Error = Null pointer with inner class

[jonytek]

Hi,
I am trying out using innerclasses with GUI but I keep getting the following error msg. Can someone please help me figure it out. Thanks..

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at Ex2201$Listen.actionPerformed(Ex2201.java:42)
at javax.swing.AbstractButton.fireActionPerformed(Unk nown Source)
at javax.swing.AbstractButton$Handler.actionPerformed (Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed (Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
at javax.swing.plaf.basic.BasicButtonListener.mouseRe leased(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(U nknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unkno wn 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.pumpOneEventForFilter s(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(U nknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarch y(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

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

public class Ex2201 
{
	JTextField j1,j2;
	JLabel label;
	
	public static void main(String[] args)
	{
		Ex2201 add = new Ex2201();
	}
	public Ex2201()
	{
		JFrame frame = new JFrame("Add numbers");
		frame.setSize(300, 300);
		frame.setLocation(500, 500);
		frame.setDefaultCloseOperation(frame.EXIT_ON_CLOSE);
		JPanel panel = new JPanel();
		JTextField j1 = new JTextField(5);
		JTextField j2 = new JTextField(5);
		JButton button = new JButton("Calculate");
		JLabel label = new JLabel();
		
		frame.add(panel);
		panel.add(j1);
		panel.add(j2);
		panel.add(button);
		panel.add(label);
		frame.setVisible(true);
		
		Listen listen = new Listen();
		button.addActionListener(listen);
	}
	public class Listen implements ActionListener
	{
		public void actionPerformed(ActionEvent e)
		{

			double d1 = Double.parseDouble(j1.getText());
			double d2 = Double.parseDouble(j2.getText());
			label.setText("The sum is " + (d1 + d2));
			
		}
	}
	
}

[Fubarable]

Hi,
I am trying out using innerclasses with GUI but I keep getting the following error msg. Can someone please help me figure it out. Thanks..

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

public class Ex2201 
{
	JTextField j1,j2; // [color="red"] *** LOCATION #1 ***[/color]
	JLabel label;
	
	public static void main(String[] args)
	{
		Ex2201 add = new Ex2201();
	}
	public Ex2201()
	{
		JFrame frame = new JFrame("Add numbers");
		frame.setSize(300, 300);
		frame.setLocation(500, 500);
		frame.setDefaultCloseOperation(frame.EXIT_ON_CLOSE);
		JPanel panel = new JPanel();
		JTextField j1 = new JTextField(5); // [color="red"] *** LOCATION #2 ***[/color]
		JTextField j2 = new JTextField(5);
		JButton button = new JButton("Calculate");
		JLabel label = new JLabel();
        ///.....

	public class Listen implements ActionListener
	{
		public void actionPerformed(ActionEvent e)
		{
			// [color="red"] *** LOCATION #3 ***[/color]
			double d1 = Double.parseDouble(j1.getText());
			double d2 = Double.parseDouble(j2.getText());
			label.setText("The sum is " + (d1 + d2));
			
		}
	}
	
}

Some observations:

• LOCATION #1: You declare the j1 and j2 variables in the class. These variables are never initialized.
• LOCATION #2: You re-declare the j1 and j2 variables in the constructor and instantiate them. These are separate unique objects completely unrelated to the j1 and j2 variables in the class.
• LOCATION #3: You try to use the class j1 and j2 variables, the ones that have never been initiated!

See the problem now and can you figure out its simple solution?

[Junky]

Classic example of local variables shadowing instance variables. Look very closely at your JTextFields.

[jonytek]

Thankyou guys. Your both legends!

New code:

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

public class Ex2201 
{
	JTextField j1 = new JTextField(5);
	JTextField j2 = new JTextField(5);
	JLabel label = new JLabel();
	
	public static void main(String[] args)
	{
		Ex2201 add = new Ex2201();
	}
	public Ex2201()
	{
		JFrame frame = new JFrame("Add numbers");
		frame.setSize(300, 300);
		frame.setLocation(500, 500);
		frame.setDefaultCloseOperation(frame.EXIT_ON_CLOSE);
		JPanel panel = new JPanel();
		JButton button = new JButton("Calculate");
		
		
		frame.add(panel);
		panel.add(j1);
		panel.add(j2);
		panel.add(button);
		panel.add(label);
		frame.setVisible(true);
		
		Listen listen = new Listen();
		button.addActionListener(listen);
	}
	public class Listen implements ActionListener
	{
		public void actionPerformed(ActionEvent e)
		{

			double d1 = Double.parseDouble(j1.getText());
			double d2 = Double.parseDouble(j2.getText());
			label.setText("The sum is " + (d1 + d2));
			
		}
	}
	
}