Using innerclass to evaluate checkbox event is not working

Question

I am learning Java. I created a Program that has a text area and a checkbox. Every time I check or uncheck, I want it to write the checkbox's state on the text area. I learn about innerclasses and Action Events. The following code seems legit, but it is showing an error:

Exception in thread "main" java.lang.NullPointerException
    at TextArea1.go(TextArea1.java:26)
    at TextArea1.main(TextArea1.java:14)

I am not sure what I am doing wrong.

Line 14 is gui.go(); Line 26 is text = new JTextArea(10, 20);

Another question. I know I can get million results in google. But can you suggest a good tutorial to learn debugging in Eclipse.

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

public class TextArea1   
{
    JTextArea text;
    JCheckBox check;

    public static void main (String[] args)
    {
        TextArea1 gui = new TextArea1();
        gui.go();   
    }

    public void go()
    {
        JFrame frame = new JFrame();
        JPanel panel = new JPanel();

        new JCheckBox("Goes to 11");

        check.addItemListener(new CheckListener());

        text = new JTextArea(10, 20);
        text.setLineWrap(true);

        JScrollPane scroller = new JScrollPane(text);
        scroller.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
        scroller.setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_NEVER);

        panel.add(scroller);

        frame.getContentPane().add(BorderLayout.CENTER,panel);
        frame.getContentPane().add(BorderLayout.SOUTH,check);

        frame.setSize(350,300);
        frame.setVisible(true);

    }

    class CheckListener implements ItemListener
    {
        public void itemStateChanged(ItemEvent ev)
        {
            String onOrOff = "off";
            if (check.isSelected()) 
                onOrOff = "on";
            text.append("Check = " + onOrOff);
        }
    }


}

Answer 1

The problem is:

new JCheckBox("Goes to 11");

Is never set to to your variable check

check = new JCheckBox("Goes to 11");

Answer 2

Modified your code, you weren't initializing your checkbox

public class TextArea1 {
    JTextArea text;

    JCheckBox check;

    public static void main(String[] args) {
        TextArea1 gui = new TextArea1();
        gui.go();
    }

    public void go() {
        JFrame frame = new JFrame();
        JPanel panel = new JPanel();

        check = new JCheckBox("Goes to 11");

        check.addItemListener(new CheckListener());

        text = new JTextArea(10, 20);
        text.setLineWrap(true);

        JScrollPane scroller = new JScrollPane(text);
        scroller.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
        scroller.setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_NEVER);

        panel.add(scroller);

        frame.getContentPane().add(BorderLayout.CENTER, panel);
        frame.getContentPane().add(BorderLayout.SOUTH, check);

        frame.setSize(350, 300);
        frame.setVisible(true);

    }

    class CheckListener implements ItemListener {
        @Override
        public void itemStateChanged(ItemEvent ev) {
            String onOrOff = "off";
            if (check.isSelected())
                onOrOff = "on";
            text.append("\nCheck = " + onOrOff);
        }
    }

}