A problem about pushing a exit button to exit my java swing program

[blackdiz]

I try to write a simple text editor.
When I click the exit button ,I get a exception as blow:

Exception in thread "AWT-EventQueue-0" java.lang.Error: Unresolved compilation problem: 

	at com.black.System.exit(System.java:959)
	at com.black.HomeWork09$4.actionPerformed(HomeWork09.java:98)
	at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
	at javax.swing.AbstractButton$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$000(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)

And here's my code :

package com.black;

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

public class HomeWork09 {
	private boolean fileSaved = false;
	private JFrame frame;
	private JTextArea textArea;
	private JMenuItem newFile;
	private JMenuItem openFile;
	private JMenuItem saveFile;
	private JMenuItem exit;

	public HomeWork09() {
		frame = new JFrame("Simple Text Editor");
		textArea = new JTextArea();
		textArea.setLineWrap(true);
		textArea.setWrapStyleWord(true);

		JMenuBar bar = new JMenuBar();
		JMenu file = new JMenu("File");
		newFile = new JMenuItem("Open New File");
		openFile = new JMenuItem("Open File");
		saveFile = new JMenuItem("Save File");
		exit = new JMenuItem("Exit");

		newFile.addActionListener(newFileListener);
		openFile.addActionListener(OpenFileListener);
		saveFile.addActionListener(SaveFileListener);
		exit.addActionListener(ExitListener);

		file.add(newFile);
		file.add(openFile);
		file.add(saveFile);
		file.add(exit);

		bar.add(file);

		frame.setJMenuBar(bar);
		frame.add(textArea);
		frame.setSize(500, 600);
		frame.setVisible(true);
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
	}

	public static void main(String[] args) {
		new HomeWork09();
	}

	ActionListener newFileListener = new ActionListener() {
		public void actionPerformed(ActionEvent event) {
			int n = 0;
			if (fileSaved == false && !textArea.getText().isEmpty()) {
				n = JOptionPane.showConfirmDialog(frame, "You haven't saved a file,sure to open a new one?",
						"Simple Text Editor", JOptionPane.YES_NO_OPTION);
				if (n == JOptionPane.YES_OPTION) {
					textArea.setText(null);
					fileSaved = false;
				}
			} else {
				textArea.setText(null);
				fileSaved = false;
			}
		}
	};

	ActionListener OpenFileListener = new ActionListener() {
		public void actionPerformed(ActionEvent event) {
			JFileChooser openFileDialog = new JFileChooser();
			openFileDialog.setDialogTitle("Open File");
			openFileDialog.showOpenDialog(frame);

			open(openFileDialog.getSelectedFile());
		}
	};

	ActionListener SaveFileListener = new ActionListener() {
		public void actionPerformed(ActionEvent event) {
			JFileChooser saveFile = new JFileChooser();
			saveFile.setDialogTitle("Save New File");
			saveFile.showSaveDialog(frame);

			save(saveFile.getSelectedFile());
		}
	};

	ActionListener ExitListener = new ActionListener() {
		public void actionPerformed(ActionEvent event) {
			System.exit(0);
		}
	};

	private void open(File file) {
		if (file != null) {
			try {
				String str;
				StringBuilder s = new StringBuilder();
				BufferedReader br = new BufferedReader(new FileReader(file));
				while ((str = br.readLine()) != null) {
					s.append(str);
					s.append("\n");
				}
				textArea.setText(s.toString());
			} catch (IOException e) {
				e.printStackTrace();
			}
		}
	}

	private void save(File file) {
		if (file != null) {
			try {
				String s = textArea.getText();
				BufferedWriter bf = new BufferedWriter(new FileWriter(file));
				bf.write(s);
				bf.close();
				fileSaved = true;
			} catch (IOException ex) {
				ex.printStackTrace();
			}
		}
	}
}

Can somebody help me to find what's wrong?
Thank you,Best regards!

[KevinWorkman]

That means you have a compiler error. Track down what that is before trying to run your program.

[blackdiz]

Thank you for your answering
But I can't see any warning icon in my Eclipse.
Is there any other way to find that compiler error?

[blackdiz]

Well....That's really weird...
I change the workspace to another directory.
Then this works just perfectly fine.
So the problem solved.
I think the reason may because I put my src file in dropbox , and the file's synchronization has problem.So after I copy the file to another directory , it will work. Just guessing.
Anyway , thank you for answering again.

[Tolls]

com.black.System.exit(System.java:959)
You haven't got a System class have you by any chance?

[blackdiz]

Oh, Yes~ I put the System.java in my package(just curious what's inside that, so I copy it into my package and use eclipse to look) , the error happens.
After I delete it , all works!!
So the problem is I put the System.java in my package, when I call System.exit(0) , it will look into the System.java in my package rather than the System.java in the Java src.zip , right?
I really do something silly
Thank you. Best Regards!

[Tolls]

Yep, or at least that's when it notices the System.java is different (and non-compilable as its package declaration and location don't match).
If you want Eclipse to bring up the source code then CTRL-left click on the classname in your code (or method name) and it'll either bring up the source code or ask you where the source code is located.

[blackdiz]

Yep, or at least that's when it notices the System.java is different (and non-compilable as its package declaration and location don't match).
If you want Eclipse to bring up the source code then CTRL-left click on the classname in your code (or method name) and it'll either bring up the source code or ask you where the source code is located.

Oh, OK , I 've got it , thank you for teaching me that~