Null Pointer Exception in program
<p>The Below code does not fail to compile but however at runtime it says java.lang.NullPointerException at Line at line number 20 and 41. Also i am little bit  curious to know what is Null Pointer Exception, what happens at runtime ?</p>

<pre><code>import java.awt.*;
import java.awt.event.*;
import javax.swing.*;


public class Tool 
    {
     private JToolBar toolbar1;
     private JToolBar toolbar2;
     private JPanel panel;
     public Tool()
        {
         JFrame frame= new JFrame();
         panel = new JPanel();
         panel.setLayout(new BoxLayout(panel,BoxLayout.Y_AXIS));
         JButton one = new JButton("one");
         JButton two = new JButton("two");
         JButton three = new JButton("three");
         JButton four = new JButton("four");
             toolbar1 = new JToolBar();
             toolbar2 = new JToolBar();
         toolbar1.add(one);
         toolbar1.add(two);
         toolbar2.add(three);
         toolbar2.add(four);
         toolbar1.setAlignmentX(0);
         toolbar2.setAlignmentX(0);
         panel.add(toolbar1);
         panel.add(toolbar2);
         frame.add(panel,BorderLayout.NORTH);

         frame.setDefaultCloseOperation(frame.EXIT_ON_CLOSE);
         frame.setSize(400,300);
         frame.setTitle("ZOOP");
         frame.setVisible(true);
        }




        public static void main (String args[])
        {
         Tool zoop = new Tool();
        }


    }
</code></pre>
