HELP! Run-time Error with this code.

Question

I'm having problem with the code below. But if I were to remove the writer class and instances of it (writeman), then there are no problems. Can some1 pls tell me why the writer class is giving problems. Btw, no compilation errors, only errors at run-time..........
import java.applet.*;
import java.awt.*;
import java.awt.event.MouseListener;
import java.awt.event.*;
public class HelloWorld extends Applet
public static String MyString = new String("Hello");
Graphics f;
public void init()
Changer Changer1 = new Changer();
writer writeman = new writer();
setBackground(Color.red);
setForeground(Color.green);
addMouseListener(Changer1);
writeman.paintit(f);
public void paint(Graphics g)
g.drawString(MyString,10 ,10);
public class Changer implements MouseListener
public void mouseEntered(MouseEvent e)
setBackground(Color.blue);
MyString = "HI";
paint(f);
repaint();
public void mouseExited(MouseEvent e)
setBackground(Color.red);
repaint();
public void mousePressed(MouseEvent e){};
public void mouseReleased(MouseEvent e){};
public void mouseClicked(MouseEvent e){};
public class writer
public void paintit(Graphics brush)
brush.drawString("can u see me", 20, 20);

Answer 1

I assume the exception you are getting is a NullPointerException...
When you applet is loaded, it is initialised with a call to init... the following will then occur...
HelloWorld.init()
writeman.paintit(f)
// f has not been initialised, so is null
brush.drawString("can u see me", 20, 20)
// brush == f == null, accessing a null object causes a NullPointerException!
The simplest way to rectify this is to not maintain your own reference to the Graphics object. Move the writer.paintit(f) method to the HelloWorld.paint(g) method, and pass in the given Graphics object. Also, change the paint(f) call in Changer to repaint(), which will cause the paint method to be called with a valid Graphics object - which will then be passed correctly to writer.
Hope this helps,
-Troy