Adding model to JTable - NullPointerException thrown

**Zyril** · 09-18-2012, 05:01 PM

SOLUTION:
I just solved the problem, it's a really silly one. The table never gets initialized.
Following code solves it completely.

Java Code:

        table = new JTable(model);

Original post below for interested people!

Hello all!

I'm having some trouble with a GUI I'm coding right now. Not the best way to have fun, but it has to be done.
I'm adding a DefaultTableModel to a JTable object, but I keep getting a NullPointerException thrown. I've tried debugging, but I can't really figure out what the Unknown Source indicates.

The troublesome code is in the very end of the file.

EDIT:
Alright, I've shortened down the code so it's more easy to run! I'm sorry for pasting all of it and just waiting for answers, I was in a hurry and wanted to get it posted. Next time I'll know it's better to do it correctly from the start!

1) Why does your class extend JFrame? (misuse of inheritance: favor composition.)
2) Why is menuBar static? (it shouldn't be)
3) Why all the setPreferredSize(...) calls? (use layout managers correctly and they are not needed. Moreover, setting the preferred size to something that looks right on your computer can mess up royally when the code is run on another system.)
4) Removed inane comments before posting on a forum. Your comments make the code more, not less, difficult to follow.

1. I created a visual class in Eclipse named AddressBookGUI. It works as my mainframe, therefor it extends JFrame. Why is that less desirable than creating a class, and then a JFrame within it?

2. True, I don't know why it is. Might be the result of a copy/paste, though that means there is another file in another project with the same mistake.

3. I'm using FlowLayout where the buttons are used, where most of the setPreferredSize are. I want them to be of the same size and I don't know how a layoutmanager can do that for me. I've read about them and I am using different layoutmanagers for the panels.

4. Comments removed.

Java Code:

import java.awt.*;
import javax.swing.*;
import javax.swing.UIManager.LookAndFeelInfo;
import javax.swing.border.EmptyBorder;
import javax.swing.table.DefaultTableModel;
 
public class AddressBookGUI extends JFrame {

    private JPanel contentPane;
    private JPanel panelTable;
    private JPanel panelLeft;
     
    private JScrollPane tableContainer;
    private JTable table;
    private DefaultTableModel model;
     
 
    private JMenuBar menuBar;
    private JMenu fileMenu;
    private JMenuItem openMenu;
    private JMenuItem exitMenu;
     
    /**
     * Launch the application.
     */
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    for (LookAndFeelInfo info : UIManager.getInstalledLookAndFeels()) {
                        if ("Nimbus".equals(info.getName())) {
                            UIManager.setLookAndFeel(info.getClassName());
                            break;
                        }
                    }
                } catch (Exception e) {
                    e.printStackTrace();
                }
                try {
                    AddressBookGUI frame = new AddressBookGUI();
                    frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }
 
    /**
     * Create the frame.
     */
    @SuppressWarnings("serial")
    public AddressBookGUI() {
        setTitle("Address Book");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setBounds(100, 100, 600, 530);
        contentPane = new JPanel();
        contentPane.setBackground(Color.WHITE);
        contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
        contentPane.setLayout(new BorderLayout(0, 0));
        setContentPane(contentPane);
        

        panelLeft = new JPanel();   
        panelLeft.setBackground(Color.WHITE);
        contentPane.add(panelLeft, BorderLayout.WEST);
        panelLeft.setBorder(BorderFactory.createLineBorder(Color.BLACK));
        panelLeft.setPreferredSize(new Dimension(150,0));
        panelLeft.setLayout(new FlowLayout(FlowLayout.CENTER, 5, 5));
         

        panelTable = new JPanel();
        contentPane.add(panelTable, BorderLayout.EAST);
        tableContainer = new JScrollPane(table);
        panelTable.add(tableContainer);
        panelTable.setBorder(BorderFactory.createLineBorder(Color.BLACK));
        panelTable.setPreferredSize(new Dimension(420,0));
 

        model = new DefaultTableModel(){
            public boolean isCellEditable(int row, int column) {
                return false;
            }
        };
//      model = new DefaultTableModel();
        model.setColumnIdentifiers(new String[] {"Förnamn", "Efternamn", "Telefonnummer"});
        table.setModel(model);
    }
}

Am I messing things up with my different JPanels, perhaps missing a step somewhere that makes it impossible for the table to find the model?

I also tried to declare the model differently:

Java Code:

        DefaultTableModel model = new DefaultTableModel(){
        	public boolean isCellEditable(int row, int column) {
                return false;
        	}
        };
//		model = new DefaultTableModel();
        model.setColumnIdentifiers(new String[] {"Förnamn", "Efternamn", "Telefonnummer"});
        table.setModel(model);

This is the stacktrace:

Java Code:

java.lang.NullPointerException
	at AddressBookGUI.<init>(AddressBookGUI.java:87)
	at AddressBookGUI$1.run(AddressBookGUI.java:40)
	at java.awt.event.InvocationEvent.dispatch(Unknown Source)
	at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
	at java.awt.EventQueue.access$000(Unknown Source)
	at java.awt.EventQueue$1.run(Unknown Source)
	at java.awt.EventQueue$1.run(Unknown Source)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
	at java.awt.EventQueue.dispatchEvent(Unknown Source)
	at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
	at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
	at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
	at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
	at java.awt.EventDispatchThread.run(Unknown Source)

**JavaAdviser** · 09-18-2012, 05:12 PM

Could you maybe please post the stacktrace as well?

**DarrylBurke** · 09-18-2012, 05:28 PM

To get better help sooner, post a SSCCE (Short, Self Contained, Correct (Compilable), Example) that demonstrates the problem. Not code that has dependencies that aren't available to members who might like to try to help.

Other notes:
1) Why does your class extend JFrame? (misuse of inheritance: favor composition.)
2) Why is menuBar static? (it shouldn't be)
3) Why all the setPreferredSize(...) calls? (use layout managers correctly and they are not needed. Moreover, setting the preferred size to something that looks right on your computer can mess up royally when the code is run on another system.)
4) Removed inane comments before posting on a forum. Your comments make the code more, not less, difficult to follow.

Adn JavaAdvisor is right, without compilable code and/or a stack trace, we would be just guessing at possible causes. Probably wouldn't help to do that.

db

**doWhile** · 09-18-2012, 05:31 PM

Posting the stack trace will help. I would start by reading that, figuring out which line in your code the exception is thrown, then add some println's to determine which object is null. You posted a bit of code to wade through (an SSCCE does a world of good in you getting help), but the first variable I would look at is 'table' - where is it instantiated?

**Zyril** · 09-18-2012, 07:36 PM

Updated the original thread starting post!
Sorry for the rushed post earlier, I should have prepared it better to be expect answers!

I also included the stacktrace in the end. I put breakpoints on line 87 and 40, and the exception is thrown on line 87. Unknown source? What is the source it is looking for, is it the object called model or something else?

Cheers,
Z!