Problema con GridBagLayout in Java Swing

Question

Ho un problema con il mio semplice JPanel / GridBagLayout:

import javax.swing.*;
import java.awt.*;



public class gridfenster extends JFrame {

    private static final long serialVersionUID = 1L;
    private JPanel jContentPane = null;

    private JButton b1=null;
    private GridBagLayout gbl = null; // i tried it without this line!

    /**
     * @param args
     */
    public  void main() {
        // TODO Auto-generated method stub

        SwingUtilities.invokeLater(new Runnable()
                {
        public void run(){
            gridfenster thisClass = new gridfenster();
            thisClass.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE );
            thisClass.setVisible(true);
        }
    });

    }

    /**
     * @param owner
     */
    public  gridfenster() {
        super();
        initialize();
    }

    /**
     * This method initializes this
     * 
     * @return void
     */
    private void initialize() {
        this.setSize(300, 200);
        this.setContentPane(getJContentPane());
        this.setTitle("testgridbag");
    }

    /**
     * This method initializes jContentPane
     * 
     * @return javax.swing.JPanel
     */
    private JPanel getJContentPane() {
        if (jContentPane == null) {
            jContentPane = new JPanel();
            GridBagLayout gbl = new GridBagLayout();
            jContentPane.setLayout(gbl);
            GridBagConstraints gc = new GridBagConstraints();



            //konkrete Elemente:
            gc.fill =GridBagConstraints.HORIZONTAL ;
            gc.gridx =0; gc.gridy=0;
            gbl.setConstraints(b1,gc);
            jContentPane.add(b1);

        }
        return jContentPane;
    }

}

Poi ho il seguente errore:

IWAV0052E Invocation Target Exception creating gridfenster
java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
    at java.lang.reflect.Constructor.newInstance(Unknown Source)
    at org.eclipse.ve.internal.java.vce.launcher.remotevm.JFCLauncher$1.run(JFCLauncher.java:59)
    at java.awt.event.InvocationEvent.dispatch(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)
Caused by: java.lang.NullPointerException
    at java.awt.Container.addImpl(Unknown Source)
    at java.awt.Container.add(Unknown Source)
    at gridfenster.getJContentPane(gridfenster.java:71)
    at gridfenster.initialize(gridfenster.java:49)
    at gridfenster.<init>(gridfenster.java:39)
    ... 13 more

Answer 1

Non hai inizializzato il campo b1. Non è possibile aggiungere nulla ad un contenitore.

Answer 2

È necessario inizializzare sottocomponenti prima di aggiungerli al loro contenitore. Il riquadro dei contenuti si sta mettendo in B1 non ha idea di come gestire il pulsante, perché in realtà non esiste ancora.

Answer 3

ha dichiarato che 'B1' è un JButton, ma non avete mai istanziato esso.

Da qualche parte prima di gbl.setConstraints(b1,gc), è necessario dire b1=new JButton();