Returning A String Using A Method

[Extraho Vinco]

Hi, I am working on a Java Lab for school and trying to have it done before Monday, but I have hit a small problem.

Basically I am trying to return a string after an action event using a method called from a separate class, but I am running into a runtime error no matter how I return it. Its driving me up a wall and I have been trying different things for an hour now.

Here are my classes:

import javax.swing.JFrame;

public class Driver08 {
	
	public static void main (String[] args) throws Exception
	{
        JFrame frame = new JFrame("Unit4, Lab08: Dictionary");
        frame.setSize(300, 110);
        frame.setLocation(150, 100);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setContentPane(new Panel08());
        frame.setVisible(true);
	}

}

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

@SuppressWarnings("serial") // Suppression for an extension warning
public class Panel08 extends JPanel {
	
	JLabel output;
	Listener08 Listener;
	public Panel08() throws Exception
	{
		setLayout(new BorderLayout());
		add(new JLabel(" "), BorderLayout.NORTH);
		output = new JLabel("-----", SwingConstants.CENTER);
		add(output, BorderLayout.NORTH);

		Listener08 Listener = new Listener08();
		add(Listener, BorderLayout.CENTER);
		
		JPanel panel = new JPanel();
        panel.setLayout(new FlowLayout());
        addButton(panel, "Ok", new Listener1());
        addButton(panel, "Cancel", new Listener2());
        add(panel, BorderLayout.SOUTH);
	}
	
    private void addButton(JPanel p, String s, ActionListener a)
    {
       JButton b = new JButton(s);
       b.addActionListener(a);
       p.add(b);
    }
    
    private class Listener1 implements ActionListener
    {
       public void actionPerformed(ActionEvent e)
       {
    	   output.setText(Listener.checkWord());
       }
    }
    private class Listener2 implements ActionListener
    {
       public void actionPerformed(ActionEvent e)
       {
    	   System.out.println("Good-Bye.");
    	   System.exit(0);
       }
    }   
}

import java.awt.GridLayout;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.SwingConstants;

@SuppressWarnings("serial") //Same as above
public class Listener08 extends JPanel {

	private JTextField input;
	
	public Listener08() throws Exception
	{
		setLayout(new GridLayout(1,1));
		input = new JTextField();
		add(new JLabel("Enter Word: ", SwingConstants.CENTER));
		add(input);
	}

	public String checkWord() { //This Is supposed to check the string against an array, but the runtime error is preventing a return.
		
		String my_string = input.getText(); 
		return my_string;
		
	}
}

Thanks in advance! :

[Extraho Vinco]

Actually, I think the error falls within the calling of checkWord(), I tried to print a line within that method to the console and nothing appeared.

[Wnt2bsleepin]

Can you print the stack trace when you get the error?

[Extraho Vinco]

Here you go!
Defiantly broke something...

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
	at Panel08$Listener1.actionPerformed(Panel08.java:39)
	at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2028)
	at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2351)
	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:6375)
	at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)
	at java.awt.Component.processEvent(Component.java:6140)
	at java.awt.Container.processEvent(Container.java:2083)
	at java.awt.Component.dispatchEventImpl(Component.java:4737)
	at java.awt.Container.dispatchEventImpl(Container.java:2141)
	at java.awt.Component.dispatchEvent(Component.java:4565)
	at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4619)
	at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4280)
	at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4210)
	at java.awt.Container.dispatchEventImpl(Container.java:2127)
	at java.awt.Window.dispatchEventImpl(Window.java:2482)
	at java.awt.Component.dispatchEvent(Component.java:4565)
	at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:684)
	at java.awt.EventQueue.access$000(EventQueue.java:85)
	at java.awt.EventQueue$1.run(EventQueue.java:643)
	at java.awt.EventQueue$1.run(EventQueue.java:641)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
	at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:98)
	at java.awt.EventQueue$2.run(EventQueue.java:657)
	at java.awt.EventQueue$2.run(EventQueue.java:655)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
	at java.awt.EventQueue.dispatchEvent(EventQueue.java:654)
	at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:296)
	at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:211)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:201)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:196)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:188)
	at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)

[Wnt2bsleepin]

Yeah, it's throwing the exception at that line. What is the value of the JTextField input when you put it through the program?

[Extraho Vinco]

Yeah, it's throwing the exception at that line. What is the value of the JTextField input when you put it through the program?

"Test", I Have even tried instantiating a string within checkWord and returning that, but I'm still getting an exception thrown.

[Wnt2bsleepin]

Try this:

  public String checkWord() { //This Is supposed to check the string against an array, but the runtime error is preventing a return.
         
        return "Hello World"; 
         
    }

[Extraho Vinco]

Yup same error...
Maybe it has something to do with how its called...

[Wnt2bsleepin]

Ok, we know the error is probably not with the checkWord method. It could be what you said, but in case it's not, let's try this:

 private class Listener1 implements ActionListener
    {
       public void actionPerformed(ActionEvent e)
       {
           output.setText("Hello Again Glorious World");
       }
    }

[Extraho Vinco]

Worked like a charm.

[Wnt2bsleepin]

Ignore what I said before about how the problem is not in checkWord. We were able to pass a String to the Label, but unable to get the String from the method even if we directly returned it. If I had to guess, it would be that because Listener1 is a class, Listener may have not been initialized to anything and is pointing to null.

[Extraho Vinco]

Ignore what I said before about how the problem is not in checkWord. We were able to pass a String to the Label, but unable to get the String from the method even if we directly returned it. If I had to guess, it would be that because Listener1 is a class, Listener may have not been initialized to anything and is pointing to null.

This is where my knowledge of java breaks down as this is becoming very abstract... ^_^
But basically I gather that I did it wrong xD

So, how can I "fix" it?

EDIT: But wait isn't The Listener initialized by -

Listener08 Listener = new Listener08();

[Wnt2bsleepin]

That's in a different class, so I don't think it passes through. I got it to work.



If you can't access the variable Listener, how would you make a call to this method?

[Extraho Vinco]

That's in a different class, so I don't think it passes through. I got it to work.



If you can't access the variable Listener, how would you make a call to this method?

Excuse me while I go bash my head on my desk a couple times

output.setText(Listener08.checkWord());
and
public static String checkWord(){
return "hello world";
}

Works now! Thanks for the help, it was greatly appreciated!

[Wnt2bsleepin]

You are most welcome. I find that a different set of eyes can help in these situations.

[brynpttrsn]

Well..... while i kind of read the explanation given...

public class Panel08 extends JPanel {
     
    JLabel output;
    Listener08 Listener;
    public Panel08() throws Exception
    {
        setLayout(new BorderLayout());
        add(new JLabel(" "), BorderLayout.NORTH);
        output = new JLabel("-----", SwingConstants.CENTER);
        add(output, BorderLayout.NORTH);
 
        Listener08 Listener = new Listener08();
        add(Listener, BorderLayout.CENTER);
         
        JPanel panel = new JPanel();
        panel.setLayout(new FlowLayout());
        addButton(panel, "Ok", new Listener1());
        addButton(panel, "Cancel", new Listener2());
        add(panel, BorderLayout.SOUTH);
    }

... are you supposed to declare Listener 2 times?
(lines 4 and 12)