Thread: nullpointer exception trying to write to file

I've been closely following a tutorial series, and as far as I can tell my code is exactly the same, but not functioning. The app throws a nullpointer exception here on "controller.saveToFile(fileChooser.getSelectedFile ());"

// Export data item
		exportDataItem.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent e) {
				if (fileChooser.showSaveDialog(MainFrame.this) == JFileChooser.APPROVE_OPTION) {
					try {
						controller.saveToFile(fileChooser.getSelectedFile());
 
					} catch (IOException e1) {
						JOptionPane.showMessageDialog(MainFrame.this,
								"Could not save data to file.", "Error",
								JOptionPane.ERROR_MESSAGE);
					}
 
				}
 
			}
		});

controller save method:

public void saveToFile(File file) throws IOException {
		db.saveToFile(file);
	}

database save method:

	public void saveToFile(File file) throws IOException {
		FileOutputStream fos = new FileOutputStream(file);
		ObjectOutputStream oos = new ObjectOutputStream(fos);
 
		Person[] persons = people.toArray(new Person[people.size()]);
 
		oos.writeObject(persons);
 
		oos.close();
 
	}

there are no errors popping up in the code, and I dont understand why there is a null pointer exception when Im saving this file.

please let me know if I need to upload more code, or if I just did something stupid here.

thanks!

Welcome to the Forum! Thanks for taking the time to learn to post code correctly, and if you haven't already, please read this topic to see other useful info for newcomers.

Please post the entire error message/stack trace, copied and pasted into a post from just as it appears at your end. See if you can pare the code down to a simple runnable example you could post to demonstrate the problem: open a file using fileChooser() and write a single line to it.

Here is the stacktrace:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
	at gui.MainFrame$6.actionPerformed(MainFrame.java:145)
	at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
	at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
	at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
	at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
	at javax.swing.AbstractButton.doClick(Unknown Source)
	at javax.swing.plaf.basic.BasicMenuItemUI.doClick(Unknown Source)
	at javax.swing.plaf.basic.BasicMenuItemUI$Handler.mouseReleased(Unknown Source)
	at java.awt.Component.processMouseEvent(Unknown Source)
	at javax.swing.JComponent.processMouseEvent(Unknown Source)
	at java.awt.Component.processEvent(Unknown Source)
	at java.awt.Container.processEvent(Unknown Source)
	at java.awt.Component.dispatchEventImpl(Unknown Source)
	at java.awt.Container.dispatchEventImpl(Unknown Source)
	at java.awt.Component.dispatchEvent(Unknown Source)
	at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
	at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
	at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
	at java.awt.Container.dispatchEventImpl(Unknown Source)
	at java.awt.Window.dispatchEventImpl(Unknown Source)
	at java.awt.Component.dispatchEvent(Unknown Source)
	at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
	at java.awt.EventQueue.access$200(Unknown Source)
	at java.awt.EventQueue$3.run(Unknown Source)
	at java.awt.EventQueue$3.run(Unknown Source)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
	at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
	at java.awt.EventQueue$4.run(Unknown Source)
	at java.awt.EventQueue$4.run(Unknown Source)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.ProtectionDomain$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)

Very helpful.

Based on these lines (and a little experience):

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
	at gui.MainFrame$6.actionPerformed(MainFrame.java:145)
	at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)

look at line 145 which should be in the actionPerformed() method. It appears a button is being fired (I'll call it 'myButton') that has not been initialized before being used, as in:

myButton = new JButton();

Figure out which button (or whatever it is) is being complained about and backtrack through the code to determine why it was declared but not initialized (in the constructor, maybe) and fix it.

ok I initialized my controller which fixed the nullpointer exception. my save method seems to work (creates a file on my desktop). but when I try and import and then refresh my table model, the code runs but nothing displays. my tableModel.refresh method fires the data changed method in my table.

--- Update ---

Load method in MainFrame:

// Import data item
		importDataItem.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent e) {
				if (fileChooser.showOpenDialog(MainFrame.this) == JFileChooser.APPROVE_OPTION) {
					try {
						Controller controller = new Controller();
						controller.loadFromFile(fileChooser.getSelectedFile());
						tablePanel.refresh();
						System.out.println("tableModel refreshed");
					} catch (IOException e1) {
						JOptionPane.showMessageDialog(MainFrame.this,
								"Could not load data from file.", "Error",
								JOptionPane.ERROR_MESSAGE);
					}
				}
				tablePanel.refresh();
			}
		});

load method in controller:

public void loadFromFile(File file) throws IOException {
		db.loadToFile(file);
	}

load method in database db:

	public void loadToFile(File file) throws IOException {
		FileInputStream fis = new FileInputStream(file);
		ObjectInputStream ois = new ObjectInputStream(fis);
 
		Person[] persons;
		try {
			people.clear();
			persons = (Person[]) ois.readObject();
			people.addAll(Arrays.asList(persons));
		} catch (ClassNotFoundException e) {
			e.printStackTrace();
		}
 
		ois.close();
 
	}

when I try and import and then refresh my table model, the code runs but nothing displays

Does the table display before the refresh? The table is a component, the table model is a behind the scenes description of the table, so the first step is to reliably display the component. Can you do the first step?

Originally Posted by GregBrannon

Does the table display before the refresh? The table is a component, the table model is a behind the scenes description of the table, so the first step is to reliably display the component. Can you do the first step?

Here is my TablePanel.java:

package gui;
 
import java.awt.BorderLayout;
import java.util.List;
 
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTable;
 
import model.Person;
 
public class TablePanel extends JPanel {
 
	private static final long serialVersionUID = -5068640091114058027L;
 
	private JTable table;
	private PersonTableModel tableModel;
 
	public TablePanel() {
 
		tableModel = new PersonTableModel();
		table = new JTable(tableModel);
 
		setLayout(new BorderLayout());
 
		add(new JScrollPane(table), BorderLayout.CENTER);
	}
 
	public void setData(List<Person> db) {
		tableModel.setData(db);
	}
 
	public void refresh() {
		tableModel.fireTableDataChanged();
	}
 
}

This is starting to drive me insane, I can't figure this shit out. When I add an element and use refresh it works just fine. I don't know if it's just not displaying or if it's not loading anything at all.

--- Update ---

Originally Posted by GregBrannon

Does the table display before the refresh? The table is a component, the table model is a behind the scenes description of the table, so the first step is to reliably display the component. Can you do the first step?

The table column names display but no data.

--- Update ---

Solved! This was a response from John Purcell of caveofprogramming.com:

You're doing this in MainFrame.java: final Controller controller = new Controller();
So you're creating a new controller, creating a new variable at local scope overshadowing the one at class scope that you actually need to be using. What you need is this: controller = new Controller();
The controller that you then try to use is the instance variable, which was not set to new anything.
Then to fix the null pointer, you added more new controller statements, creating new databases all over the place
I think every programmer has made this mistake sometime -- declaring a new local variable by mistake, instead of initializing the class-level one. You have to pay careful attention to what those null pointer exceptions tell you --- otherwise they extract dire payment. They are usually easy to fix once you know how.