Problem mit GridBagLayout in Java Swing

Question

Ich habe ein einfaches Problem mit meinem 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;
    }

}

Dann bekomme ich folgende Fehlermeldung:

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

Sie haben das b1 Feld initialisiert. Sie können nicht null auf einen Behälter hinzuzufügen.

Answer 2

Sie müssen Subkomponenten initialisieren, bevor sie zu ihrem Container hinzufügen. Der Inhaltsbereich sind Sie b1 Indienststellung hat keine Ahnung, wie die Schaltfläche zu handhaben, weil es tatsächlich existiert noch nicht.

Answer 3

Sie erklärt, dass 'b1' ist ein JButton, aber Sie instanziiert nie.

Irgendwo vor gbl.setConstraints(b1,gc), müssen Sie b1=new JButton(); sagen