1 of 8 global variables not visible

[Bobbo]

I am writing a fairly simple program in the attempt to teach myself JAVA. I have 8 global variables, 3 JComboBoxes, 3 Calendar.instances, 1 int, and 1 JTextArea declared. All have been initialized. My ActionListener routine that services the JComboBoxes accesses or tries to access all of the global variables. It is successful on 7 of them, but I get a NULL reference pointer error at runtime on the JTextArea access. What am I doing wrong?

TIA,
Bobbo

[Fubarable]

What am I doing wrong?

1) You've got a bug in your code.
2) You're assuming that we can guess what that bug is without seeing code or knowing which line throws the exception.

[Fubarable]

But seriously... show us the code and the exception, and the line that throw the exception, and we'll likely be able to help you rather than guess.

Much luck!

[Bobbo]

LINK REMOVED BY BOBBO

is a link to the source file. I am declaring the JTextArea variable jta on line 13. I initialize the jta variable on line 72. I am getting the error message at runtime on line 169. (It is currently commented out so it will run.) The error message is

Exception in thread "main" java.lang.NullPointerException
at working.Working.actionPerformed(Working.java:161)
at javax.swing.JComboBox.fireActionEvent(Unknown Source)
at javax.swing.JComboBox.setSelectedItem(Unknown Source)
at javax.swing.JComboBox.setSelectedIndex(Unknown Source)
at working.Working.run(Working.java:64)
at working.Working.main(Working.java:22)

The actual line giving the error is
jta.setText(result);
which puts the String variable result into the JTextArea for display.

[Fubarable]

Best to post your code here, not at a second site. Here is your code.

import java.awt.FlowLayout;
import java.awt.event.*;
import java.util.Calendar;
import javax.swing.*;

public class Working implements ActionListener {
   JComboBox mcb;
   JComboBox ycb;
   JComboBox dcb;
   JTextArea jta;
   JScrollPane sP;
   Calendar now;
   Calendar later;
   Calendar permanent;
   int flag = 1;

   public static void main(String[] args) {
      Working me = new Working();
      me.run();
   }

   public void run() {
      now = Calendar.getInstance();
      later = Calendar.getInstance();
      permanent = Calendar.getInstance();
      permanent.set(2010, 10, 23);

      JFrame jf = new JFrame("Working");
      jf.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
      jf.setSize(640, 480);
      jf.setLayout(new BoxLayout(jf.getContentPane(), BoxLayout.Y_AXIS));

      JLabel ml = new JLabel("Month");
      String[] Months = {"January", "February", "March", "April", "May", "June", "July", "August",
         "September", "October", "November", "December"};
      mcb = new JComboBox(Months);
      mcb.setName("Month");
      mcb.setSelectedIndex(now.get(Calendar.MONTH));
      mcb.addActionListener(this);
      JPanel mp = new JPanel();
      mp.setLayout(new BoxLayout(mp, BoxLayout.Y_AXIS));
      mp.add(ml);
      mp.add(mcb);

      JLabel yl = new JLabel("Year");
      ycb = new JComboBox(setYear(now.get(Calendar.YEAR)));
      ycb.setName("Year");
      ycb.setSelectedItem(((Integer) (now.get(Calendar.YEAR))).toString());
      ycb.addActionListener(this);
      JPanel yp = new JPanel();
      yp.setLayout(new BoxLayout(yp, BoxLayout.Y_AXIS));
      yp.add(yl);
      yp.add(ycb);

      JLabel dl = new JLabel("Day");
      dcb = new JComboBox(setDay());
      dcb.setName("Day");
      dcb.addActionListener(this);
      flag = 0;
      dcb.setSelectedIndex(now.get(Calendar.DAY_OF_MONTH) - 1);
      
      JPanel dp = new JPanel();
      dp.setLayout(new BoxLayout(dp, BoxLayout.Y_AXIS));
      dp.add(dl);
      dp.add(dcb);

      jta = new JTextArea(2, 20);
      jta.setEditable(false);

      sP = new JScrollPane(jta, JScrollPane.VERTICAL_SCROLLBAR_AS_NEEDED,
               JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED);

      JPanel fp = new JPanel();
      fp.setLayout(new FlowLayout());
      fp.add(mp);
      fp.add(dp);
      fp.add(yp);
      jf.add(fp);
      jf.add(sP);

      // jf.pack();
      jf.setVisible(true);
   }

   private void fillYear(int yr) {
      String[] local = setYear(yr);
      flag++;
      ycb.removeAllItems();
      for (int i = 0; i < local.length; i++) {
         ycb.addItem(local[i]);
      }
      ycb.setSelectedIndex(1);
      flag--;
   }

   private String[] setYear(int y) {
      y = (y - (y % 10)) - 1;
      String[] local = new String[12];
      local[0] = "Lower";
      local[11] = "Higher";
      for (int i = 1; i < 11; i++) {
         local[i] = ((Integer) (y + i)).toString();
      }
      return local;
   }

   private void fillDay() {
      flag++;
      int a = dcb.getSelectedIndex();
      dcb.removeAllItems();
      String[] local = setDay();
      for (int i = 0; i < local.length; i++) {
         dcb.addItem(local[i]);
      }
      if (a >= local.length)
         a = local.length - 1;
      flag--;
      dcb.setSelectedIndex(a);
   }

   private String[] setDay() {
      final Integer[] month = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
      String[] local;
      int d = month[mcb.getSelectedIndex()] + LY();
      local = new String[d];
      for (int i = 0; i < d; i++) {
         local[i] = ((Integer) (i + 1)).toString();
      }
      return local;
   }

   private int LY() {
      if (mcb.getSelectedIndex() == 1) {
         int lly = Integer.parseInt(((String) ycb.getSelectedItem()));
         if ((lly % 400) == 0)
            return 1;
         if ((lly % 100) == 0)
            return 0;
         if ((lly % 4) == 0)
            return 1;
      }
      return 0;
   }

   public void actionPerformed(ActionEvent e) {
      String result;
      String[] dow = {"Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday", "Monday",
         "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday", "Monday"};
      JComboBox JCB = (JComboBox) e.getSource();
      if (JCB.getName() == "Day") {
         if (flag == 0) {
            later.set(Integer.parseInt((String) ycb.getSelectedItem()), mcb.getSelectedIndex(), dcb
                     .getSelectedIndex() + 1);
            long count = (later.getTimeInMillis() - permanent.getTimeInMillis()) / 86400000;
            count = ((count % 14) + 14) % 14;
            switch ((int) count) {
            case 0:
               ;
            case 1:
               ;
            case 2:
               ;
            case 3:
               ;
            case 4:
               ;
            case 5:
               ;
            case 6:
               result = "Bob is working.";
               break;
            default:
               result = "Bob is off.";
               break;
            }

            jta.setText(result);
            result = "Today is " + dow[(int) count] + ".";
            System.out.println(result);
         }
         return;
      }
      if (JCB.getName() == "Year") {
         int b = ycb.getSelectedIndex();
         switch (b) {
         case 0: {
            int i = Integer.parseInt((String) ycb.getItemAt(1)) - 10;
            fillYear(i);
            break;
         }
         case 11:
            int i = Integer.parseInt((String) ycb.getItemAt(1)) + 10;
            fillYear(i);
            break;
         }
         fillDay();
      }
      if (JCB.getName() == "Month") {
         fillDay();
      }
   }
}

Your problem is that you're adding ActionListeners to non-rendered components, and then triggering the ActionListeners with this line:

dcb.setSelectedIndex(now.get(Calendar.DAY_OF_MONTH) - 1);

but since the components have not been rendered by setVisible(true) or pack, a NullPointerException will occur when you try to set the text in the null JTextArea. The solution is add the listeners after the line of code above like so:

      mcb.setSelectedIndex(now.get(Calendar.MONTH));
      //!! mcb.addActionListener(this);
      JPanel mp = new JPanel();
      mp.setLayout(new BoxLayout(mp, BoxLayout.Y_AXIS));
      mp.add(ml);
      mp.add(mcb);

      JLabel yl = new JLabel("Year");
      ycb = new JComboBox(setYear(now.get(Calendar.YEAR)));
      ycb.setName("Year");
      ycb.setSelectedItem(((Integer) (now.get(Calendar.YEAR))).toString());
      //!! ycb.addActionListener(this);
      JPanel yp = new JPanel();
      yp.setLayout(new BoxLayout(yp, BoxLayout.Y_AXIS));
      yp.add(yl);
      yp.add(ycb);

      JLabel dl = new JLabel("Day");
      dcb = new JComboBox(setDay());
      dcb.setName("Day");
      //!! dcb.addActionListener(this);
      flag = 0;
      dcb.setSelectedIndex(now.get(Calendar.DAY_OF_MONTH) - 1);
      
      //!!
      mcb.addActionListener(this);
      ycb.addActionListener(this);
      dcb.addActionListener(this);
      //!!

[Bobbo]

I have studied that code for days, and could have studied it for weeks without finding that. You are a genius. That fixed the problem.

Thanks, Bobbo :)

[Fubarable]

You're welcome.

Happy coding!

[Bobbo]

Actually, I had placed that addActionListener statement prior to the dcb.setIndex() statement intentionally so the frame would open with the TextArea populated with that day's information. What I did to fix it was to move the instantiation of the JTextArea to the front of the routine so its pointer would no longer be NULL.

Bobbo