Java jList NullPointerException error

Question

Error:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at test.factory.MainWindow.setFuncList(MainWindow.java:160)
    at test.factory.MainWindow.<init>(MainWindow.java:22)
    at test.factory.MainWindow$2.run(MainWindow.java:151)
    at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:251)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:721)
    at java.awt.EventQueue.access$200(EventQueue.java:103)
    at java.awt.EventQueue$3.run(EventQueue.java:682)
    at java.awt.EventQueue$3.run(EventQueue.java:680)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:691)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:244)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:163)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:151)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:147)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:139)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:97)

Code:

TestFactory tf = new TestFactory();
ArrayList<Function> fList = tf.getFunctions();
DefaultListModel<Function> dFuncList = new DefaultListModel();
fListPane.setModel(dFuncList);
for(Function f : fList) {
    dFuncList.addElement(f);
}

Question:

Now, if you find the error that's great, but my question is. How do I parse the error text to find where my error originated? I'm used to things like missing ';' at line 24 of C:\filename

Update: fList has two elements, so not null.

for better help sooner post an SSCCE, short, runnable, compilable, with hardcoded value in local variable, instead of FileIO — mKorbel
– mKorbel, Commented Feb 21, 2013 at 21:46
That doesn't seem to be enough code. Is there a call to setFuncList in your MainWindow.java class (on line 160)? — Slartibartfast
– Slartibartfast, Commented Feb 21, 2013 at 21:47
inside the () the light blue is the class and :## is the line number so(MainWindow.java:160) says line 160 of MainWindow.java — orangegoat
– orangegoat, Commented Feb 21, 2013 at 21:48
I don't have the complete code but based on what you posted, you are not checking for fList , to see if it's null. Also check for f value too make sure none of those are null. just a simple if X != null may help you. — grepit
– grepit, Commented Feb 21, 2013 at 21:52
I suppose I should have just posted the exception. I really only wanted a good answer for how to read those things, but @Bridgey gave me a pretty good one. Thanks — Eric Fossum
– Eric Fossum, Commented Feb 21, 2013 at 23:48

Bridgey · Accepted Answer · 2013-02-21 23:49:38Z

3

The error dump is a stack trace, so I tend to find it's always best to start at the top and work down. In this case it looks like your setFuncList at line 160 of MainWindow.java is trying to work with an object that is null (maybe not yet initialised?).

UPDATE: Example of code that works

class Function {

    int i;

    public Function(int myI) {
        this.i = myI;
    }

    @Override
    public String toString() {
        return "i=" + this.i;
    }
}

Used with:

ArrayList<Function> fList = new ArrayList<>();
fList.add(new Function(1));
fList.add(new Function(2));

DefaultListModel<Function> dFuncList = new DefaultListModel();
jList2.setModel(dFuncList);
for(Function f : fList) {
    dFuncList.addElement(f);
}

edited Feb 21, 2013 at 23:49

answered Feb 21, 2013 at 21:48

Bridgey

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8 Comments

Eric Fossum Over a year ago

Line 160 is the setModel() line, but the example I used does the same thing; new then set. I have moved it after the for loop and it's still null. Is there some code to set the String to be displayed in the jList?

Eric Fossum Over a year ago

Do I have to use a String in DefaultListModel?

Bridgey Over a year ago

You're code looks ok. Can you dump the contents of dFuncList after the for loop - just to check that it contains what you expect?

Bridgey Over a year ago

I don't think you need to use String in DefaultListModel. I would expect it to call the object's toString() method in order to display.

Eric Fossum Over a year ago

Yeah, I feel dumb now. Just before reading this I realized my JList was being initialized after this function... doh!

|

Adrian · Accepted Answer · 2013-02-21 22:20:44Z

0

So basically look through the stack trace from the top, it will list the calls that have occurred which led to the error you received. Look carefully at the lines in your code that are listed. If you can't see any obvious errors you can add some extra tests based on the error. Ie check some objects are not null before the line which caused the error, I find printouts a simple approach. You can also use a debugger, I use jswat but only break it out when I really need to.

Hope that was what you were after

@orangegoat gave a good breakdown of how to interpret the stack trace if that's what you wanted

Also a link to jswat http://code.google.com/p/jswat/

edited Feb 21, 2013 at 22:20

answered Feb 21, 2013 at 22:08

Adrian

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