尝试停止重复J内部形式时的空指针异常

Question

I wanted to stop repeating J internal form by clicking the label in Home J Frame. And I'm trying to add "<<" to the label after I load the form. Now, I'm getting null pointer exception when I'm trying to load the second J internal form. This is my method to load J internal form:

public void lordForm(JInternalFrame frame) { 
        jDesktopPane1.add(frame);
        frame.setVisible(true);
    }


    public void load(String ss, JInternalFrame in, JLabel jl) {

        JInternalFrame[] j = jDesktopPane1.getAllFrames();
        boolean b = false;

       for (JInternalFrame jI : j) {
            if (jI.getName().equals(ss)) {
                b = true;
                break;
            } 
        } 
        if (!b) {
            LF(in);
            jl.setText(ss + " >>"); 
            jl.setEnabled(false);

        } else { 
          in.moveToFront();
        }
    } 

I'm Using moveToFront() method to move the selected J internal form.

These are the cords to label mouse releasing events:

private void lblinvoiceMouseReleased(java.awt.event.MouseEvent evt) <br/> {                                         
load("Invoice", new Invoice(), lblinvoice);

    }   
  private void lblReservationMouseReleased(java.awt.event.MouseEvent evt){                                             
   load("Reservation", new Reservation(), lblReservation);

    }   

Stack trace is given below.

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException at 
viewer.MainFrame.load(MainFrame.java:39) at 
viewer.MainFrame.lblinvoiceMouseReleased(MainFrame.java:186) at 
viewer.MainFrame.access$000(MainFrame.java:18) at 
viewer.MainFrame$1.mouseReleased(MainFrame.java:82)

And viewer.MainFrame.load(MainFrame.java:39) is if (jI.getName().equals(ss)) { at viewer.MainFrame.lblinvoiceMouseReleased(MainFrame.java:186) is load("Invoice", new Invoice(), lblinvoice);

Answer 1

I suspect the NPE might be in this block:

for (JInternalFrame jI : j) { // The first time j length == 0, so the loop is not executed
    if (jI.getName().equals(ss)) { // In this line, if jI.getName() == null, you'll get NPE
        b = true;
        break;
    } 
 } 

Are you sure you're setting the name of the JInternalFrame objects when you create them? This would explain why the first time you don't get NPE but the second time you do. If not so, please post the NPE's stack trace.

Update : based on stack trace recently added, the problem is the line I pointed above. So, you should make following changes to avoid NPE:

private void lblinvoiceMouseReleased(java.awt.event.MouseEvent evt) {
    String name = "Invoice";
    Invoice invoice = new Invoice();
    invoice.setName(name); 
    load(name, invoice, lblinvoice);
}

private void lblReservationMouseReleased(java.awt.event.MouseEvent evt){
    String name = "Reservation";
    Reservation reservation = new Reservation();
    reservation.setName(name);
    load(name, reservation, lblReservation);

}