AWT-EventQueue-0" - need Help

[Beth Laguardia]

I am new to Swing and I wonder if somebody can explain to me what is wrong with the program I am making:

Here's the code:

  package firstGui;
import javax.swing.*;
import java.awt.event.*;
 
public class SimpleGui_2 implements ActionListener {
 
JButton button1;

public static void main(String[] args) {
// TODO Auto-generated method stub

SimpleGui_2 newGui = new SimpleGui_2();
newGui.go();
}

public void go()
{
JFrame frame = new JFrame();
button1  = new JButton(" click me");

button1.addActionListener(this);

frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.getContentPane().add(button1);

frame.setSize(300,300);
frame.setVisible(true);
}

public void actionPerformed(ActionEvent e)
{
button1.setText("You click me!");
}
 
}
  

Here's the error:

  Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at firstGui.SimpleLayout.actionPerformed(SimpleLayout.java:40)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(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)
  

Please explain to me what's wrong.. ANybody? Thank you..

[Brian Cole]

Your firstGui.SimpleGui_2 code looks ok to me,
though I haven't actually tried running it.

The error complains about the actionPerformed()
method in the firstGui.SimpleLayout class, which
you haven't shown us.

[Campbell Ritchie]

Have you managed to pass null to anything? This two-year old thread resurfaced yesterday; I wrote about how I look for NullPointerExceptions, which are a frequent problem, so you need to know about them.

[Beth Laguardia]

Brian: Yeah, sorry. The first program that I posted is a working program.

Here's the not working program.. sorry for the inconvenience..

  package firstGui;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
 
 
public class SimpleLayout implements ActionListener {
 
JFrame frame;
JButton button1,button2;
public static void main(String[] args) {
SimpleLayout newLayout = new SimpleLayout();
newLayout.go();
 
}

public void go()
{
  frame = new JFrame();
  JPanel panel = new JPanel();
  JButton button1 = new JButton("Click Me");
  JButton button2 = new JButton("Should be me");
  
  frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  panel.setBackground(Color.green);
  
  panel.setLayout(new BoxLayout(panel, BoxLayout.Y_AXIS));
  panel.add(button1);
  panel.add(button2);
  
  button1.addActionListener(this);
  frame.getContentPane().add(BorderLayout.EAST, panel);
  frame.setSize(300,300);
  frame.setVisible(true);
  
}

public void actionPerformed (ActionEvent e)
{
button1.setText("Ouch!");
Font newFont = new Font("Arial",Font.BOLD,15);
button1.setFont(newFont);

}
 
 
 
}  

Please somebody explain what's wrong? Thank you all..

[ August 28, 2008: Message edited by: Beth Laguardia ]
[ August 28, 2008: Message edited by: Beth Laguardia ]

[Norm Radder]

SimpleLayout.actionPerformed(SimpleLayout.java:40)

The error message says an object reference (pointer) at source line 40 in SimpleLayout is null. Look at that line of code and see what pointer could be null. I bet its button1.

   JButton button1 = new JButton("Click Me"); 

In the go method you've "hidden" the definition of button1 with a local one and have NOT initialized the global one referenced where the NPE occurs.
[ August 28, 2008: Message edited by: Norm Radder ]

[Beth Laguardia]

Your right!

I change this

 JButton button1 = new JButton("Click Me");  

to ths:

   button1 = new JButton("Click Me"); 

Thank you!