difficult design problem?

[gerrymcc]

// I want to place a panel which contains two canvases (a red one and
a blue one) on screen,
// along with two buttons. When a button is pressed the appropriate
message should be
// printed in the canvas. The code compiles okay, but I get a
run-time NullPointerException.
// I don't understand why - could anyone please explain the problem
and suggest a solution?
// Thanks very much for any help,
// Gerard
import java.awt.*;
import java.applet.Applet;
import java.awt.event.*;

class TypeOneApplet extends Applet implements ActionListener{

public final int CLEAR = 1;
public final int DULL = 2;
Button b1, b2;
Panel pan;
P p;
int theAction = 1;

public void actionPerformed( ActionEvent e){
if( e.getSource() == b1)
theAction = CLEAR;
else
theAction = DULL;
}

void methodOne(){ System.out.println(" method One"); }
void methodTwo(){ System.out.println(" method Two"); }
}

public class TypeTwoApplet extends TypeOneApplet {

public void init(){
b1 = new Button("clear");
b2 = new Button("dull");
b1.addActionListener(this);
b2.addActionListener(this);
this.add( b1);
this.add( b2);
pan = new Panel();
p = new P( this);
pan.add( p);
this.add( pan);
System.out.println("exit init");
}

void methodTwo(){ System.out.println(" method Two B"); }
}

class M extends Canvas implements MouseListener{
TypeOneApplet toa;
String message;

public M(){}
public M( TypeOneApplet t){
toa = t;
message = "ready";
}

public void mousePressed( MouseEvent evt){
if( toa.theAction == toa.CLEAR)
message = "clear";
else
message = "dull";
repaint();
}
public void mouseClicked( MouseEvent e) {}
public void mouseEntered( MouseEvent e) {}
public void mouseExited( MouseEvent e) {}
public void mouseReleased( MouseEvent e) {}

public void paint( Graphics g){
setBackground( Color.red);
setForeground( Color.black);
g.drawString( message, 5, 5);
}
}

class M2 extends M{
TypeTwoApplet atoa;

public M2( TypeTwoApplet t){
atoa = t;
}

public void mousePressed( MouseEvent evt){
if( atoa.theAction == toa.CLEAR)
message = "clear 2";
else
message = "dull 2";
repaint();
}

public void paint( Graphics g){
setBackground( Color.blue);
setForeground( Color.black);
g.drawString( message, 5, 5);
}
}

class P extends Panel{
M [] m;

public P( TypeTwoApplet t){
m[0] = new M(t);
m[1] = new M2(t);
add( m[0]);
add( m[1]);
}
}

[VisionSet]

<ge******@indigo.ie> wrote in message
news:vy*******************@news.indigo.ie...

The code compiles okay, but I get a
run-time NullPointerException.

Please give the 1st lines of the exception that apply to your code and
indicate which line of the code the line numbers in the exception trace
refer to.

--
Mike W

[gerrymcc]

Thanks for the suggestion about including error message. Here's the
run-time NullPointerException, which reads:
at P.(init)(TypeTwoApplet.java:107)
at TypeTwoApplet.init(TypeTwoApplet.java:43)

Regarding the previous subject line, I've stripped down and extracted
the idea of a much larger program to show the overall design, as I
wasn't certain that I was using the classes correctly. For
instance, as init() seems to be causing a problem, I wondered if it is
necessary to have a default init() in an applet in the same way that I
must supply a default constructor for class M. And I wondered if
class M2 will communicate with the applet subclasses (as I hope it
will).

As for posting to the three newsgroups, I'm unsure if it is supposed
to be better etiquette to post to one, read the replies, then post to
another, read the replies there, etc? I've found in the past that by
posting to two or three closely related groups at once I get
different, useful replies in each group.

Thanks again for any help,
Gerard
import java.awt.*;
import java.applet.Applet;
import java.awt.event.*;

class TypeOneApplet extends Applet implements ActionListener{

public final int CLEAR = 1;
public final int DULL = 2;
Button b1, b2;
Panel pan;
P p;
int theAction = 1;

public void actionPerformed( ActionEvent e){
if( e.getSource() == b1)
theAction = CLEAR;
else
theAction = DULL;
}

void methodOne(){ System.out.println(" method One"); }
void methodTwo(){ System.out.println(" method Two"); }
}

public class TypeTwoApplet extends TypeOneApplet {

public void init(){
b1 = new Button("clear");
b2 = new Button("dull");
b1.addActionListener(this);
b2.addActionListener(this);
this.add( b1);
this.add( b2);
pan = new Panel();
p = new P( this);
pan.add( p);
this.add( pan);
System.out.println("exit init");
}

void methodTwo(){ System.out.println(" method Two B"); }
}

class M extends Canvas implements MouseListener{
TypeOneApplet toa;
String message;

public M(){}
public M( TypeOneApplet t){
toa = t;
message = "ready";
}

public void mousePressed( MouseEvent evt){
if( toa.theAction == toa.CLEAR)
message = "clear";
else
message = "dull";
repaint();
}
public void mouseClicked( MouseEvent e) {}
public void mouseEntered( MouseEvent e) {}
public void mouseExited( MouseEvent e) {}
public void mouseReleased( MouseEvent e) {}

public void paint( Graphics g){
setBackground( Color.red);
setForeground( Color.black);
g.drawString( message, 5, 5);
}
}

class M2 extends M{
TypeTwoApplet atoa;

public M2( TypeTwoApplet t){
atoa = t;
}

public void mousePressed( MouseEvent evt){
if( atoa.theAction == toa.CLEAR)
message = "clear 2";
else
message = "dull 2";
repaint();
}

public void paint( Graphics g){
setBackground( Color.blue);
setForeground( Color.black);
g.drawString( message, 5, 5);
}
}

class P extends Panel{
M [] m;

public P( TypeTwoApplet t){
m[0] = new M(t);
m[1] = new M2(t);
add( m[0]);
add( m[1]);
}
}

[Chris]

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ge******@indigo.ie wrote:

[snip]

class P extends Panel{
M [] m;

public P( TypeTwoApplet t){
m[0] = new M(t);
m[1] = new M2(t);
add( m[0]);
add( m[1]);
}
}

Hi,
I see the problem. You declared a reference to an array of M, but you
never created the array before trying to store objects in it. You
should change the declaration to also create the actual array:

M[] m = new M[2];

I think this should be the only problem.

- --
Chris
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[Roedy Green]

On Fri, 08 Aug 2003 13:26:35 GMT, ge******@indigo.ie wrote or quoted :

. The code compiles okay, but I get a
run-time NullPointerException.

When it throws the null pointer exception you will get a stack trace
that tells you exactly where it occurred and how you got there. That
it what you have to look at.
--
Canadian Mind Products, Roedy Green.
Coaching, problem solving, economical contract programming.
See http://mindprod.com/jgloss/jgloss.html for The Java Glossary.