NullPointerException by ActionListener

[YouGina]

Hello everybody,

At the moment I am trying to build an application that's using lot's of different classes. I am trying to create event handlers to handle the events from the different classes. Because the source is some kind of big, i have made a sort of simulation, with the same problem.

When you click the Ok button, there will be a NullPointerException. Can anybody tell me how I can solve this without rebuilding the whole source?

Here is the source of the `simulation`:

import java.awt.*;
import java.awt.event.*;

import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.JToolBar;
import java.awt.LayoutManager;

public class TijdsRegistratie extends JFrame {   
    public static void main(String[] args) {  	
    	JFrame frame = new TijdsRegistratie();
    	frame.setSize(500, 500);
    	frame.setDefaultCloseOperation(frame.EXIT_ON_CLOSE);
    	frame.setTitle("Tijds Registratie");
    	frame.setContentPane(new RegistratiePanel());
    	frame.setVisible(true);
    }
}

class RegistratiePanel extends JPanel {
	private JToolBar bar;
	private JButton Ok, Cancel, Start, Stop;
	public JTextField tijdStart, tijdEind, Voor, Van;
	
	public RegistratiePanel() {
		
		setLayout(null);
		
		Ok = new JButton("Ok");
		Cancel = new JButton("Cancel");
		Start = new JButton("Start");
		Stop = new JButton("Stop");
		
		Ok.addActionListener(new okHandler());
		
		bar = new JToolBar();
		
		bar.add(Ok);
		bar.add(Cancel);
		bar.add(Start);
		bar.add(Stop);
		
		add(bar);		
		bar.setFloatable(false);
		bar.setRollover(true);
		bar.setBounds(0, 0, 500, 30);
		
		tijdStart = new JTextField(10);
		tijdEind = new JTextField(10);
		Voor = new JTextField(10);
		Van = new JTextField(10);
		
		tijdStart.setBounds(100, 50, 300, 20);
		tijdEind.setBounds(100, 150, 300, 20);
		Voor.setBounds(100, 250, 300, 20);
		Van.setBounds(100, 350, 300, 20);
		
		add(tijdStart);
		add(tijdEind);
		add(Voor);
		add(Van);
					
	}
}
class okHandler implements ActionListener {
	RegistratiePanel reg;
	public void actionPerformed(ActionEvent e) {
		reg.tijdStart.setText("Er is op Ok gedrukt");
	}
}

Thanks in advance.

P.S. Sorry for my bad English, and non-English variables.

[RamyaSivakanth]

Hi,
First u should put what ur code is going to do in comments.Then only,it will be helpful for debugging.

class okHandler implements ActionListener {
RegistratiePanel reg;
public void actionPerformed(ActionEvent e) {
reg.tijdStart.setText("Er is op Ok gedrukt");
}
}

In the above piece of code u are tring to call the setText method with null reference without instantiating.Please check it.

[YouGina]

class okHandler implements ActionListener {
public void actionPerformed(ActionEvent e) {
RegistratiePanel reg = new RegistratiePanel();
reg.tijdStart.setText("Dit is een test");
}
}
Is this what you mean?

When I try this, nothing happens, the text "Dit is een test" is not sended to the JTextField, but there's also no error, although I would expect a stackoverflow?

The source with a couple of comments:

import java.awt.*;
import java.awt.event.*;

import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.JToolBar;
import java.awt.LayoutManager;

public class TijdsRegistratie extends JFrame {   
    public static void main(String[] args) {  	
    	JFrame frame = new TijdsRegistratie();
    	frame.setSize(500, 500);
    	frame.setDefaultCloseOperation(frame.EXIT_ON_CLOSE);
    	frame.setTitle("Tijds Registratie");
    	frame.setContentPane(new RegistratiePanel());
    	frame.setVisible(true);
    }
}

class RegistratiePanel extends JPanel {
	private JToolBar bar;
	private JButton Ok, Cancel, Start, Stop;
	public JTextField tijdStart, tijdEind, Voor, Van;
	
	public RegistratiePanel() {
		
		setLayout(null);
		
		Ok = new JButton("Ok"); // This button is going to be used
		Cancel = new JButton("Cancel");
		Start = new JButton("Start");
		Stop = new JButton("Stop");
		
		Ok.addActionListener(new okHandler()); // The actionhandler is called
		
		bar = new JToolBar();
		
		bar.add(Ok);  // Button added to the JToolBar
		bar.add(Cancel);
		bar.add(Start);
		bar.add(Stop);
		
		add(bar);		
		bar.setFloatable(false);
		bar.setRollover(true);
		bar.setBounds(0, 0, 500, 30);
		
		tijdStart = new JTextField(10); // when Ok button is pushed, text should appear in here.
		tijdEind = new JTextField(10);
		Voor = new JTextField(10);
		Van = new JTextField(10);
		
		tijdStart.setBounds(100, 50, 300, 20);
		tijdEind.setBounds(100, 150, 300, 20);
		Voor.setBounds(100, 250, 300, 20);
		Van.setBounds(100, 350, 300, 20);
		
		add(tijdStart);
		add(tijdEind);
		add(Voor);
		add(Van);
					
	}
}
class okHandler implements ActionListener {
	public void actionPerformed(ActionEvent e) {
		RegistratiePanel reg = new RegistratiePanel(); // I guess this is instantiating the RegistratiePanel?
		reg.tijdStart.setText("Dit is een test"); // This is the text which should appear in the JTextField tijdStart
	}
}

[RamyaSivakanth]

Hi,
Something u want to experiment with Java Compiler and make everyone to get :confused: ..Just kidding.

1.My question is when actionPerformed is called again u are instantiating and attaching the actionlistener with this object and totally u are doing the same thing make it stack overflow.

2.Why u need separate handler class for this?

I have corrected ur code.please check it.

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

public class Test extends JFrame {   
    public static void main(String[] args) {      
        JFrame frame = new Test();
        frame.setSize(500, 500);
        frame.setDefaultCloseOperation(frame.EXIT_ON_CLOSE);
        frame.setTitle("Tijds Registratie");
        frame.setContentPane(new RegistratiePanel());
        frame.setVisible(true);
    }
}

class RegistratiePanel extends JPanel implements ActionListener {
    private JToolBar bar;
    private JButton Ok, Cancel, Start, Stop;
    public JTextField tijdStart, tijdEind, Voor, Van;
    
    public RegistratiePanel() {
        
        setLayout(null);
        
        Ok = new JButton("Ok");
        Cancel = new JButton("Cancel");
        Start = new JButton("Start");
        Stop = new JButton("Stop");
        
        Ok.addActionListener(this);
        
        bar = new JToolBar();
        
        bar.add(Ok);
        bar.add(Cancel);
        bar.add(Start);
        bar.add(Stop);
        
        add(bar);        
        bar.setFloatable(false);
        bar.setRollover(true);
        bar.setBounds(0, 0, 500, 30);
        
        tijdStart = new JTextField(10);
        tijdEind = new JTextField(10);
        Voor = new JTextField(10);
        Van = new JTextField(10);
        
        tijdStart.setBounds(100, 50, 300, 20);
        tijdEind.setBounds(100, 150, 300, 20);
        Voor.setBounds(100, 250, 300, 20);
        Van.setBounds(100, 350, 300, 20);
        
        add(tijdStart);
        add(tijdEind);
        add(Voor);
        add(Van);
                    
    }

	public void actionPerformed(ActionEvent e) {
        tijdStart.setText("Er is op Ok gedrukt");
    }
}

[YouGina]

Do I have to do anything with something like this?:
(JTextField) Beans.instantiate(getClass().getClassLoader(), JTextField.class.getName());

Found this in a source on google, but i can't post the link, because i haven't 20 posts or more.

If I need this, how can I submit this to my classes?

[RamyaSivakanth]

Have u run my code?

[YouGina]

Yes i have. I don't know if this is the right solution for my problem. Is it possible to instantiate and use the objects at the way I shown in the source?

[RamyaSivakanth]

Just gothru ur code piece.When u instantiate again u are trying to create the components and again u are trying to register the event results to stack overflow....Just rethink ............U shouldn't confuse yourself and make others also to get confused.

U have to experiment but u shouldn't go out of focus.Just a freindly advice from me.

-Regards
Ramya

[YouGina]

I've found out how to do this.

I thank you for your quick replies. I think you're hint about instantiating helped me farther. I didn't no the English word for that, and Yesterday evening I googled and solved the problem.

Thank you for your help again.