为什么在启动GUI时出现NullPointerException？

Question

I have coded a gui for my CRUD Program, and when I want to run it I get:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at gui.guimain$1.run(guimain.java:477)
    at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:597)
    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)

Thats the code where the NPE comes from:

public static void main(String[] args){
    javax.swing.SwingUtilities.invokeLater(new Runnable() {
        public void run(){
                maingui.showStart();// thats the line with the Exception
        }
    });
}

and thats the method showStart():

public void showStart(){
    mainFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    mainFrame.getContentPane().removeAll();
    tabstructure.removeAll();
    Produkt filter=new Produkt();

    JPanel P=new JPanel();
    P=Produktgui(0, filter);
    JPanel R=new JPanel();
    R=Billgui(0);
    JPanel nR=new JPanel();
    nR=Billgui(0);

    tabstructure.addTab("e", P);
    tabstructure.addTab("Bills", R);
    tabstructure.addTab("Pay bill", nR);
    mainFrame.getContentPane().add(tabstructure);
    mainFrame.validate();
    mainFrame.repaint();
    mainFrame.pack();
    mainFrame.setVisible(true);
}

Why do I get a NullPointerException ?

Answer 1

You will have to create an instance of your MainGui .

public static void main(String[] args) {
    final MainGui maingui = new MainGui();
    javax.swing.SwingUtilities.invokeLater(new Runnable() {
        public void run(){
                maingui.showStart();// thats the line with the Exception
        }
    });
}

Answer 2

Because something that you're calling a method on is null .

Look carefully at the error message. It says that the exception happens on line 477 of guimain.java .

At that point, maingui is null .

Answer 3

You must instantiate objects before you can invoke methods on them. In this case, maingui hasn't been instantiated.

public static void main(String[] args) {
    javax.swing.SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            MainGUI maingui = new MainGUI();
            maingui.showStart();
        }
    });
}