Null pointer Exception. after a bit of execution!! Plz help me

[rohan]

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Component;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Toolkit;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.AbstractAction;
import javax.swing.Icon;
import javax.swing.ImageIcon;
import javax.swing.JApplet;
import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JPanel;
import javax.swing.JTextPane;
import javax.swing.JToolBar;
import javax.swing.KeyStroke;
import javax.swing.border.BevelBorder;

import javax.swing.Timer;
import javax.swing.Timer;

class CA_1D2S3N {

private byte[][]rows;

private int curRow=0,nextRow=1;
private int rowSize=50;
private int cellStates=2;

private byte[] rules={0,1,1,1,1,0,0,0};

public CA_1D2S3N(){}

public CA_1D2S3N(int rSize){

rowSize=rSize;
rows=new byte[2][];
rows[curRow] =new byte[rowSize];
rows[nextRow] =new byte[rowSize];

reset();
}
public int getRowSize(){
return rowSize;
}
public void reset(){
for(int i=curRow;i<=nextRow;++i){
for(int j=0;j<rowSize;j++){
rows[i][j]=0;
}
}
rows[curRow][(int)(rowSize/2)]=1;


}
public void TimeStep(){
for(int i=1;i<rowSize-2;++i){
int state=Map(rows[curRow],i);

rows[1][i]=rules[state];
}
for(int i=0;i<rowSize;i++){
rows[0][i]=rows[1][i];

}

}

int Map(byte []row,int i){

int x=4*row[i-1] + 2*row[i] + 1*row[i+1];

return x;
}
byte[] getcurrentRow(){

return rows[curRow].clone();
}
}


public class MainFrame extends JApplet implements ActionListener{

public JPanel pane;
CA_1D2S3N ca=new CA_1D2S3N (50);
int timeStep=101;
public JMenuBar menuBar;

public JToolBar toolBar;
JButton Tstart;
JButton Tstop;

public static Timer timer=new Timer(100,null);


public void timerEvent(ActionEvent e){

ca.TimeStep();

ShowCurrentRow(ca.getcurrentRow(),++timeStep);
if (timeStep>100) timer.stop();

}



public MainFrame() {
menuBar = new JMenuBar();

JMenu formatMenu = new JMenu("Timer");
formatMenu.add("Start");
formatMenu.add("Stop");

menuBar.add(formatMenu);

Tstart=new JButton("Start");
Tstop=new JButton("Stop");

toolBar = new JToolBar("Formatting");
toolBar.add("Start",Tstart);
Tstart.addActionListener(this);
formatMenu.addActionListener(this);
timer.addActionListener(new Main_Frame_Timer_ActionAdapter(this));

toolBar.addSeparator();


}

public void start() {

MainFrame example = new MainFrame();
example.pane = new JPanel();
example.pane.setPreferredSize(new Dimension(500, 500));
example.pane.setBorder(new BevelBorder(BevelBorder.LOWERED));
example.toolBar.setMaximumSize(example.toolBar.get Size());

JFrame frame = new JFrame("CA Example");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOS E);
frame.setJMenuBar(example.menuBar);
frame.getContentPane().add(example.toolBar, BorderLayout.NORTH);
frame.getContentPane().add(example.pane, BorderLayout.CENTER);
frame.pack();
frame.setVisible(true);
}
public void stop() {

}
public void destroy() {

}
public void clear(){
Graphics g=pane.getGraphics();
g.setColor(Color.WHITE);
g.fillRect(0, 0, pane.getWidth(), pane.getHeight());
}
public void ShowCurrentRow(byte[] row,int time){
Graphics g=pane.getGraphics();
for(int i=0;i<row.length;++i){
if(row[i]>0)
g.fillRect(3*(10+i), 3*(time+5), 3, 3);
}
}



public void actionPerformed(ActionEvent e) {
// TODO Auto-generated method stub

if(e.getSource()==Tstart){

if(timer.isRunning()) timer.stop();
else{
if(timeStep>100){
ca.reset();
timeStep=0;
clear();
ShowCurrentRow(ca.getcurrentRow(),timeStep);
}
timer.start();
}

}

}

}



class Main_Frame_Timer_ActionAdapter implements ActionListener{
MainFrame adaptee;
public Main_Frame_Timer_ActionAdapter(MainFrame ad){
this.adaptee=ad;
}

public void actionPerformed(ActionEvent e) {
// TODO add timer event
adaptee.timerEvent(e);

}
}

//-----------Error----------
Exception in thread "AWT-EventQueue-1" java.lang.NullPointerException
at MainFrame.ShowCurrentRow(MainFrame.java:166)
at MainFrame.timerEvent(MainFrame.java:108)
at Main_Frame_Timer_ActionAdapter.actionPerformed(Mai nFrame.java:207)
at javax.swing.Timer.fireActionPerformed(Unknown Source)
at javax.swing.Timer$DoPostEvent.run(Unknown Source)
at java.awt.event.InvocationEvent.dispatch(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForHierar chy(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarch y(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

[hardwired]

//  <applet code="MFApplet" width="500" height="500"></applet>
//  run this at the prompt with: >appletviewer MFApplet.java
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.Timer;
import javax.swing.border.BevelBorder;

public class MFApplet extends JApplet implements ActionListener{
    public JPanel pane;
    CA_1D2S3N ca=new CA_1D2S3N (50);
    int timeStep=101;
    public JMenuBar menuBar;

    public JToolBar toolBar;
    JButton Tstart;
    JButton Tstop;

    public static Timer timer=new Timer(100,null);

    public void timerEvent(ActionEvent e){
        ca.TimeStep();

        ShowCurrentRow(ca.getcurrentRow(),++timeStep);
        if (timeStep>100) timer.stop();
    }

//    public MFApplet() {  // this is a constructor for an app

    public void init() {   // applet construction code goes here
        menuBar = new JMenuBar();

        JMenu formatMenu = new JMenu("Timer");
        formatMenu.add("Start");
        formatMenu.add("Stop");

        menuBar.add(formatMenu);

        Tstart=new JButton("Start");
        Tstop=new JButton("Stop");

        toolBar = new JToolBar("Formatting");
        toolBar.add("Start",Tstart);
        Tstart.addActionListener(this);
        formatMenu.addActionListener(this);
        timer.addActionListener(new Main_Frame_Timer_ActionAdapter(this));

        toolBar.addSeparator();
        pane = new JPanel();
        pane.setPreferredSize(new Dimension(500, 500));
        pane.setBorder(new BevelBorder(BevelBorder.LOWERED));
        Dimension size = toolBar.getSize();
        Dimension prefSize = toolBar.getPreferredSize();
        System.out.printf("toolBar size = [%2d, %2d]%n" +
                          "    prefSize = [%2d, %2d]%n",
                           size.width, size.height,
                           prefSize.width, prefSize.height);
        toolBar.setMaximumSize(toolBar.getSize()); 
        setJMenuBar(menuBar);
        getContentPane().add(toolBar, BorderLayout.NORTH);
        getContentPane().add(pane, BorderLayout.CENTER);
    }

    public void start() {
        // This method is called everytime an applet is restored.
        // To run this applet as an application move the code that
        // was here off to a main method.
    }

    public void stop() {}

    public void destroy() {}

    public void clear(){
        Graphics g=pane.getGraphics();
        g.setColor(Color.WHITE); 
        g.fillRect(0, 0, pane.getWidth(), pane.getHeight()); 
    }

    public void ShowCurrentRow(byte[] row,int time){
        Graphics g=pane.getGraphics();
        for(int i=0;i<row.length;++i){
            if(row[i]>0)
                g.fillRect(3*(10+i), 3*(time+5), 3, 3);
        }
    }

    public void actionPerformed(ActionEvent e) {
        if(e.getSource()==Tstart){
            if(timer.isRunning()) timer.stop();
            else{
                if(timeStep>100){
                    ca.reset();
                    timeStep=0;
                    clear();
                    ShowCurrentRow(ca.getcurrentRow(),timeStep);
                }
                timer.start();
            }
        }
    }

    /** Convenience main to run applet as an aplication. */
    public static void main(String[] args) {
        MFApplet applet = new MFApplet();
        Dimension d = applet.getPreferredSize();
        System.out.printf("applet prefSize = [%d, %d]%n",
                           d.width, d.height);
        applet.setPreferredSize(new Dimension(500, 500));
        JFrame frame = new JFrame("CA Example");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(applet);
        frame.pack();
        applet.init();
        frame.setVisible(true);
    }
}

class Main_Frame_Timer_ActionAdapter implements ActionListener{
    MFApplet adaptee;

    public Main_Frame_Timer_ActionAdapter(MFApplet ad){
        this.adaptee=ad;
    }

    public void actionPerformed(ActionEvent e) {
        // TODO add timer event
        adaptee.timerEvent(e);
    }
}

class CA_1D2S3N {
    private byte[][]rows;

    private int curRow=0,nextRow=1;
    private int rowSize=50;
    private int cellStates=2;

    private byte[] rules={0,1,1,1,1,0,0,0};

    public CA_1D2S3N(){}

    public CA_1D2S3N(int rSize){
        rowSize=rSize;
        rows=new byte[2][]; 
        rows[curRow] =new byte[rowSize];
        rows[nextRow] =new byte[rowSize];

        reset();
    }

    public int getRowSize(){
        return rowSize;
    }

    public void reset(){
        for(int i=curRow;i<=nextRow;++i){
            for(int j=0;j<rowSize;j++){
                rows[i][j]=0;
            }
        }
        rows[curRow][(int)(rowSize/2)]=1;
    }

    public void TimeStep(){
        for(int i=1;i<rowSize-2;++i){
            int state=Map(rows[curRow],i);

            rows[1][i]=rules[state];
        }
        for(int i=0;i<rowSize;i++){
            rows[0][i]=rows[1][i];
        }
    }

    int Map(byte []row,int i){
        int x=4*row[i-1] + 2*row[i] + 1*row[i+1];

        return x;
    }

    byte[] getcurrentRow(){
        return rows[curRow].clone();
    }
}

Note what happens to your graphics after the applet is minimized/restored or partially un/covered. This happens when you draw inside event code. To avoid
this do your drawing inside the JPanel "pane"s paintComponent method.
In java pseudocode:

    ...
    public void init() {
        ...
        pane = new Pseudo();
        ...

class Pseudo extends JPanel {
   // Use booleans or other member variables to control
   // the state of this component. Set up the drawing
   // code inside [i]paintComponent[/i] so it can/will
   // draw the current state of this compoinent at any time.

    /** override this method for custom drawing */
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        // do custom drawing here.
    }
}

Then you alter/control/set the state/member variables in this class from your applet event code and tell it to repaint.

[DonCash]

Hello rohan,

Welcome to the Java Forums.

In future, please put the tags around your code! It stops errors and makes it much easier to read.