Question

I’ve got a problem connected with my previous question: JList: sorting by Up/down buttons
My Up/Down buttons work properly now, but here’s the other thing:
I need to make Top/Bottom buttons also. They seem to be harder. I use actionPerformed to button “Top”:

private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {
 int indexOfSelected = jList1.getSelectedIndex();
 swapElements2(indexOfSelected, 0);
 indexOfSelected = 0;
 jList1.setSelectedIndex(indexOfSelected );
 jList1.updateUI();
}

And swapElements2 method looks like that:

private void swapElements2(int pos1, int pos2) {
  String tmp1 =  (String) listModel.get(pos1);
  String tmp2 =  (String) listModel.get(pos2);
  listModel = (DefaultListModel) jList1.getModel();
  int sizeOfList = listModel.getSize();

  listModel.set(pos2, listModel.get(sizeOfList-1));
  for (int i=sizeOfList-2;i>0;i--){
    listModel.set(i+1, listModel.get(i));
  }

  listModel.set(pos2, tmp1);
  listModel.set(pos2+1, tmp2);
}

Of course, it works only if the last element of jList is selected. Has anyone an idea how to make an universal method which will move the element from selected position to 0 position? Method should also move all other elements forwards and the last element should stay at it’s place if it’s not selected.

I’d be grateful for any help.

Edit from comments, which also doesn’t work:

private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {
 String temp = (String) listModel.remove(jList1.getSelectedIndex());
 listModel.add(0, temp);
}

Error:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
name podziwiaj.jpg
at java.io.File.<init>(File.java:222)

Which is:

public File(String pathname) {
if (pathname == null) {
    throw new NullPointerException();
}
this.path = fs.normalize(pathname);
this.prefixLength = fs.prefixLength(this.path);
}

List stores path names of files (images) and displays is, when item is selected.

Answer 1

My own preference is to work with the list’s model to do this. Something like:

DefaultListModel model = (DefaultListModel)jList1.getModel();
File temp = model.remove(jList1.getSelectedIndex());
model.add(0, temp);

To see what methods are available to the list’s model, please check out the DefaultListModel API.

Edit: code works fine.

Edit 2: My sscce

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

public class TestList {

   private static void createAndShowGui() {
      String[] data = {"Mon", "Tues", "Wed", "Thurs", "Fri", "Sat", "Sun"};
      final DefaultListModel<String> model = new DefaultListModel<String>();
      for (String datum : data) {
         model.addElement(datum);
      }
      final JList<String> myList = new JList<String>(model);      
      JButton toTop = new JButton(new AbstractAction("To Top") {

         @Override
         public void actionPerformed(ActionEvent arg0) {
            int selectedIndex = myList.getSelectedIndex();
            if (selectedIndex < 0) {
               return;
            }
            String temp = model.remove(selectedIndex);
            model.add(0, temp);
         }
      });



      JPanel mainPanel = new JPanel();
      mainPanel.add(new JScrollPane(myList));
      mainPanel.add(toTop);


      JFrame frame = new JFrame("TestList");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.getContentPane().add(mainPanel);
      frame.pack();
      frame.setLocationByPlatform(true);
      frame.setVisible(true);
   }

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }
}

If you’re still stuck after reviewing this, then consider reading about SSCCE, and posting your own here that we can modify and correct.

Edit 3
Per our discussions in chat,
Your problem is when you call the To Top JButton, you are deleting the selected item from the list, while the ListSelectionListener is still listening, and this will cause a NPE to be thrown. A possible solution is to remove the ListSelectionListener before removing or adding any items, and then re-adding the listener after performing these actions. This will probably be necessary when you update your list model as well. Please see my SSCCE below. Also see how it is much easier to read and understand SSCCE code than your big code.

import java.awt.BorderLayout;
import java.awt.Component;
import java.awt.Dimension;
import java.awt.Image;
import java.awt.event.*;
import java.io.File;
import java.io.IOException;

import javax.imageio.ImageIO;
import javax.swing.*;
import javax.swing.event.*;

@SuppressWarnings("serial")
public class MyToolView extends JPanel {

   public File[] file;
   public static int i = 0;

   final static JFileChooser fc = new JFileChooser();
   private static final int PREF_W = 900;
   private static final int PREF_H = 750;
   final DefaultListModel listModel = new DefaultListModel();
   final JList myList = new JList(listModel);
   private JLabel imageLabel = new JLabel();
   private javax.swing.JButton openButton;
   private JButton toTopBtn;
   private javax.swing.JList imageJList;
   private javax.swing.JScrollPane jScrollPane1;
   private javax.swing.JPanel mainPanel;

   public MyToolView() {
      initComponents();
   }

   private void initComponents() {
      jScrollPane1 = new javax.swing.JScrollPane();
      imageJList = new javax.swing.JList(listModel);
      openButton = new javax.swing.JButton();
      toTopBtn = new JButton("To Top");

      JPanel topPanel = new JPanel();
      topPanel.add(jScrollPane1);
      topPanel.add(openButton);
      topPanel.add(toTopBtn);

      setLayout(new BorderLayout());
      add(topPanel, BorderLayout.PAGE_START);
      add(new JScrollPane(imageLabel), BorderLayout.CENTER);

      toTopBtn.addActionListener(new ActionListener() {

         @Override
         public void actionPerformed(ActionEvent e) {
            toTopBtnActionPerformed(e);
         }
      });

      jScrollPane1.setFocusable(false);
      jScrollPane1.setName("jScrollPane1"); // NOI18N

      imageJList.setModel(new javax.swing.AbstractListModel() {
         String[] strings = { "AAAA.JPG", "BBBB.JPG" };

         public int getSize() {
            return strings.length;
         }

         public Object getElementAt(int i) {
            return strings[i];
         }
      });
      imageJList.setName("jList1"); // NOI18N
      imageJList.setCellRenderer(new JavaRenderer());
      jScrollPane1.setViewportView(imageJList);
      imageJList.addListSelectionListener(listSelectionListener);

      openButton.setFocusable(false);
      openButton.setName("openButton"); // NOI18N
      openButton.setAction(new OpenAction("Open"));

   }

   @Override
   public Dimension getPreferredSize() {
      return new Dimension(PREF_W, PREF_H);
   }

   private void toTopBtnActionPerformed(java.awt.event.ActionEvent evt) {

      // ******** here **********
      imageJList.removeListSelectionListener(listSelectionListener);
      int selectedIndex = imageJList.getSelectedIndex();
      if (selectedIndex < 0) {
         return;
      }
      String temp = (String) listModel.remove(selectedIndex);
      listModel.add(0, temp);

      // ******** here **********
      imageJList.addListSelectionListener(listSelectionListener);
   }

   ListSelectionListener listSelectionListener = new ListSelectionListener() {
      public void valueChanged(ListSelectionEvent listSelectionEvent) {
         System.out.print("First index: " + listSelectionEvent.getFirstIndex());
         System.out.print(", Last index: " + listSelectionEvent.getLastIndex());

         String path = (String) imageJList.getSelectedValue();
         System.out.println("path should be here: " + path);
         Image img;
         try {
            System.out.println("path is null: " + (path == null));
            File imageFile = new File(path);
            img = ImageIO.read(imageFile);
            imageLabel.setIcon(new ImageIcon(img));
            imageLabel.setText("");
         } catch (IOException ex) {
            ex.printStackTrace();
         }
      }
   };

   private class OpenAction extends AbstractAction {
      public OpenAction(String text) {
         super(text);
      }

      public void actionPerformed(ActionEvent evt) {

         fc.setMultiSelectionEnabled(true);
         int returnVal = fc.showDialog(null, "Open");
         int len = 0;

         if (returnVal == JFileChooser.APPROVE_OPTION) {
            imageJList.removeListSelectionListener(listSelectionListener); //!!
            file = fc.getSelectedFiles();
            len = file.length;
            System.out.println("length = " + len);

            // this code should be inside the if block
            for (i = 0; i < len; i++) {
               String path = file[i].getPath();
               System.out.printf("%d: %s%n", i, path);
               listModel.add(i, path);
            }
            imageJList.setModel(listModel);
            imageJList.addListSelectionListener(listSelectionListener); //!!
         }
         // imageJList.updateUI(); //!! you shouldn't call this.
      }
   }

   private static void createAndShowGui() {
     MyToolView mainPanel = new MyToolView();

      JFrame frame = new JFrame("MyToolView");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.getContentPane().add(mainPanel);
      frame.pack();
      frame.setLocationByPlatform(true);
      frame.setVisible(true);
   }

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }
}

class JavaRenderer extends DefaultListCellRenderer {
   private int a;

   @Override
   public Component getListCellRendererComponent(JList jList1, Object value,
         int index, boolean isSelected, boolean hasFocus) {

      if (value instanceof String) {
         String name = (String) value;
         String[] splitedArray = null;
         splitedArray = name.split("\\\\");
         a = splitedArray.length - 1;
         name = splitedArray[a];
         System.out.println("name " + name);
         return super.getListCellRendererComponent(jList1, name, index,
               isSelected, hasFocus);// what we display
      } else {
         return super.getListCellRendererComponent(jList1, value, index,
               isSelected, hasFocus);// what we take
      }
   }
}