null pointer exception

[depak khatri]

i am making a prog to enter values in a textfield array and sum all those values but
i am getting null pointer exception when i click on sum button and clear button
please help me out

  
import java.awt.*;
import java.applet.*;
import java.awt.event.*;
//<applet code="M"width="700"height="700"></applet>
public class M extends Applet implements ActionListener
{
Button sum;
Button avg;
Button clear;
static final int n=4;
int s[]=new int[n];
int p;
 String f= "";
TextField t[][]=new TextField[n][n];
public void init()
{

setLayout(new GridLayout(n,n));
setFont(new Font("SansSerif",Font.BOLD,14));
for(int i=0;i<n;i++)
{
for(int j=0;j<n;j++)
{
t=new TextField[n][n];
t[i][j]=new TextField();
add(t[i][j]);
}
}

sum=new Button("Column Sum");
avg=new Button("Column average");
clear=new Button("Clear");

add(sum);
add(avg);
add(clear);
sum.addActionListener(this);
avg.addActionListener(this);
clear.addActionListener(this);


}
public void actionPerformed(ActionEvent e)
{Object obj=e.getSource();
if(obj==clear)
    {
    for(int i=0;i<n;i++)
         {
         for(int j=0;j<n;j++)
            {
            
               t[i][j].setText(f);
            }
        }
    }
if(obj==sum)
{

for(int j=0;j<n;j++)
{
for(int i=0;i<n;i++)
{
    int a = Integer.parseInt(t[i][j].getText());
    s[j]=a;
}
}
for(int i=0;i<n;i++)
{
for(int j=0;j<n;j++)
{
t[i][j].setText("");
}
}
p=n-1;
for(int j=0;j<n;j++)
{
    String a=Integer.toString(s[j]);

t[p][j].setText(a);
}
}

}

}  

[Ernest Friedman-Hill]

In this code

 for(int i=0;i<n;i++)  
 {  
     for(int j=0;j<n;j++)  
     {  
         t=new TextField[n][n];  
         t[i][j]=new TextField();  
         add(t[i][j]);  

For each value of i and j, you're creating a whole new 2-dimensional array, and putting a TextField into one element. The next time around the loop, you create another new array, discarding the first one, and put a TextField into a different element; etc. You get NullPointerExceptions because the end result is an array which contains just one TextField (the very last one) and a lot of nulls.

Just remove that "t=new TextField[n][n];" line. You create the array when the member variable is declared, so you don't need to create it again!>