Array Exception

// ArrayIndexOutOfBoundsException is raised when
// a[40] = 40 is attempted
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.applet.*;

public class ArrayProblem extends Applet implements ActionListener {
   JTextArea out;
   JButton button;
   int a[] = new int[40];
   
   public void init() {
      setLayout(new FlowLayout());
      add(new JScrollPane(out = new JTextArea(16,30)));
      add(button = new JButton("Press Me"));
      button.addActionListener(this);
   }

   public void doit() {
      for (int i=0 ; ; i++) {
         a[i] = i;
         out.append(i+"\n");
      }
   }

   public void actionPerformed (ActionEvent evt) { doit(); }
}