Path: csiph.com!usenet.pasdenom.info!news.albasani.net!fu-berlin.de!uni-berlin.de!individual.net!not-for-mail
From: blmblm@myrealbox.com <blmblm.myrealbox@gmail.com>
Newsgroups: comp.lang.java.programmer
Subject: problems with editable JList
Date: 17 Feb 2014 15:53:16 GMT
Organization: None
Lines: 365
Message-ID: <bmepjbFoa1fU1@mid.individual.net>
X-Trace: individual.net WDvlnaNVXVBH4Mhkx5921wOtjxmQZVmzd07DEGmFob0ZOO/v6M
X-Orig-Path: not-for-mail
Cancel-Lock: sha1:9QlRpQ0F0qFGlLN8ysa28iEDuyY=
X-Newsreader: trn 4.0-test76 (Apr 2, 2001)
Xref: csiph.com comp.lang.java.programmer:27533

Maybe one of the experts here can help ....  Apologies in advance for
the length of this post; I've tried to make it as short as I could, 
but oh well.

I'm trying to work up a prototype for a JList that supports some simple
editing operations:  add a new element, replace an existing element,
remove a selected element, and do some simple reordering.  My code
is working pretty well, *except* that when I try to set the selection
after an "add" or "remove", it doesn't quite work the way I want.

What I want:

After adding a new element, the new element is selected.

After removing an element, the preceding element is selected,
unless the removed element was the first one, in which case I want
the first one to be selected, or the only one, in which case I want
nothing selected.

How I'm trying to accomplish this:

In my extension of JList, I attach a ListDataListener to the list
data model, and in the listener's methods that deal with addition and
removal events I use setSelectedIndex() or clearSelection() to change
the selection to what I want.  And it seems to work, based on what's
reported by debug prints -- *EXCEPT* that something [*] subsequently
changes the selection in a way that I don't want:

After an "add", I end up with a selected index of getModel().getSize(),
which of course is not valid.

After a "remove" of the first element, I end up with nothing selected,
even if there are elements remaining.

[*] Based on attempts to find out what's going on by adding a
lot of debug-print code and poring over source code for various
javax.swing classes, I *think* something in the ListDataListener in
plaf.basic.BasicListUI is responsible.

I've stripped down the prototype to what I think is just about the
smallest program that demonstrates the problem (well, okay, plus some
debug-print code), which appears below.  Other than not being very short,
it's an SSCCE.

Am I going about this wrong?  Have I tripped over a bug?  Initial
development was done with Java 1.6.something, but the code (mis)behaves
the same way with 1.7.something.  

Help appreciated!!  the problem's not a show-stopper for my use case,
but it *is* annoying, and puzzling ....



/*
   Attempt at an editable JList, supporting allow adding, removing, and 
   changing elements.

   Works as desired except for (some) attempts to modify programmatically 
   what element is selected:

   "add" should result in new item being selected (but does not).

   "remove" should result in preceding element being selected if there is
   one (and that works), or element 0 being selected if the removed element
   was the one at index 0 (and that does not work).
*/

import java.awt.*;
import java.awt.event.*;

import javax.swing.*;
import javax.swing.event.*;


public class TryListMain extends JFrame {

    // ---- variables ----

    private static boolean DEBUG = true;

    private DefaultListModel model;
    private TryList list;

    private static int counter = 0;

    // ---- constructor ----

    public TryListMain() {

        super();

        model = new DefaultListModel();

        list = new TryList(model);

        setLayout(new BorderLayout());

        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        add(new ButtonPanel(), BorderLayout.NORTH);
        add(list, BorderLayout.CENTER);
    }

    // ---- methods ----

    public TryList getList() {
        return list;
    }
    public DefaultListModel getModel() {
        return model;
    }

    public static void debugPrint(String msg) {
        if (DEBUG) {
            if (msg.length() > 0) {
                System.out.println("DBG "+msg);
            } else {
                System.out.println();
            }
        }
    }
    public static void debugPrint() {
        debugPrint("");
    }

    private String generateItem() {
        ++counter;
        return "Element "+counter;
    }

    // ---- main ----

    public static void main(String[] args) {

        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {

                UIManager.put("Button.defaultButtonFollowsFocus", true);
                TryListMain theFrame = new TryListMain();

                System.out.println("data listeners:");
                for (ListDataListener ll : 
                    theFrame.getModel().getListDataListeners())
                {
                    System.out.println(ll.getClass().getName());
                }
                System.out.println();

                System.out.println("list selection listeners:");
                for (ListSelectionListener ll : 
                    theFrame.getList().getListSelectionListeners())
                {
                    System.out.println(ll.getClass().getName()); 
                }
                System.out.println();

                theFrame.setSize(new Dimension(400,200));
                theFrame.setVisible(true);
            }
        });
    }

    // ---- classes ----

    private class ButtonPanel extends JPanel {

        private Action removeAction;
        private Action replaceAction;
        private Action newAction;
        private Action showAction;

        private void setActionsEnabled() {
            int si = list.selectedIndex();
            removeAction.setEnabled(si >= 0);
            replaceAction.setEnabled(si >= 0);
        }

        public ButtonPanel() {

            super(new FlowLayout());

            removeAction = new AbstractAction("Remove") {
                @Override
                public void actionPerformed(ActionEvent e) {
                    int si = list.selectedIndex();
                    TryListMain.debugPrint();
                    TryListMain.debugPrint("== remove item at "+si);
                    if (si >= 0) {
                        model.remove(si);
                    } else {
                        // FIXME should never happen
                        System.err.println("no selected item to remove");
                    }
                }
            };
    
            replaceAction = new AbstractAction("Replace") {
                @Override
                public void actionPerformed(ActionEvent e) {
                    int si = list.selectedIndex();
                    if (si >= 0) {
                        String item = generateItem();
                        TryListMain.debugPrint();
                        TryListMain.debugPrint("== replace item at "+si);
                        model.set(si, item);
                    } else {
                        // FIXME should never happen
                        System.err.println("no selected item to replace");
                    }
                }
            };

            newAction = new AbstractAction("New") {
                @Override
                public void actionPerformed(ActionEvent e) {
                    String item = generateItem();
                    TryListMain.debugPrint();
                    TryListMain.debugPrint("== add new item");
                    model.addElement(item);
                }
            };

            showAction = new AbstractAction("Show selected") {
                @Override
                public void actionPerformed(ActionEvent e) {
                    int si = list.selectedIndex();
                    if (si < 0) {
                        System.out.println("No selected value");
                    } else {
                        System.out.println(
                                String.format("Selected value (%d of %d) '%s'",
                                    si+1, model.getSize(), model.get(si)));
                    }
                }
            };
 
            list.addListSelectionListener(new ListSelectionListener() {
                @Override
                public void valueChanged(ListSelectionEvent e) {
                    setActionsEnabled();
                }
            });

            setActionsEnabled();

            add(new JButton(removeAction));
            add(new JButton(replaceAction));
            add(new JButton(newAction));
            add(new JButton(showAction));
        }
    }
}

class TryList extends JList {

    // ---- constructor ----

    public TryList(DefaultListModel lModel) {
        super(lModel);

        getSelectionModel().setSelectionMode(
                ListSelectionModel.SINGLE_SELECTION);

        getModel().addListDataListener(new ListDataListener() {

            @Override
            public void intervalAdded(ListDataEvent e) {
                printEvent("add", e);
                setSelectedIndex(e.getIndex1());
                ensureIndexIsVisible(e.getIndex1());
                printNewSelection("add");
                /*
                   HERE things seem okay -- desired element selected --
                   but then something changes the selection to 
                   getModel().getSize(), which of course(?) is not valid
                */
                TryListMain.debugPrint();
            }

            @Override
            public void intervalRemoved(ListDataEvent e) {
                printEvent("remove", e);
                if (getModel().getSize() == 0) {
                    clearSelection();
                } else {
                    int newSelect = Math.max(0, e.getIndex1()-1);
                    setSelectedIndex(newSelect);
                    ensureIndexIsVisible(newSelect);
                }
                printNewSelection("remove");
                /*
                   HERE things seem okay -- desired element selected --
                   but then if the removed element was the first one 
                   something changes the selection from (0) to ()
                */
                TryListMain.debugPrint();
            }

            @Override
            public void contentsChanged(ListDataEvent e) {
                printEvent("change", e);
                setSelectedIndex(e.getIndex1());
                ensureIndexIsVisible(e.getIndex1());
                printNewSelection("change");
                TryListMain.debugPrint();
            }
        });

        addListSelectionListener(new ListSelectionListener() {
            @Override
            public void valueChanged(ListSelectionEvent e) {
                TryListMain.debugPrint(String.format(
                        "selection changed to %s, source %s",
                        selectedIndicesToString(),
                        e.getSource().getClass().getName()));
            }
      });
    }

    // ---- methods ----

    // return -1 if nothing selected, or out of range
    public int selectedIndex() {
        int[] si = getSelectedIndices();
        if ((si.length == 1) && (si[0] < getModel().getSize())) {
            return si[0];
        } else {
            return -1;
        } 
    }

    private void printEvent(String name, ListDataEvent e) {
        TryListMain.debugPrint();
        TryListMain.debugPrint(
                String.format("%s (%s, %d, %d), %d items (selected %s)",
                name,
                e.getSource().getClass().getName(),
                e.getIndex0(), e.getIndex1(), 
                getModel().getSize(),
                selectedIndicesToString()));
    }

    private void printNewSelection(String name) {
        TryListMain.debugPrint(
                String.format("after %s, selection %s",
                    name,
                    selectedIndicesToString()));
        TryListMain.debugPrint();
    }

    private String selectedIndicesToString() {
        StringBuffer sb = new StringBuffer();
        for (int i : getSelectedIndices()) {
            if (sb.length() > 0) sb.append(' ');
            sb.append(i);
        }
        return "("+sb.toString()+")";
    }
}


-- 
B. L. Massingill
ObDisclaimer:  I don't speak for my employers; they return the favor.