Trying to change the text on a button when clicked - crasches....

[Peter Hammar]

Hi!

The code below compiles, but when I click on the spelarJB the program crashes.

  

package bkjakob;

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.Action;
import javax.swing.JButton;
import javax.swing.JFrame;

public class welCome implements ActionListener {

JButton spelarJB;

    public static void main (String [] args) {

        welCome wc = new welCome();
        wc.createGUI();
    }

    public void createGUI() {
     
        JFrame frame = new JFrame("FC Arsenal");
        frame.setSize(250, 250);
        
        //skapar knapparna
        
        JButton spelarJB = new JButton("Players");
        spelarJB.addActionListener(this);
                
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(spelarJB);
        frame.setVisible(true);
    }

    /**
     * @param e
     */
    public void actionPerformed(ActionEvent e) {
        spelarJB.setText("yeah!!");
    }
    }   
        
  

The error messages:

  

D:\Oracle\Middleware\jdk160_14_R27.6.5-32\bin\javaw.exe -client -classpath D:\JDeveloper\mywork\bkjakob\bkjakob\classes -Djavax.net.ssl.trustStore=D:\Oracle\Middleware\wlserver_10.3\server\lib\DemoTrust.jks bkjakob.welCome
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
	at bkjakob.welCome.actionPerformed(welCome.java:39)
	at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
	at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
	at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
	at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
	at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
	at java.awt.Component.processMouseEvent(Component.java:6263)
	at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)
	at java.awt.Component.processEvent(Component.java:6028)
	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:4574)
	at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4238)
	at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4168)
	at java.awt.Container.dispatchEventImpl(Container.java:2085)
	at java.awt.Window.dispatchEventImpl(Window.java:2475)
	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)
Process exited with exit code 0.

  

What have I missed?

Regards
H

[Campbell Ritchie]

You have declared the button twice, once as a field and once as a local variable. The local variable is initialised, goes out of scope, and the field is left as null.

[Peter Hammar]

But otherwise I get this:

  

Error(39,9):  cannot find variable spelarJB

Where should I create the button so that the actionPerformed method can see it?

[Ernest Friedman-Hill]

There are 2 distinct variables named spelarJB -- one is a member, shared by all methods, the other declared in the createGUI() method. This latter one is the one being initialized, but your event handler is expecting the former one to be. In CreateGUI, don't say "JButton spelarJB =" -- just say "spelarJB =". This is a common mistake!

[Peter Hammar]

Thanks a lot for your time and help it works now

[Campbell Ritchie]

Well done