candyList.getSelectedValue())
;
                          ^
Note: ClientApp.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
3 errors

Answer 1

I can think of a few ways. First, make the reference of the candyList final

final JList candyList = new JList(candy);

This should allow the inner class to see the list. Personally, I don’t like this approach, but that’s just me.

The other thing you might try is to reference the list via the event object

ListSelectionListener sl = new ListSelectionListener() {
    public void valueChanged(ListSelectionEvent e) {
        JList candyList = (JList)e.getSource();
        if (e.getValueIsAdjusting() == false) {
            CardLayout.show(infoPanel, (String)candyList.getSelectedValue());
        }
       }
    };

Or construct a new inner class that can have the list past to it as parameter

public class MyInnerListener implements ListSelectionListener {

    private JList list;

    public MyInnerListener(JList list) {

        this.list = list;

    }
    public void valueChanged(ListSelectionEvent e) {
        if (e.getValueIsAdjusting() == false) {
            CardLayout.show(infoPanel, (String)list.getSelectedValue());
        }
    }

}

Just some ideas 😉