Question

I’m trying to create a form with a JList and a button
What I want to do, is select an item from the JList, and then press the button to perform an action dependant on the selection. However, as soon as the button is clicked, the JList loses focus, and the selection disappears, making the button unable to determine what element was selected in the JList.

Is there a solution to this?
Thanks in advance 🙂

Answer 1

• I can’t see any issue with example from Oracle JList tutorial

• have look at ListSelectionListener

• maybe carefully with SelectionModes

• can you please to describe whats is your goal

code example

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

public class ListDemo extends JPanel implements ListSelectionListener {

    private JList list;
    private DefaultListModel listModel;
    private static final String hireString = "Hire";
    private static final String fireString = "Fire";
    private JButton fireButton;
    private JTextField employeeName;

    public ListDemo() {
        super(new BorderLayout());
        listModel = new DefaultListModel();
        listModel.addElement("Jane Doe");
        listModel.addElement("John Smith");
        listModel.addElement("Kathy Green");
        list = new JList(listModel);
        list.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
        list.setSelectedIndex(0);
        list.addListSelectionListener(this);
        list.setVisibleRowCount(5);
        JScrollPane listScrollPane = new JScrollPane(list);
        JButton hireButton = new JButton(hireString);
        HireListener hireListener = new HireListener(hireButton);
        hireButton.setActionCommand(hireString);
        hireButton.addActionListener(hireListener);
        hireButton.setEnabled(false);
        fireButton = new JButton(fireString);
        fireButton.setActionCommand(fireString);
        fireButton.addActionListener(new FireListener());
        employeeName = new JTextField(10);
        employeeName.addActionListener(hireListener);
        employeeName.getDocument().addDocumentListener(hireListener);
        String name = listModel.getElementAt(list.getSelectedIndex()).toString();
        JPanel buttonPane = new JPanel();
        buttonPane.setLayout(new BoxLayout(buttonPane, BoxLayout.LINE_AXIS));
        buttonPane.add(fireButton);
        buttonPane.add(Box.createHorizontalStrut(5));
        buttonPane.add(new JSeparator(SwingConstants.VERTICAL));
        buttonPane.add(Box.createHorizontalStrut(5));
        buttonPane.add(employeeName);
        buttonPane.add(hireButton);
        buttonPane.setBorder(BorderFactory.createEmptyBorder(5, 5, 5, 5));
        add(listScrollPane, BorderLayout.CENTER);
        add(buttonPane, BorderLayout.PAGE_END);
    }

    class FireListener implements ActionListener {

        public void actionPerformed(ActionEvent e) {
            int index = list.getSelectedIndex();
            listModel.remove(index);
            int size = listModel.getSize();
            if (size == 0) { //Nobody's left, disable firing.
                fireButton.setEnabled(false);
            } else { //Select an index.
                if (index == listModel.getSize()) {
                    index--;
                }
                list.setSelectedIndex(index);
                list.ensureIndexIsVisible(index);
            }
        }
    }

    class HireListener implements ActionListener, DocumentListener {

        private boolean alreadyEnabled = false;
        private JButton button;

        public HireListener(JButton button) {
            this.button = button;
        }

        public void actionPerformed(ActionEvent e) {
            String name = employeeName.getText();
            if (name.equals("") || alreadyInList(name)) {
                Toolkit.getDefaultToolkit().beep();
                employeeName.requestFocusInWindow();
                employeeName.selectAll();
                return;
            }
            int index = list.getSelectedIndex(); //get selected index
            if (index == -1) { //no selection, so insert at beginning
                index = 0;
            } else {           //add after the selected item
                index++;
            }
            listModel.insertElementAt(employeeName.getText(), index);
            //listModel.addElement(employeeName.getText());
            employeeName.requestFocusInWindow();
            employeeName.setText("");
            list.setSelectedIndex(index);
            list.ensureIndexIsVisible(index);
        }

        protected boolean alreadyInList(String name) {
            return listModel.contains(name);
        }

        public void insertUpdate(DocumentEvent e) {
            enableButton();
        }

        public void removeUpdate(DocumentEvent e) {
            handleEmptyTextField(e);
        }

        public void changedUpdate(DocumentEvent e) {
            if (!handleEmptyTextField(e)) {
                enableButton();
            }
        }

        private void enableButton() {
            if (!alreadyEnabled) {
                button.setEnabled(true);
            }
        }

        private boolean handleEmptyTextField(DocumentEvent e) {
            if (e.getDocument().getLength() <= 0) {
                button.setEnabled(false);
                alreadyEnabled = false;
                return true;
            }
            return false;
        }
    }

    public void valueChanged(ListSelectionEvent e) {
        if (e.getValueIsAdjusting() == false) {

            if (list.getSelectedIndex() == -1) {
                fireButton.setEnabled(false);
            } else {
                fireButton.setEnabled(true);
            }
        }
    }

    private static void createAndShowGUI() {
        JFrame frame = new JFrame("ListDemo");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        JComponent newContentPane = new ListDemo();
        newContentPane.setOpaque(true); //content panes must be opaque
        frame.setContentPane(newContentPane);
        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        javax.swing.SwingUtilities.invokeLater(new Runnable() {

            public void run() {
                createAndShowGUI();
            }
        });
    }
}