Question

I would like to display the audio information to jList after I selected a lists of songs from JFileChooser. Here is what I get as far, but it throws a NullPointerException.

boolean openFile() throws FileNotFoundException{
    JFileChooser jfc = new JFileChooser();
    jfc.setMultiSelectionEnabled(true);
    jfc.setDialogTitle("Open File");
    jfc.setFileSelectionMode(JFileChooser.FILES_ONLY);
    jfc.setCurrentDirectory(new File ("."));
    jfc.setFileFilter(audiofilter);
    int result = jfc.showOpenDialog(this);
    if(result == JFileChooser.CANCEL_OPTION){
        return true;
    }else if(result == JFileChooser.APPROVE_OPTION){
        File[] file = jfc.getSelectedFiles();//fFile = jfc.getSelectedFile();
        String file_string = fFile.getAbsolutePath();
        for(int i=0;i<file.length;i++){
            if(file[i].isFile()){
                String[] filesInDirectory = file[i].list();
                for(int ii=0;ii<filesInDirectory.length;ii++){
                    list.addElement(filesInDirectory[ii]);
                }
            }else{
                list.addElement(file[i]);
            }
        }

        if(file_string != null){
            fTextArea.setText(file_string);
        }else{
            return false;
        }
        this.jList1.setModel(list);
    }
    return true;
}

the error says:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at SoundsTrip.SoundBytePlaying.openFile(SoundBytePlaying.java:267)
        at SoundsTrip.SoundBytePlaying.jButton1MouseClicked(SoundBytePlaying.java:210)
        at SoundsTrip.SoundBytePlaying.access$000(SoundBytePlaying.java:43)
        at SoundsTrip.SoundBytePlaying$2.mouseClicked(SoundBytePlaying.java:135)
        at java.awt.AWTEventMulticaster.mouseClicked(AWTEventMulticaster.java:253)
        at java.awt.Component.processMouseEvent(Component.java:6270)
        at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)
        at java.awt.Component.processEvent(Component.java:6032)
        at java.awt.Container.processEvent(Container.java:2041)
        at java.awt.Component.dispatchEventImpl(Component.java:4630)
        at java.awt.Container.dispatchEventImpl(Container.java:2099)
        at java.awt.Component.dispatchEvent(Component.java:4460)
        at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4577)
        at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4247)
        at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4168)
        at java.awt.Container.dispatchEventImpl(Container.java:2085)
        at java.awt.Window.dispatchEventImpl(Window.java:2478)
        at java.awt.Component.dispatchEvent(Component.java:4460)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)

How is this caused and how can I solve it?

Answer 1

Read the 1st line of the exception stacktrace.

at SoundsTrip.SoundBytePlaying.openFile(SoundBytePlaying.java:267)

It’s been thrown in line 267 of class SoundsTrip.SoundBytePlaying, inside the openFile() method. Trackback it in your source code. It’ll contain something like

someObject.doSomething();

A NullPointerException means that someObject is null. Using the period operator . in an attempt to access/invoke it will then throw this exception.

There are basically two ways to fix it:

1. Instantiate it beforehand so that it’s not null.

   someObject = new SomeObject();
   someObject.doSomething();
   

2. Check it beforehand so that it can be bypassed when it’s null.

   if (someObject != null) {
       someObject.doSomething();
   }
   

Which to choose depends on the sole functional requirement.

---

Unrelated to the concrete problem, the Java naming conventions state that package names should preferably be written in all lowercase.