Exception in thread "main" java.lang.NullPointerException
at java.awt.Container.addImpl(Container.java:625)
at java.awt.Container.add(Container.java:307)
at Checkerboard.<init>(Checkerboard.java:31)
at Checkerboard.main(Checkerboard.java:95)

Can any1 help me understand what is wrong with this code ?
Here is the Code......................................................

import java.awt.*;
import java.awt.event.*;
import java.io.*;

public class Checkerboard extends Frame implements ActionListener
{
TextField checkerArray[];
Panel mainPad = new Panel();
Panel checkerRows = new Panel();
Panel fieldRow = new Panel();
TextField firstText = new TextField(5);
TextField scndText = new TextField(5);
TextField thirdText = new TextField(5);
int start, stop, step;
String temp;

public Checkerboard()
{
BufferedReader dataIn = new BufferedReader(new
InputStreamReader(System.in));
checkerArray = new TextField[16];
Button Clear = new Button("Clear");
Button Go = new Button("Go");
start = 0;
stop = 0;
step = 0;

setLayout(new BorderLayout());

checkerRows.setLayout(new GridLayout(4,4));
for(int f=0; f<checkerArray.length; f++)
checkerRows.add(checkerArray[f]);

fieldRow.setLayout(new GridLayout(2,3));
fieldRow.add(firstText, BorderLayout.SOUTH);
fieldRow.add(scndText, BorderLayout.SOUTH);
fieldRow.add(thirdText, BorderLayout.SOUTH);
fieldRow.add(Clear,BorderLayout.CENTER);
fieldRow.add(Go,BorderLayout.CENTER);

mainPad.add(checkerRows);
mainPad.add(fieldRow);
add(mainPad);


addWindowListener(
new WindowAdapter()
{
public void windowClosing(WindowEvent
e)
{
System.exit(0);
}
}
);
}
public void init()
{
for(int i = 0; i<=16; i++)
{
checkerArray[i].setText("i");
checkerArray[i].setEditable(false);
}

for(int f = 0; f < checkerArray.length; f++)
checkerArray[f].setBackground(Color.magenta);

for(int i = 0; i < checkerArray.length; i++)
checkerArray[i].addActionListener(this);

}

public void actionPerformed(ActionEvent e)
{
String arg = e.getActionCommand();
if(arg == "Go")
{
temp = firstText.getText();
start = Integer.parseInt(temp);
temp = scndText.getText();
stop = Integer.parseInt(temp);
temp = thirdText.getText();
step = Integer.parseInt(temp);

}
if(arg == "Clear")
{
firstText.setText("");
scndText.setText("");
thirdText.setText("");
}

}

public static void main(String args[])
{
Checkerboard checkerFrame = new Checkerboard();

checkerFrame.setBounds(50,100,300,400);
checkerFrame.setTitle("Checker Board");
checkerFrame.setVisible(true);
}
}