Why am I getting a NullPointerException?

[Adam Cripps]

I want to save some preferences when the user closes the JFrame. I've implemented the WindowEvent to pick this up and set setDefaultCloseOperation(JFrame.DO_NOTHING). I'm calling the applications variable prefs.flush() which is giving the NullPointerException. I'm guessing that the nullpointerexception is thrown because I'm not actually handling the instance of jreport.prefs that I think I am. I would like some help finding out where I'm going wrong with handling the jreport.prefs instance. I declare JReport jreport in JReportFrame (which is instantiated by JReport). And I pass the object of JReport when calling JReportFrame:

frame = new JReportFrame(this, version);

and the JReportFrame constructor that receives this:

public JReportFrame(JReport jreport, String version){
		//System.out.println("I have the jreport - I will construct now. ");
		setTitle(applicationName + " - writing academic reports more efficiently.    " + version);
		setDefaultCloseOperation(JFrame.DO_NOTHING_ON_CLOSE);
		addWindowListener(this); 

@Override;
	public void windowClosing(WindowEvent e) {
		// TODO Auto-generated method stub
		System.out.println("You closed on me...");
		System.out.println("preferences have been set. ");
		  //This is throwing a nullpointerexception - not sure why - but glad I'm here now. 
		  try {
					jreport.prefs.flush();
		} catch (BackingStoreException e1) {
			// TODO Auto-generated catch block
			e1.printStackTrace();
		}
		  this.dispose();
	}

TIA
Adam

[Winston Gutkowski]

Adam Cripps wrote:I want to save some preferences when the user closes the JFrame. I've implemented the WindowEvent to pick this up and set setDefaultCloseOperation(JFrame.DO_NOTHING). I'm calling the applications variable prefs.flush() which is giving the NullPointerException. I'm guessing that the nullpointerexception is thrown because I'm not actually handling the instance of jreport.prefs that I think I am. I would like some help finding out where I'm going wrong with handling the jreport.prefs instance. I declare JReport jreport in JReportFrame (which is instantiated by JReport)...

The easiest thing to do is probably to add some debugging statements to print out the contents of the possible culprits, viz:

try {
   System.out.println(jreport);
   System.out.println(jreport.prefs);
   jreport.prefs.flush();
   ...

it won't tell you why, but it may help you to narrow down the problem.

Winston

[Adam Cripps]

Thanks Winston. That gives

null

as output when run - which supports my initial guess (I'm not handling the correct version of jreport). Am I correct in thinking that if I pass the jreport instance to the constructor and that with the declaration at the beginning of the class (JReport jreport;) that the whole class should be able to see jreport?

[Joanne Neal]

Adam Cripps wrote:Am I correct in thinking that if I pass the jreport instance to the constructor and that with the declaration at the beginning of the class (JReport jreport;) that the whole class should be able to see jreport?

No. You have to assign the jreport you pass to your constructor to the member variable. In your constructor you need the line

this.jreport = jreport;