Question

i coded a little annotator in Java with a simple GUI based on Swing but i faced with a problem that is freaking me out.
The problem is: i have a jlist and two jbuttons modifying such jlist, the two buttons have the SAME listener but they don’t work the same.
All it’s supposed to work like this: once you select a row in the jlist you can tag it as ON TOPIC or OFF TOPIC (with the two buttons), the row changes color and then selects the next row but; even if the selection is on the right row (the next one) it highlights the next just for the OFF Topic button, why?

Here’s the code:

public class TweetsAnnotator {

static Boolean[] annotations = null;
@SuppressWarnings("rawtypes")
static JList jl;
static JButton offbutton = new JButton("OFF Topic");
static JButton onbutton = new JButton("ON Topic");
static String file = "inception_TweetList";

public TweetsAnnotator() {
}

/**
 * @param args
 * @throws IOException 
 * @throws FileNotFoundException 
 * @throws ClassNotFoundException 
 */
@SuppressWarnings({ "unchecked", "rawtypes" })
public static void main(String[] args) throws FileNotFoundException, IOException, ClassNotFoundException {

    // Read Tweets from file
    ObjectInputStream load = new ObjectInputStream(new FileInputStream(file));
    ArrayList<String> list = (ArrayList<String>) load.readObject();
    load.close();
    System.out.println(list.size() + " Tweets read from: " + file);

    // Check and read annotations
    File fileannot = new File(file + "Annotations");
    if (fileannot.exists()) {
        System.out.println("esiste, leggo");
        ObjectInputStream loadannot = new ObjectInputStream(new FileInputStream(file + "Annotations"));
        annotations = (Boolean[]) loadannot.readObject();
        loadannot.close();
    } else {
        System.out.println("non esiste, creo poi leggo");
        ObjectOutputStream save = new ObjectOutputStream(new FileOutputStream(file + "Annotations"));
        Boolean[] creatannotations = new Boolean[list.size()];
        for (int i=0; i<list.size(); i++) {
            creatannotations[i] = (Boolean) null;
        }
        save.writeObject(creatannotations);
        save.close();
        ObjectInputStream loadannot = new ObjectInputStream(new FileInputStream(file + "Annotations"));
        annotations = (Boolean[]) loadannot.readObject();
        loadannot.close();
    }
    System.out.println(annotations.length + " Annotations loaded");     

    // Buttons
    offbutton.setActionCommand("off");
    offbutton.addActionListener(new ButtonListener());
    offbutton.setEnabled(false);
    onbutton.setActionCommand("on");
    onbutton.addActionListener(new ButtonListener());
    onbutton.setEnabled(false);

    // ButtonPanel
    JPanel buttonPane = new JPanel();
    buttonPane.setLayout(new BoxLayout(buttonPane, BoxLayout.X_AXIS));
    buttonPane.add(onbutton);
    buttonPane.add(offbutton);

    // JList
    jl = new JList((Object[])list.toArray());
    jl.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
    jl.setLayoutOrientation(JList.VERTICAL);
    jl.setVisibleRowCount(-1);
    jl.setCellRenderer(new MyCellRenderer());
    ListSelectionListener listSelectionListener = new ListSelectionListener() {

        public void valueChanged(ListSelectionEvent e) {
            if (e.getValueIsAdjusting() == false) {
                if (jl.getSelectedIndex() == -1) {
                    //No selection, disable buttons.
                    onbutton.setEnabled(false);
                    offbutton.setEnabled(false);
                } else {
                    //Selection, enable buttons.
                    onbutton.setEnabled(true);
                    offbutton.setEnabled(true);
                }
            }
        }
    };
    jl.addListSelectionListener(listSelectionListener);

    // JScrollPane
    JScrollPane listScroller = new JScrollPane(jl);

    // JFrame
    JFrame frame = new JFrame(file);
    frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
    frame.setExtendedState(frame.getExtendedState()|JFrame.MAXIMIZED_BOTH);
    frame.addWindowListener(new WindowCloseHandler());

    // Add and show
    frame.getContentPane().add(listScroller, BorderLayout.CENTER);
    frame.getContentPane().add(buttonPane, BorderLayout.PAGE_END);
    frame.pack();
    frame.setVisible(true);

}

private static class MyCellRenderer extends DefaultListCellRenderer {   

    private static final long serialVersionUID = 1L;

    @SuppressWarnings("rawtypes")
    public Component getListCellRendererComponent( JList list, Object value, int index, boolean isSelected, boolean cellHasFocus ) {   
        Component c = super.getListCellRendererComponent( list, value, index, isSelected, cellHasFocus );   
        if ( annotations[index] == null ) {   
            c.setBackground( Color.white );   
        }   
        else if (annotations[index] == true) {   
            c.setBackground( Color.green );   
        } else {
            c.setBackground( Color.red);
        }
        return c;   
    } 
}

private static class ButtonListener implements ActionListener {

    public void actionPerformed(ActionEvent e) {
        int ind = jl.getSelectedIndex() +1;
        if (e.getActionCommand().equals("on")) {
            System.out.println("ON");
            annotations[jl.getSelectedIndex()] = true;
        }
        if (e.getActionCommand().equals("off")) {
            System.out.println("OFF");
            annotations[jl.getSelectedIndex()] = false;

        }
        jl.clearSelection();
        jl.setSelectedIndex(ind);
    }
}

private static class WindowCloseHandler extends WindowAdapter {

    public void windowClosing(WindowEvent evt) {
        ObjectOutputStream save = null;
        try {
            save = new ObjectOutputStream(new FileOutputStream(file + "Annotations"));
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        try {
            save.writeObject(annotations);
        } catch (IOException e) {
            e.printStackTrace();
        }
        try {
            save.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
        System.out.println("Saved.");
    }
}
}

Even though jl.requestFocus(); works fine, i tried another thing that make it works: i swapped these two lines

buttonPane.add(onbutton);
buttonPane.add(offbutton);

but, why?
Sorry if i ask again but it’s really strange, isn’t it?

Answer 1

In your cell render, try this instead

if (annotations[index] == null) {
    //c.setBackground(Color.white);
} else if (annotations[index] == true) {
    c.setBackground(Color.green);
} else {
    c.setBackground(Color.red);
}

For something a “little” extended, you could also try

private static class MyCellRenderer extends DefaultListCellRenderer {

    private static final long serialVersionUID = 1L;

    private static final Border SELECTION_BORDER = new LineBorder(UIManager.getColor("List.selectionBackground"));
    private static final Border EMPTY_BORDER = new EmptyBorder(1, 1, 1, 1);

    @SuppressWarnings("rawtypes")
    public Component getListCellRendererComponent(JList list, Object value, int index, boolean isSelected, boolean cellHasFocus) {
        Component c = super.getListCellRendererComponent(list, value, index, isSelected, cellHasFocus);
        setBorder(EMPTY_BORDER);
        if (annotations[index] == null) {

            if (isSelected) {

                setBorder(SELECTION_BORDER);

            }

            c.setBackground(Color.white);
        } else if (annotations[index] == true) {
            c.setBackground(Color.green);
        } else {
            c.setBackground(Color.red);
        }
        return c;
    }
}

Basically, what you are doing is changing the selection highlight color, hiding the selection. As to why you did get a paint artifact that “seemed” to highlight the selected row is still a slight mystery to me 😛

UPDATED with Knowing why

The paint artifact you are sometimes getting is a result of the hasFocus parameter of the cell renderer, painting a focus rectangle.

Now, if you want to keep your existing cell renderer, then try this in the ActionEvent

 //jl.clearSelection();
 jl.setSelectedIndex(ind);
 jl.requestFocus();

From what I can tell, it seems to a slight qwerk with the repaint manager. If I added jl.repaint() into you existing code (under the setSelectedIndex call) I can get it to never paint the focus rectangle 😛